A new giant outburst of the Be X-ray binary RX J0520.5-6932 was detected and subsequently observed with several space-borne and ground-based instruments. This study presents a comprehensive analysis of the optical and X-ray data, focusing on the spectral and timing characteristics of selected X-ray observations. A joint fit of spectra from simultaneous observations performed by the X-ray telescope (XRT) on the Neil Gehrels Swift Observatory (Swift) and Nuclear Spectroscopic Telescope ARray (NuSTAR) provides broad-band parameter constraints, including a cyclotron resonant scattering feature (CRSF) at $32.2_{-0.7}^{+0.8}$ keV with no significant energy change since 2014, and a weaker Fe line. Independent spectral analyses of observations by the Lobster Eye Imager for Astronomy, Einstein Probe (EP), Swift-XRT, and NuSTAR demonstrate the consistency of parameters across different bands. Luminosity variations during the current outburst were tracked. The light curve of the Optical Gravitational Lensing Experiment (OGLE) aligns with the X-ray data in both 2014 and 2024. Spin evolution over 10 yr is studied after adding Fermi Gamma-ray Burst Monitor data, improving the orbital parameters, with an estimated orbital period of 24.39 d, slightly differing from OGLE data. Despite intrinsic spin-up during outbursts, a spin-down of $\sim$0.04 s over 10.3 yr is suggested. For the new outburst, the pulse profiles indicate a complicated energy-dependent shape, with decreases around 15 and 25 keV in the pulsed fraction, a first for an extragalactic source. Phase-resolved NuSTAR data indicate variations in parameters such as flux, photon index, and CRSF energy with rotation phase.

Super-Eddington accretion onto stellar-mass compact objects powers fast outflows in Ultra-Luminous X-ray sources (ULXs). Such outflows, which can reach mildly relativistic velocities, are often observed forming bubble structures. Wind bubbles are expected to develop strong wind termination shocks, sites of great interest for diffusive shock acceleration. We develop a model of diffusive shock acceleration in the wind bubbles powered by ULXs. We find that the maximum energy in these objects can easily reach the PeV range, promoting ULX winds as a new class of PeVatrons. We specialize our model in the context of the Galactic source SS 433 and show that high-energy protons in the bubble might explain the highest energy photons (>100 TeV) and their morphology recently observed by LHAASO. We discuss the detectability of such a source in neutrinos and we analyze the possible radio counterpart of ULXs focusing on the case of W50, the nebula surrounding SS 433. We finally discuss the possible contribution of Galactic ULXs to the cosmic-ray flux at the knee concluding that their role might be substantial.

An X-ray brightening of a source likely associated with the Be X-ray binary candidate RX J0032.9-7348 in the SMC (Kahabka, Pietsch 1996, A & A 312, 919) was recently discovered with the Einstein Probe mission (EP, ATel#16880).

The Magellanic Clouds are our closest star-forming galaxies with low Galactic foreground absorption. This makes them a unique laboratory to study the population of high-energy sources. The SMC hosts a large population of Be/X-ray binaries associated with high star formation activity 25-40 Myr ago. It has been proposed that the HMXB population in the LMC is associated with more recent star formation. However, due to the large angular extent and resulting insufficient coverage of the LMC, this association with SFR is not well established yet. An essential asset for studying the HMXB population in the entire LMC was the launch of eROSITA. eROSITA scans the sky in great circles crossing at the ecliptic poles. Due to the vicinity of the south-ecliptic pole, sources in the LMC are monitored for up to several weeks during each all-sky survey, leading to a deep total exposure and the possibility of studying long-term temporal behaviour. This allowed us to discover several new HMXBs, verify candidate HMXBs and construct a complete, flux-limited catalogue. During my presentation, I will first focus on HMXB population properties in the LMC. Then I will discuss individual systems we discovered with eROSITA, such as a Be-WD and an SFXT candidate.

LeHaMoC simulates high-energy astrophysical sources. It simulates the behavior of relativistic pairs, protons interacting with magnetic fields, and photons in a spherical region. The package contains numerous physical processes, including synchrotron emission and self-absorption, inverse Compton scattering, photon-photon pair production, and adiabatic losses. It also includes proton-photon pion production, proton-photon (Bethe-Heitler) pair production, and proton-proton collisions. LeHaMoC can model expanding spherical sources with a variable magnetic field strength. In addition, three types of external radiation fields can be defined: grey body or black body, power-law, and tabulated.

The Magellanic Clouds are our closest star-forming galaxies with low Galactic foreground absorption and well determined distances. In addition, a low metallicity environment makes them a unique laboratory to study the population of high-energy sources. The SMC hosts a large population of Be/X-ray binaries associated with high star formation activity 25-40 Myr ago. The HMXB population in the LMC is associated with a star formation period at an earlier epoch and a lower HMXB formation efficiency. The Magellanic Bridge is thought to be a product of the tidal interaction between the Large and Small Magellanic Clouds (LMC and SMC). It contains both gas and stellar components, with young stellar components (therefore HMXBs) which is thought to have formed in situ, as well as an older population of stars (e.g. LMXBs) mostly stripped from the SMC by the LMC. The recent eROSITA all-sky survey marks the first comprehensive X-ray coverage of the entire Magellanic system, offering a broad band X-ray coverage in 0.2-10 keV. Proximity to the south-ecliptic pole facilitates extended monitoring of LMC sources during each survey, enabling a deep total exposure and the exploration of long-term temporal behavior. This presentation will unveil the findings from our study of the X-ray binary population across the entire Magellanic system through the SRG/eROSITA all-sky survey. Additionally, we will showcase unique discoveries, including an X-ray burster in the Magellanic Bridge and an ultra-compact binary system in the direction of the LMC.

Context. During four all-sky surveys (eRASS1--4), eROSITA, the soft X-ray instrument aboard Spektrum-Roentgen-Gamma (SRG) detected a new supersoft X-ray source, eRASSU J060839.5-704014, in the direction of the Large Magellanic Cloud (LMC). Methods. We arranged follow-up observations in the X-ray and optical wavelengths and further searched in archival observations to reveal the nature of the object. Results. We discover pulsations at ~374 s with a pulse profile consistent with 100% modulation. We identify two other periodicities in the eROSITA data, which we establish as aliases due to the sampling of the eROSITA light curve. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The timing and spectral characteristics of the source are consistent with a double degenerate ultra-compact binary system in the foreground of the LMC. eRASSU J060839.5-704014 belongs to a rare class of AM CVns, which are important to study in the context of progenitors of SN Ia and for persistent gravitational wave detection. Conclusions. We identify eRASSU J060839.5-704014 as a new double degenerate ultra-compact binary located in the foreground of the LMC.

Blazars—a subclass of active galaxies—are intrinsically time-variable broadband sources of electromagnetic radiation. In this contribution, we explored relativistic proton (hadronic) signatures in the time domain blazar emission and searched for those parameter combinations that unveil their presence during flaring epochs. We generated time series for key model parameters, like magnetic field strength and the power-law index of radiating particles, which were motivated from a simulated time series with statistical properties describing the observed GeV gamma-ray flux. We chose the TeV blazar Mrk 501 as our test case, as it had been the study ground for extensive investigations during individual flaring events. Using the code LeHaMoC, we computed the electromagnetic and neutrino emissions for a period of several years that contained several flares of interest. We show that for both of those particle distributions the power-law index variations that were tied to moderate changes in the magnetic field strength of the emitting region might naturally lead to hard X-ray flares with very-high-energy γ-ray counterparts. We found spectral differences measurable by the Cherenkov Telescope Array Observatory at sub-TeV energies, and we computed the neutrino fluence over 14.5 years. The latter predicted ∼0.2 muon and anti-muon neutrinos, consistent with the non-detection of high-energy neutrinos from Mrk 501.

We model the spectral formation occurring in the binary X-ray pulsar (XRP) RX J0209.6‑7427 during the 2019 super-Eddington outburst. Using a theoretical model previously developed by the authors, we are able to produce spectra that closely resemble the phase-averaged X-ray spectra observed using the Nuclear Spectroscopic Telescope Array and Insight-HXMT during low- and high-luminosity states of the outburst, respectively. The theoretical model simulates the accretion of fully ionized gas in a dipole magnetic field and includes a complete description of the radiation hydrodynamics, matter distribution, and spectral formation. Type II X-ray outbursts provide an opportunity to study accretion over a large range of luminosities for the same neutron star. The analysis performed here represents the first time both the outburst low and high states of an accretion-powered XRP are modeled using a physics-based model rather than standard phenomenological fitting with arbitrary mathematical functions. We find that the outer polar cap radius remains constant and the column is more fully filled with increasing luminosity, Comptonized bremsstrahlung dominates the formation of the phase-averaged X-ray spectrum, and a negative correlation exists between cyclotron centroid energy and luminosity, as expected. The super-Eddington nature of the outburst is rendered possible owing to the low scattering cross section for photons propagating parallel to the magnetic field. We also find that emission through the column top dominates in both the low and high states, implying that the pulse profiles should have a roughly sinusoidal shape, which agrees with observed properties of ultraluminous XRPs.

Context. During four all-sky surveys (eRASS1-4), eROSITA, the soft X-ray instrument aboard Spektrum-Roentgen-Gamma (SRG) detected a new supersoft X-ray source, eRASSU J060839.5−704014, in the direction of the Large Magellanic Cloud (LMC). Aims: We arranged follow-up observations in the X-ray and optical wavelengths and further searched in archival observations to reveal the nature of the object. Methods: Using X-ray observations with XMM-Newton we investigated the temporal and spectral behaviour of the source. Results: We discover pulsations at 374 s with a pulse profile consistent with 100% modulation. We identify two other periodicities in the eROSITA data, which we establish as aliases due to the sampling of the eROSITA light curve. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The timing and spectral characteristics of the source are consistent with a double degenerate ultra-compact binary system in the foreground of the LMC. eRASSU J060839.5−704014 belongs to a rare class of AM CVns, which are important to study in the context of progenitors of SN Ia and for persistent gravitational wave detection. Conclusions: We identify eRASSU J060839.5−704014 as a new double degenerate ultra-compact binary located in the foreground of the LMC. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

An X-ray outburst was recently detected by the Einstein Probe mission and designated EP J0052.9-7230 (ATel #16631). Swift's S-CUBED survey (Kennea et al. 2018) then localized the event to the X-ray source CXOU J005245.0-722844 and noted that the soft spectrum is consistent with a Be X-ray binary (BeXRB) system with a white dwarf as the compact object (ATel #16633).

HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<10 arcsec FWHM) and broad spectral coverage (0.1-150 keV) with an effective area far superior to current facilities (including XMM-Newton and NuSTAR) to enable revolutionary new insights into a variety of important astrophysical problems. HEX-P will characterize X-ray binary systems over an unprecedented range of fluxes, energies, and time-scales, allowing us to answer questions about binary evolution, neutron star physical properties, and feedback into their environment. We present spectral simulations of accreting neutron stars to demonstrate the power of the HEX-P observatory. We show that HEX-P will (1) detect cyclotron line features above 80 keV, directly measuring B-fields stronger than ever before; (2) provide constraints on neutron star radii with higher accuracy using X-ray reflection modeling; (3) yield unique information about the accretion geometry and accretion state in these systems for the whole range of mass accretion rates from ULX-like luminosities to the onset of the propeller effect or quiescence; and (4) characterize the diverse environments these systems live in, including stellar winds and dust shrouds, intervening Be-star disks or warped accretion disks, as well as outflows. More information on HEX-P, including the full team list, is available at hexp.org.

We report on NICER observations of the Be X-ray binary 4U 0115+63 during the rise of the source's current historic outburst, which began around 2023 March 28 and is the brightest recorded in 27 years.

Neutron star high mass X-ray binaries that exhibit superorbital variability offer an opportunity to study the geometry and stability of warped accretion disks. The high mass X-ray binary SMC X-1 is an ideal system in which to investigate these questions because the supeorbital period has epochs of instability known as excursions likely caused by disk instability. Using the high resolution spectral and timing capabilities of the Neutron Star Interior Composition Explorer (NICER) we examined the high state of four different superorbital cycles of SMC X-1 to search for short term changes in spectral shape and any connection to the unstable accretion disk geometry. We performed phase averaged and phase resolved spectroscopy, as well as principal component analysis to closely compare the spectral characteristics and any cycle-to-cycle variations. While soft, disk-related spectral components showed variations with time, the accretion column related parameters (i.e. photon index) remained mostly constant, indicating that the disk instability does not significantly change SMC X-1's accretion process.

We report on Neutron Star Interior Composition Explorer (NICER), Nuclear Spectroscopic Telescope Array (NuSTAR) and Fermi Gamma-ray Burst Monitor (GBM) observations of the Be-X-ray binary pulsar 1A 0535+262 performed during its giant Type II outburst in 2020, which peaked at a 15-50keV flux of ~12Crab. From the Fermi GBM, NICER and NuSTAR measurements, we find the neutron star rotation period decreases from 103.622 ± 0.004 s to 103.24 ± 0.02 s, suggesting that it rapidly spins-up as the outburst progresses. The NICER and NuSTAR observations, which monitor the evolution of the spectral shape, show the 1-79 keV luminosity peaks at (1.354 ± 0.005)×1038 erg s-1, which is above the inferred critical luminosity of ~6.3×1037 erg s-1 in the accretion column. We find that the pulse profiles in each observation show a strong energy dependence as well as a strong dependence on luminosity. Emission lines from Fe Kα, Fe XXV Kα and Fe XXVI Kα are observed in the joint NICER and NuSTAR spectra. The line strengths are correlated with the 7.2- 10 keV unabsorbed continuum flux. The Fe fluorescent emission lines show no evidence of pulsations, which suggests that the size of the Fe fluorescent emission region is larger than the Alfv ́en radius. Even when the measured 1-79 keV luminosity is above the critical value, we find that the spectral shape hardens with increasing luminosity. The pulse-phase-averaged NuSTAR data show that the energy of the CRSF around ~45keV remains unchanged with respect to X-ray flux, but the line depth decreases with increasing X-ray flux. This may be a result of photon spawning.

Compact objects and supernova remnants provide nearby laboratories to probe the fate of stars after they die, and the way they impact, and are impacted by, their surrounding medium. The past five decades have significantly advanced our understanding of these objects, and showed that they are most relevant to our understanding of some of the most mysterious energetic events in the distant Universe, including Fast Radio Bursts and Gravitational Wave sources. However, many questions remain to be answered. These include: What powers the diversity of explosive phenomena across the electromagnetic spectrum? What are the mass and spin distributions of neutron stars and stellar mass black holes? How do interacting compact binaries with white dwarfs - the electromagnetic counterparts to gravitational wave LISA sources - form and behave? Which objects inhabit the faint end of the X-ray luminosity function? How do relativistic winds impact their surroundings? What do neutron star kicks reveal about fundamental physics and supernova explosions? How do supernova remnant shocks impact cosmic magnetism? This plethora of questions will be addressed with AXIS - the Advanced X-ray Imaging Satellite - a NASA Probe Mission Concept designed to be the premier high-angular resolution X-ray mission for the next decade. AXIS, thanks to its combined (a) unprecedented imaging resolution over its full field of view, (b) unprecedented sensitivity to faint objects due to its large effective area and low background, and (c) rapid response capability, will provide a giant leap in discovering and identifying populations of compact objects (isolated and binaries), particularly in crowded regions such as globular clusters and the Galactic Center, while addressing science questions and priorities of the US Decadal Survey for Astronomy and Astrophysics (Astro2020).

Neutron star high-mass X-ray binaries with superorbital modulations in luminosity host warped inner accretion disks that occult the neutron star during precession. In SMC X-1, the instability in the warped disk geometry causes superorbital period "excursions": times of instability when the superorbital period decreases from its typical value of 55 to ~40 days. Disk instability makes SMC X-1 an ideal system in which to investigate the effects of variable disk geometry on the inner accretion flow. Using the high-resolution spectral and timing capabilities of the Neutron Star Interior Composition Explorer, we examined the high state of four different superorbital cycles of SMC X-1 to search for changes in spectral shape and connections to the unstable disk geometry. We performed pulse phase-averaged and phase-resolved spectroscopy to closely compare the changes in spectral shape and any cycle-to-cycle variations. While some parameters, including the photon index and absorbing column density, show slight variations with superorbital phase, these changes are most evident during the intermediate state of the superorbital cycle. Few spectral changes are observed within the high state of the superorbital cycle, possibly indicating the disk instability does not significantly change SMC X-1's accretion process.

Context. Using data from eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma (SRG), we report the discovery of two new hard transients, eRASSU J050810.4-660653 and eRASSt J044811.1-691318, in the Large Magellanic Cloud. We also report the detection of the Be/X-ray binary RX J0501.6-7034 in a bright state. Aims: We initiated follow-up observations to investigate the nature of the new transients and to search for X-ray pulsations coming from RX J0501.6-7034. Methods: We analysed the X-ray spectra and light curves from our XMM-Newton observations, obtained optical spectra using the South African Large Telescope to look for Balmer emission lines and utilised the archival data from the Optical Gravitational Lensing Experiment (OGLE) for the long-term monitoring of the optical counterparts. Results: We find X-ray pulsations for eRASSU J050810.4-660653, RX J0501.6-7034, and eRASSt J044811.1-691318 of 40.6 s, 17.3 s, and 784 s, respectively. The Hα emission lines with equivalent widths of −10.4 Å (eRASSU J050810.4-660653) and −43.9 Å (eRASSt J044811.1-691318) were measured, characteristic for a circumstellar disc around Be stars. The OGLE I- and V-band light curves of all three systems exhibit strong variability. A regular pattern of deep dips in the light curves of RX J0501.6-7034 suggests an orbital period of ∼451 days. Conclusions: We identify the two new hard eROSITA transients eRASSU J050810.4-660653 and eRASSt J044811.1-691318 and the known Be/X-ray binary RX J0501.6-7034 as Be/X-ray binary pulsars.

Context. During the third all-sky survey (eRASS3), eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma, detected a new hard X-ray transient, eRASSt J040515.6 − 745202, in the direction of the Magellanic Bridge. Aims: We arranged follow-up observations and searched for archival data to reveal the nature of the transient. Methods: Using X-ray observations with XMM-Newton, NICER, and Swift, we investigated the temporal and spectral behaviour of the source for over about 10 days. Results: The X-ray light curve obtained from the XMM-Newton observation with an ∼28 ks exposure revealed a type-I X-ray burst with a peak bolometric luminosity of at least 1.4 × 1037 erg s−1. The burst energetics are consistent with a location of the burster at the distance of the Magellanic Bridge. The relatively long exponential decay time of the burst of ∼70 s indicates that it ignited in a H-rich environment. The non-detection of the source during the other eROSITA surveys, twelve and six months before and six months after eRASS3, suggests that the burst was discovered during a moderate outburst which reached 2.6 × 1036 erg s−1 in persistent emission. During the NICER observations, the source showed alternating flux states with the high level at a similar brightness as during the XMM-Newton observation. This behaviour is likely caused by dips as also seen during the last hour of the XMM-Newton observation. Evidence for a recurrence of the dips with a period of ∼21.8 h suggests eRASSt J040515.6 − 745202 is a low-mass X-ray binary (LMXB) system with an accretion disk seen nearly edge on. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The spectral energy distribution is consistent with radiation from an accretion disk which dominates the UV and from a cool late-type star detected in the optical to infrared wavelengths. Conclusions: After the discovery of X-ray bursts in M 31, the Magellanic Bridge is only the second location outside of the Milky Way where an X-ray burster was found. The burst uniquely identifies eRASSt J040515.6 − 745202 as an LMXB system with a neutron star. Its location in the Magellanic Bridge confirms the existence of an older stellar population which is expected if the bridge was formed by tidal interactions between the Magellanic Clouds, which stripped gas and stars from the clouds.

Introduction: Ultraluminous X-ray sources (ULXs) represent an extreme class of accreting compact objects: from the identification of some of the accretors as neutron stars to the detection of powerful winds travelling at 0.1–0.2 c, the increasing evidence points towards ULXs harbouring stellar-mass compact objects undergoing highly super-Eddington accretion. Measuring their intrinsic properties, such as the accretion rate onto the compact object, the outflow rate, the masses of accretor/companion-hence their progenitors, lifetimes, and future evolution-is challenging due to ULXs being mostly extragalactic and in crowded fields. Yet ULXs represent our best opportunity to understand super-Eddington accretion physics and the paths through binary evolution to eventual double compact object binaries and gravitational-wave sources. Methods: Through a combination of end-to-end and single-source simulations, we investigate the ability of HEX-P to study ULXs in the context of their host galaxies and compare it to XMM-Newton and NuSTAR, the current instruments with the most similar capabilities.Results: HEX-P's higher sensitivity, which is driven by its narrow point-spread function and low background, allows it to detect pulsations and broad spectral features from ULXs better than XMM-Newton and NuSTAR.Discussion: We describe the value of HEX-P in understanding ULXs and their associated key physics, through a combination of broadband sensitivity, timing resolution, and angular resolution, which make the mission ideal for pulsation detection and low-background, broadband spectral studies.

Context. The Magellanic Clouds are our nearest star-forming galaxies. While the population of high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud is relatively well studied, our knowledge about the Large Magellanic Cloud (LMC) is far from complete given its large angular extent and the insufficient coverage with X-ray observations. Aims: We conducted a search for new HMXBs in the LMC using data from eROSITA, the soft X-ray instrument on board the Spektrum-Roentgen-Gamma satellite. Methods: After confirming the nature of eRASSU J052914.9−662446 as a hard X-ray source that is positionally coincident with an early-type star, we followed it up with optical spectroscopic observations from the South African Large Telescope (SALT) and a dedicated NuSTAR observation. Results: We study the broadband timing and spectral behaviour of the newly discovered HMXB eRASSU J052914.9−662446 through eROSITA, Swift, and NuSTAR data in X-rays and the Optical Gravitational Lensing Experiment (OGLE) and SALT RSS data at the optical wavelength. We report the detection of a spin period at 1412 s and suggest that the orbital period of the system is ∼151 days. We thereby establish that eRASSU J052914.9−662446 is an accreting pulsar. Furthermore, through optical spectroscopic observations and the detection of Hα emission, the source is identified as a Be X-ray binary pulsar in the LMC. We also investigated the variability of the source in the optical and X-ray regime over the past decades and provide estimates of the possible magnetic field strength of the neutron star.

Accreting neutron stars (NSs) represent a unique laboratory for probing the physics of accretion in the presence of strong magnetic fields (B ≳ 108 G). Additionally, the matter inside the NS itself exists in an ultra-dense, cold state that cannot be reproduced in Earth-based laboratories. Hence, observational studies of these objects are a way to probe the most extreme physical regimes. Here we present an overview of the field and discuss the most important outstanding problems related to NS accretion. We show how these open questions regarding accreting NSs in both low-mass and high-mass X-ray binary systems can be addressed with the High-Energy X-ray Probe (HEX-P) via simulated data. In particular, with the broad X-ray passband and improved sensitivity afforded by a low X-ray background, HEX-P will be able to 1) distinguish between competing continuum emission models; 2) provide tighter upper limits on NS radii via reflection modeling techniques that are independent and complementary to other existing methods; 3) constrain magnetic field geometry, plasma parameters, and accretion column emission patterns by characterizing fundamental and harmonic cyclotron lines and exploring their behavior with pulse phase; 4) directly measure the surface magnetic field strength of highly magnetized NSs at the lowest accretion luminosities; as well as 5) detect cyclotron line features in extragalactic sources and probe their dependence on luminosity in the super-Eddington regime in order to distinguish between geometrical evolution and accretion-induced decay of the magnetic field. In these ways HEX-P will provide an essential new tool for exploring the physics of NSs, their magnetic fields, and the physics of extreme accretion.

We present a detailed compilation and analysis of the X-ray phase space of low- to intermediate-redshift (0 ≤ z ≤ 1) transients that consolidates observed light curves (and theory where necessary) for a large variety of classes of transient/variable phenomena in the 0.3-10 keV energy band. We include gamma-ray burst afterglows, supernovae, supernova shock breakouts and shocks interacting with the environment, tidal disruption events and active galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary outbursts, and ultraluminous X-ray sources. Our overarching goal is to offer a comprehensive resource for the examination of these ephemeral events, extending the X-ray duration-luminosity phase space (DLPS) to show luminosity evolution. We use existing observations (both targeted and serendipitous) to characterize the behavior of various transient/variable populations. Contextualizing transient signals in the larger DLPS serves two primary purposes: to identify areas of interest (i.e., regions in the parameter space where one would expect detections, but in which observations have historically been lacking), and to provide initial qualitative guidance in classifying newly discovered transient signals. We find that while the most luminous (largely extragalactic) and least luminous (largely Galactic) part of the phase space is well populated at t > 0.1 days, intermediate-luminosity phenomena (L X = 1034-1042 erg s-1) represent a gap in the phase space. We thus identify L X = 1034-1042 erg s-1 and t = 10-4 to 0.1 days as a key discovery phase space in transient X-ray astronomy.

We report comprehensive spectral and temporal properties of the Be/X-ray binary pulsar SMC X-2 using X-ray observations during the 2015 and 2022 outbursts. The pulse profile of the pulsar is unique and strongly luminosity dependent. It evolves from a broad-humped into a double-peaked profile above luminosity 3 × 1038 erg s-1. The pulse fraction of the pulsar is found to be a linear function of luminosity as well as energy. We also studied the spectral evolution of the source during the latest 2022 outburst with NICER. The observed photon index shows a negative and positive correlation below and above the critical luminosity, respectively, suggesting evidence of spectral transition from the sub-critical to supercritical regime. The broad-band spectroscopy of four sets of NuSTAR and XRT/NICER data from both outbursts can be described using a cut-off power-law model with a blackbody component. In addition to the 6.4 keV iron fluorescence line, an absorption-like feature is clearly detected in the spectra. The cyclotron line energy observed during the 2015 outburst is below 29.5 keV, however latest estimates in the 2022 outburst suggest a value of 31.5 keV. Moreover, an increase of 3.4 keV is detected in the cyclotron line energy at equal levels of luminosity observed in 2022 with respect to 2015. The observed cyclotron line energy variation is explored in terms of accretion induced screening mechanism or geometrical variation in line forming region.

Accretion disks around compact objects are expected to enter an unstable phase at high luminosity1. One instability may occur when the radiation pressure generated by accretion modifies the disk viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales2. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole3-5. Although there are hints of these cycles in a few isolated cases6-10, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a continuing puzzle11. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum—from radio to X-ray—of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such a new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.

X-Ray pulsars are systems powered by accretion, the majority of which is found in Be X-ray binaries (BeXRBs). The study of Giant outbursts (L_{X} > 1038 erg s -1) in such systems becomes very relevant in the advent of the recent discoveries of pulsating ultra-luminous X-ray sources (PULXs) with an apparent isotropic luminosity above the Eddington limit for a typical neutron star (NS) demonstrating that stable accretion onto NSs is possible at super - Eddington rates. Given that PULXs host magnetized NSs, several attempts have been made to estimate the magnetic field of the NS using standard torque models. At the same time theoretical studies have demonstrated that it is required to adjust these models due to changes in the accretion disc structure when exceeding the Eddington limit. Motivated by these findings we studied torque models during Giant outbursts of BeXRBs monitored by Fermi/GBM and Swift/BAT. We developed a code to estimate posterior distributions for the parameters of standard accretion models and binary orbital parameters using a nested sampling algorithm for Bayesian Parameter Estimation. Most notably we applied our method to the recently discovered Swift J0243.6+6124 (i.e., the only known Galactic PULX) and we illustrate that the standard torque models need adjustment to explain the observed spin evolution of the NS. Finally, we discuss the implications the newest GAIA distances have on the NS equation of state.

We report on Neutron Star Interior Composition Explorer (NICER), Nuclear Spectroscopic Telescope Array (NuSTAR) and Fermi Gamma-ray Burst Monitor (GBM) observations of the Be-X-ray binary pulsar 1A 0535+262 performed during its giant Type II outburst in 2020, which peaked at a 15-50 keV flux of ~12 Crab. From the Fermi GBM, NICER and NuSTAR measurements, we find the neutron star rotation period decreases from 103.622±0.004 s to 103.24±0.02 s, suggesting that the neutron star rapidly spins-up as the outburst progressed. The NICER and NuSTAR observations, which monitor the evolution of the spectral shape, show the 1-79 keV luminosity peaks at ~1.4 × 1038 erg s-1. This is clearly above ~6.3 × 1037 erg s-1, which is expected around the critical luminosity. We find that the pulse profiles in each observation show a strong energy dependence as well as a strong dependence on luminosity. Emission lines from Fe Kα, Fe XXV Kα and Fe XXVI Kα are observed in the joint NICER and NuSTAR spectra. The line strengths are correlated with the 7.2-10 keV unabsorbed continuum flux. The Fe fluorescent emission lines show no evidence of pulsations, which may suggest that the size of the Fe fluorescent emission region is larger than the Alfvén radius. Although the measured 1-79 keV luminosities are clearly in the supercritical accretion regime, we find that the spectral shape hardens with increasing luminosity. The pulse-phase-averaged NuSTAR data show that the energy of the CRSF around ~45 keV remains virtually unchanged, but the line depth decreases with increasing X-ray flux. This may be a result of photon spawning.

Blazars are the most extreme subclass of active galactic nuclei with relativistic jets emerging from a super-massive black hole and forming a small angle with respect to our line of sight. Blazars are also known to be related to flaring activity as they exhibit large flux variations over a wide range of frequency and on multiple timescales, ranging from a few minutes to several months. The detection of a high-energy neutrino from the flaring blazar TXS 0506+056 and the subsequent discovery of a neutrino excess from the same direction have naturally strengthened the hypothesis that blazars are cosmic neutrino sources. While neutrino production during gamma-ray flares has been widely discussed, the neutrino yield of X-ray flares has received less attention. Motivated by a theoretical scenario where high energy neutrinos are produced by energetic protons interacting with their own X-ray synchrotron radiation, we make neutrino predictions over a sample of a sample of X-ray blazars. This sample consists of all blazars observed with the X-ray Telescope (XRT) on board Swift more than 50 times from November 2004 to November 2020. The statistical identification of a flaring state is done using the Bayesian Block algorithm to the 1 keV XRT light curves of frequently observed blazars. We categorize flaring states into classes based on their variation from the time-average value of the data points. During each flaring state, we compute the expected muon plus anti-muon neutrino events as well as the total signal for each source using the point-source effective area of Icecube for different operational seasons. We find that the median of the total neutrino number (in logarithm) from flares with duration $<30$ d is $\mathcal{N}^{(\rm tot)}_{\nu_{\mu}+\bar{\nu}_{\mu}} \sim 0.02$.

Aims: We conducted a spectral and temporal analysis of X-ray data from the Be X-ray binary pulsar SXP 15.6 located in the Small Magellanic Cloud based on NuSTAR, NICER, and Swift observations during the 2021 outburst. Methods: We present the broadband X-ray spectra of the system based on simultaneous NuSTAR and NICER observations for the first time. Moreover, we used monitoring data to study the spectral and temporal properties of the system during the outburst. Results: Comparison of the evolution of the 2021 outburst with archival data reveals a consistent pattern of variability, with multiple peaks occurring at time intervals similar to the orbital period of the system (∼36 d). Our spectral analysis indicates that most of the energy is released at high energies above 10 keV, while we found no cyclotron absorption line in the spectrum. Analysing of the spectral evolution during the outburst, we find that the spectrum is softer when brighter, which in turn reveals that the system is probably in the super-critical regime in which the accretion column is formed. This places an upper limit on the magnetic field of the system of about 7 × 1011 G. The spin-evolution of the neutron star (NS) during the outburst is consistent with an NS with a low magnetic field (∼5 × 1011 G), while there is evident orbital modulation that we modelled, and we derived the orbital parameters. We found the orbit to have a moderate eccentricity of ∼0.3. Our estimates of the magnetic field are consistent with the lack of an electron cyclotron resonance scattering feature in the broadband X-ray spectrum.

One of the brightest X-ray pulsars in the Small Magellanic Cloud is SMC X-2. During its most recent major outburst in 2015, this transient pulsar displayed significant changes in both its accretion state and magnetosphere, particularly when it entered the low-luminosity regime of subcritical accretion. Polestar is a pulse-profile modeling code that helps in delineating the geometry of the emission as the source evolves past outburst and toward lower-luminosity states. Applying Polestar to XMM-Newton and NuSTAR pulse profiles, we constrained the most likely inclination of the spin axis of the pulsar to be i = 87° ± 4°. As the X-ray luminosity declined, an increase in the pulsed fraction was detected from Swift observations, which suggests a transition from fan- to pencil-beam emission during the later stages of the outburst. Additionally, we also performed analysis of the OGLE IV light curves, which showed strong modulation in the optical profiles during the outburst.

We study the giant outbursts of SMC X-3 and RX J0209.6-7427 (hereafter RX J0209) to compare their super-Eddington accretion regime with that of Swift J0243.6+6124 (hereafter Swift J0243). The high double-peak profile of SMC X-3 is found to be 0.25 phase offset from that below 2.3 × 1038 erg s-1, which is similar to Swift J0243 (happened around 0.9 × 1038 erg s-1). The profile of RX J0209 shows a similar 0.25 phase offset between high double-peak and low double-peak around 1.25 × 1038 erg s-1. The 0.25 phase offset corresponds to a 90° angle change of the emission beam and strongly supports for a transition from a fan beam to a pencil beam. Their critical luminosities imply a surface magnetic field ~4 × 1013 and 2 × 1013 G for SMC X-3 and RX J0209, respectively, based on the recently measured cyclotron line of Swift J0243. The spin-up rate and luminosity of SMC X-3 follows a relation of $\dot{\nu }\propto L^{0.94\pm 0.03}$, while that of RX J0209 follows $\dot{\nu }\propto L^{1.00\pm 0.03}$, which are similar to Swift J0243 and consistent with the prediction of a radiation-pressure-dominated disc. These results indicate that accretion columns are indeed formed above Eddington luminosity, and the population of ultraluminous X-ray pulsars likely corresponds to X-ray pulsars of highest magnetic field.

Measuring a pulsar's rotational evolution is crucial to understanding the nature of the pulsar. Here, we provide updated timing models for the rotational evolution of six pulsars, five of which are rotation phase-connected using primarily NICER X-ray data. For the newly discovered fast energetic young pulsar, PSR J0058-7218, we increase the baseline of its timing model from 1.4 days to 8 months and not only measure more precisely its spin-down rate $\dot{\nu }=(-6.2324\pm 0.0001)\times {10}^{-11}\,\mathrm{Hz}\,{{\rm{s}}}^{-1}$ but also for the first time the second time derivative of its spin rate $\ddot{\nu }=(4.2\pm 0.2)\times {10}^{-21}\,\mathrm{Hz}\,{{\rm{s}}}^{-2}$ . For the fastest and most energetic young pulsar, PSR J0537-6910 (with a 16 ms spin period), we detect four more glitches, for a total of 15 glitches over 4.5 yr of NICER monitoring, and show that its spin-down behavior continues to set this pulsar apart from all others, including a long-term braking index n = -1.234 ± 0.009 and interglitch braking indices that asymptote to ≲7 for long times after a glitch. For PSR J1101-6101, we measure a much more accurate spin-down rate that agrees with a previous value measured without phase connection. For PSR J1412+7922 (also known as Calvera), we extend the baseline of its timing model from our previous 1 yr model to 4.4 yr, and for PSR J1849-0001, we extend the baseline from 1.5 to 4.7 yr. We also present a long-term timing model of the energetic pulsar PSR J1813-1749, by fitting previous radio and X-ray spin frequencies from 2009-2019 and new ones measured here using 2018 NuSTAR and 2021 Chandra data.

Context. The Magellanic Clouds host a large population of high-mass X-ray binary (HMXB) systems, and although the Large Magellanic Cloud (LMC) is an order of magnitude more massive than the Small Magellanic Cloud, there are significantly fewer known HMXBs in the former. Aims: We conducted a search for new HMXBs in XMM-Newton observations that were performed with the aim of investigating supernova remnant candidates in the supergiant shells LMC5 and LMC7. The three observed fields are located in regions that have not been widely explored in the X-ray band. Methods: We analysed the XMM-Newton data to look for sources with hard X-ray spectrum and their counterparts with optical colours and brightness values that are typical of HMXBs. Results: We report the discovery of three new Be/X-ray binaries, two of them showing pulsations in their X-ray flux. With a luminosity of 6.5 × 1034 erg s−1, we see that XMMU J045315.1−693242 in LMC7 was relatively X-ray faint. The long-term OGLE I-band light curve of the V = 15.5 mag counterpart suggests a 49.6 day or 24.8 day orbital period for the binary system. Then, XMMU J045736.9−692727, which is also located in LMC7, was brighter, with a luminosity of 5.6 × 1035 erg s−1 and hard spectrum with a power-law photon index of 0.63. The X-ray flux revealed clear pulsations with a period of 317.7 s. We obtained optical high resolution spectra from the V = 14.2 mag counterpart using the SALT-HRS spectrograph. Hα and Hβ were observed in emission with complex line profiles and equivalent widths of −8.0 Å and −1.3 Å, respectively. The I-band light curve obtained from OGLE shows a series of four strong outbursts followed by a sudden drop in brightness by more than 1 mag within 73-165 days and a recovery to the level from before the outbursts. RX J0524.2−6620, previously classified as X-ray binary candidate, is located at the eastern part of LMC5. We report the discovery of 360.7 s pulsations. During the XMM-Newton observation the luminosity was at ∼4 × 1035 erg s−1 and the source showed a hard spectrum with a power-law photon index of 0.78. The Hα emission line profile obtained from SALT-HRS is characterised by two broad peaks with a separation corresponding to ∼178 km s−1, along with an equivalent width of −4.2 Å. The long-term OGLE I-band light curve of the V = 14.9 mag counterpart reveals a quasi-periodic flaring activity while the colour evolution during the flares follows a hysteresis loop with redder colour during the rise. Based on the modelling the Hα line profiles measured from XMMU J045736.9−692727 and RX J0524.2−6620, we derived constraints on the size of the Be disks. Conclusions: Our discovery of two pulsars among three new Be/X-ray binaries increases the number of known HMXB pulsars in the LMC to 25.

All disc-accreting astrophysical objects produce powerful disc winds. In compact binaries containing neutron stars or black holes, accretion often takes place during violent outbursts. The main disc wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated `soft states'1,2. By contrast, optical wind-formed lines have recently been detected in `hard states', when a hot corona dominates the luminosity3. The relationship between these signatures is unknown, and no erupting system has as yet revealed wind-formed lines between the X-ray and optical bands, despite the many strong resonance transitions in this ultraviolet (UV) region4. Here we report that the transient neutron star binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption lines associated with C IV, N V and He II in time-resolved UV spectroscopy during a luminous hard state, which we interpret as a warm, moderately ionized outflow component in this state. Simultaneously observed optical lines also display transient blue-shifted absorption. Decomposing the UV data into constant and variable components, the blue-shifted absorption is associated with the former. This implies that the outflow is not associated with the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or spatially stratified evaporative outflow from the outer disc5. This type of persistent mass loss across all accretion states has been predicted by radiation-hydrodynamic simulations6 and helps to explain the shorter-than-expected duration of outbursts7.

M51 ULX-7 is a an ultraluminous X-ray pulsar (ULXP) with a spin period of ~2.8 s, and orbital period of ~2 d, and a maximum luminosity that exceeds by more than 20 times the Eddington limit for a neutron star (NS). An open question about ULXPs, is if indeed the mass accretion rates are super-Eddington, or the emission is beamed due to the presence of strong optically thick outflows. Such outflows could originate from the disk and may form a narrow funnel allowing radiation to escape only towards the observer. We will discuss the temporal properties of the system based on the analysis of archival X-ray data collected by Swift/XRT and Chandra. We find that its X-ray flux modulates with a super-orbital period of ~40 d, while there appear to be epochs where the pulsar transitions to the propeller stage. Moreover, we report the discovery of periodic X-ray dips, with a period that matches the orbital period. The observable properties of M51 ULX-7 can be used to probe the mass accretion rate onto the NS and to constrain the orbital inclination of the binary. We conclude that the properties of the systems are evident of a small beaming factor and a wide funnel, thus demonstrating that strong beaming does not need to be invoked to explain the observed fluxes, and that super-Eddington accretion rates are possible for highly magnetized NSs.

SMC X-2 is one of the brightest pulsars in the Small Magellanic Cloud (SMC) with a maximum known Luminosity of Lx = 4.0×1038 erg s-1. This transient Be/X-ray pulsar with a spin period of Pspin ~ 2.37 s and an orbital period of Porb = 18.62 ± 0.02 days last underwent a Type-II outburst in 2015. Following its detection by MAXI, simultaneous observations were carried out by Swift, XMM-Newton and NuSTAR throughout the outburst phase extending up to two months. Its spectra showed a dominant hard cutoff power law along with additional soft blackbody and thermal components. The source is one of few SMC pulsars in which the propeller state was observed and a cyclotron resonance feature was detected at E ~ 27 keV. The onset of the propeller regime causes dramatic changes in the accretion state and the neutron star magnetosphere. This serves as impetus for trying to model the observed pulse profiles in various accretion states in order to deduce the geometry of the emitting regions. For this analysis, we use the geometrical pulse-profile modeling code Polestar. The pulsar exhibited a double-peak pulse profile during its previous giant outburst in 2000. In the 2015 data, we confirm the presence of a double peak during outburst, but there are also some profiles with a single broad peak. The pulse profile evolution from double peak to single peak probably indicates changes in the emission mechanism that can be traced by Polestar. This modeling effort will help us pinpoint the geometry of the emission and understand the energy and accretion changes as the source evolves past outburst and toward lower luminosity states.

SMC X-2 is one of the brightest pulsars in the Small Magellanic Cloud (SMC).This transient Be/X-ray pulsar with a spin period of Pspin = 2:37 s and an orbital period of Porb = 18.62 +/-0.02 days last underwent a Type-II outburst in 2015. Following its detection by MAXI, simultaneous observations were carried out by Swift, XMM-Newton, and NuSTAR throughout the outburst phase extending for up to two months. The source is one of few SMC pulsars in which the propeller state was observed and a cyclotron resonance feature was detected at E ~ 27 keV. The onset of the propeller regime causes dramatic changes in the accretion state and the neutron-star magnetosphere. This serves as impetus for trying to model the observed pulse profiles in various accretion states in order to deduce the geometry of the emitting regions. For this analysis, we use the geometrical pulse-profile modeling code Polestar. This modeling effort will help us pinpoint the geometry of the emission and understand the energy and accretion changes as the source evolves past outburst and toward lower luminosity states.

Blazars are the most extreme active galactic nuclei, having relativistic jets that are closely aligned to our line of sight. They are the most powerful persistent astrophysical sources of non-thermal electromagnetic radiation in the Universe, with spectral energy distributions (SEDs) spanning ~15 decades in energy, from radio frequencies up to high-energy γ-rays. Blazar SEDs vary both in terms of energy flux (i.e. flux variability) and spectral characteristics (i.e. color changes) on timescales ranging from minutes to years. Decade monitoring of blazars at optical and infrared (O/IR) wavelengths with the meter-class telescopes of the Small and Moderate Aperture Research Telescope System (SMARTS) in Chile and in γ-rays with the Fermi Large Area Telescope (LAT) has enabled the systematic study of multi-wavelength long-term variability in blazars. In this study we investigate, from a theoretical perspective, the long-term variability properties of blazar emission by introducing an observationally motivated time-dependence to four main parameters of the one-zone leptonic model: electron injection luminosity, magnetic field strength, Doppler factor and external photon field luminosity. For the first time, we use both the probability density function (PDF) and the power spectrum density (PSD) of the observed 10 year-long Fermi-LAT light curves to create fake γ-ray light curves and variation patterns for the model parameters in order to simulate the long-term multi-wavelength flux variability for the full time-interval of 10 years. To quantify the latter, we use standard timing tools, such as discrete correlation functions (DCFs) and fractional variabilities (FVs). Our goal is to compare the findings of our theoretical investigation with observations of two bright blazars from the SMARTS sample (PKS 2155-304 and 3C 273), and to understand the cause of the observed time lags between O/IR wavelengths and γ-rays.

We present unexpected optical and X-ray results that challenge established models of Be/X-ray binary accretion and system geometry. Be/X-ray binaries generally consist of a neutron star in a wide, eccentric orbit around a Be star. At periastron, magnetospheric accretion from the Be disk onto the neutron star magnetic poles yields X-ray pulses with increased luminosity by orders of magnitude. These periodic X-ray outbursts often coincide with optical flares. Characterizing X-ray and optical data from these systems informs our understanding of magnetospheric accretion onto objects with extreme magnetic fields (~1012 G) as well as the resulting feedback between the high-mass star and the orbiting pulsar. In June 2020, eROSITA detected Be/X-ray binary LXP 69.5 as the brightest transient in the Large Magellanic Cloud. We followed up with NICER to analyze the spectral and temporal properties of the neutron star over the course of the outburst. Pulse profile morphology is expected to change with accretion regime, which in turn is governed by system luminosity. However, we found the pulse profile from the peak of the outburst to be consistent with XMM-Newton observations from 2000, taken when the source was two orders of magnitude fainter. During the decay of the outburst, when the luminosity decreased by a third, the profile secondary peak became more prominent and the hardness increased. We also analyzed long-term optical data from OGLE, which spanned from March 2010 to March 2020. The orbital period of the system remains unknown, since no single period can describe the optical flares. Our best model suggests that the optical light curve can be split into three epochs with different periods (149, 171, and 200 days). Swift archival observations and our modeling of the I band quasi-periodicity suggest that the X-ray outbursts often occur near optical minimum. Such behavior is out of the ordinary for systems that host a face-on Be disk.

During the third all-sky survey (eRASS3), the eROSITA instrument (Predehl et al. 2021) aboard the Russian/German Spektrum-Roentgen-Gamma (SRG) mission (Sunyaev et al. 2021) discovered a new X-ray transient, designated as eRASSt J040515.6-745202, and located in the Magellanic Bridge at: RA(J2000)= 04:05:15.30 (61.31456 deg) DEC(J2000)= -74:51:58.1 (-74.86683 deg) with an estimated positional uncertainty of ~5" radius (incl.

eRASSU J050810.4-660653 is a new Be/X-ray binary in the LMC, which was discovered during the beginning of the first all-sky survey of the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission (ATel#13609).

We report the results of eROSITA and NICER observations of the 2020 June outburst of the Be/X-ray binary pulsar RX J0529.8-6556 in the Large Magellanic Cloud, along with the analysis of archival X-ray and optical data from this source. We find two anomalous features in the system's behaviour. First, the pulse profile observed by NICER during maximum luminosity is similar to that observed by XMM-Newton in 2000, despite the fact that the X-ray luminosity was different by two orders of magnitude. In contrast, a modest decrease in luminosity in the 2020 observations generated a significant change in pulse profile. Secondly, we find that the historical optical outbursts are not strictly periodic, as would be expected if the outbursts were triggered by periastron passage, as is generally assumed. The optical peaks are also not coincident with the X-ray outbursts. We suggest that this behaviour may result from a misalignment of the Be star disc and the orbital plane, which might cause changes in the timing of the passage of the neutron star through the disc as it precesses. We conclude that the orbital period of the source remains unclear.

We investigate the long-term variability of the iron K α line in the spectra of two ultracompact X-ray sources (UCXBs) with C/O-rich donors. We revisit archival observations from five different X-ray telescopes, over an ∼20-yr period. Adopting physically motivated models for the spectral continuum, we probe the long-term evolution of the source emission in a self-consistent manner enabling physical interpretation of potential variability of the primary X-ray continuum emission and/or any emission lines from reflection off the accretion disc. We find that the spectral shape and flux of the source emission (for both objects) has remained almost constant throughout all the observations, displaying only minor variability in some spectral parameters and the source flux (largest variation is an ∼25 per cent drop in the flux of Swift J1756.9-2508). We note a striking variability of the Fe K α line that fluctuates from a notable equivalent width of ∼66-100 eV in 4U 1543-624 and ∼170 eV in Swift J1756.9-2508 , to non-detections with upper limits of 2-8 eV. We argue that the disappearance of the iron line is due to the screening of the Fe K α line by the overabundant oxygen in the C/O-rich UCXBs. This effect is cancelled when oxygen becomes fully ionized in the inner disc region, resulting in the variability of the Fe K α line in an otherwise unaltered spectral shape. This finding supports earlier predictions on the consequences of H-poor, C/O-rich accretion disc on reflection-induced fluorescent lines in the spectra of UCXBs.

Context. Supergiant fast X-ray transients (SFXTs) are a peculiar class of supergiant high-mass X-ray binary (HMXB) systems characterised by extreme variability in the X-ray domain. In current models, this is mainly attributed to the clumpy nature of the stellar wind coupled with gating mechanisms involving the spin and magnetic field of the neutron star. Aims: We studied the X-ray properties of the supergiant HMXB XMMU J053108.3-690923 in the Large Magellanic Cloud to understand its nature. Methods: We performed a detailed temporal and spectral analysis of the eROSITA and XMM-Newton data of XMMU J053108.3-690923. Results: We confirm the putative pulsations previously reported for the source with high confidence, certifying its nature as a neutron star in orbit with a supergiant companion. We identify the extremely variable nature of the source in the form of flares seen in the eROSITA light curves. The source flux exhibits a total dynamic range of more than three orders of magnitude, which confirms its nature as an SFXT, and is the first such direct evidence from a HMXB outside our Galaxy exhibiting a very high dynamic range in luminosity as well as a fast flaring behaviour. We detect changes in the hardness ratio during the flaring intervals where the hardness ratio reaches its minimum during the peak of the flare and increases steeply shortly afterwards. This is also supported by the results of the spectral analysis carried out at the peak and off-flare intervals. This scenario is consistent with the presence of dense structures in the supergiant wind of XMMU J053108.3-690923 where the clumpy medium becomes photoionised at the peak of the flare leading to a drop in the photo-electric absorption. Further, we provide an estimate of the clumpiness of the medium and the magnetic field of the neutron star assuming a spin equilibrium condition.

We have analysed the X-ray spectra of all known Ultra-Compact X-ray Binaries (UCXBs), with the purpose of constraining the chemical composition of their accretion disc and donor star. Our investigation was focused on the presence (or absence) of the Fe Kα emission line, which was used as the probe of chemical composition of the disc, based on previously established theoretical predictions for the reflection of X-ray radiation off the surface of C/O-rich or He-rich accretion discs in UCXBs. We have contrasted the results of our spectral analysis to the history of type I X-ray bursts from these systems, which can also indicate donor star composition. We found that UCXBs with prominent and persistent iron Kα emission also featured repeat bursting activity. On the other hand, the UCXBs for which no iron line was detected, appear to have few or no type I X-ray bursts detected over more than a decade of monitoring. Based on Monte Carlo simulations, demonstrating a strong correlation between the Fe Kα line strength and the abundance of C and O in the accretion disc material and given the expected correlation between the H/He abundance and the recurrence rate of type I X-ray bursts, we propose that there is a considerable likelihood that UCXBs with persistent iron emission have He-rich donors, while those that do not, likely have C/O or O/Ne/Mg-rich donors. Our result strongly advocate for the development of more sophisticated simulations of X-ray reflection from hydrogen-poor accretion discs.

Context. Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic black hole binaries, their evidence in the super-Eddington regime has so far remained elusive. However, recent circumstantial evidence hinted at the presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II X-1 and NGC 5204 X-1. Aims: We aim to build on our previous work and exploit the long-term high-cadence monitoring of Swift-XRT in order to provide robust evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions. Methods: We studied the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb-Scargle periodograms and Gaussian process modelling to look for periodic variability. We also applied a physically motivated model to the combined Chandra, XMM-Newton, NuSTAR, and Swift-XRT data of each of the source spectral states. Results: We robustly show that both sources follow a clear and recurrent evolutionary pattern in the HID that can be characterised by the hard ultra-luminous (HUL) and soft ultra-luminous (SUL) spectral regimes, and a third state with characteristics similar to the super-soft ultra-luminous (SSUL) state. The transitions between the soft states seem consistent with aperiodic variability, as revealed by a timing analysis of the light curve of Holmberg II X-1; albeit, further investigation is warranted. The light curve of NGC 5204 X-1 shows a stable periodicity on a longer baseline of ∼200 days, possibly associated with the duration of the evolutionary cycle. Conclusions: The similarities between both sources provide strong evidence of both systems hosting the same type of accretor and/or accretion flow geometry. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the super-critical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the rapid and aperiodic variability responsible for the SUL-SSUL spectral changes. The nature of the longer periodicity of NGC 5204 X-1 remains unclear, and robust determination of the orbital period of these sources could shed light on the nature of the periodic modulation found. Based on the similarities between the two sources, a long periodicity should be detectable in Holmberg II X-1 with future monitoring.

We report the discovery of a new high-mass X-ray binary pulsar, XMMU J050722.1-684758, possibly associated with the supernova remnant (SNR) MCSNR J0507-6847 in the Large Magellanic Cloud, using XMM-NewtonX-ray observations. Pulsations with a periodicity of 570 s are discovered from the Be X-ray binary XMMU J050722.1-684758 confirming its nature as a HMXB pulsar. The HMXB is located near the geometric centre of the SNR MCSNR J0507-6847(0.9 arcmin from the centre) which supports the XRB-SNR association. The estimated age of the SNR is 43-63 kyr years which points to a middle aged to old SNR. The large diameter of the SNR combined with the lack of distinctive shell counterparts in optical and radio indicates that the SNR is expanding into the tenuous environment of the superbubble N103. The estimated magnetic field strength of the neutron star is B ≳ 1014 G assuming a spin equilibrium condition which is expected from the estimated age of the parent remnant and assuming that the measured mass-accretion rate remained constant throughout.

We report on the temporal properties of the ultraluminous X-ray (ULX) pulsar M51 ULX-7 inferred from the analysis of the 2018-2020 Swift/X-ray Telescope monitoring data and archival Chandra data obtained over a period of 33 days in 2012. We find an extended low flux state, which might be indicative of propeller transition, lending further support to the interpretation that the neutron star is rotating near equilibrium. Alternatively, this off-state could be related to a variable superorbital period. Moreover, we report the discovery of periodic dips in the X-ray light curve that are associated with the binary orbital period. The presence of the dips implies a configuration where the orbital plane of the binary is closer to an edge-on orientation, and thus demonstrates that favorable geometries are not necessary in order to observe ULX pulsars. These characteristics are similar to those seen in prototypical X-ray pulsars such as Her X-1 and SMC X-1 or other ULX pulsars such as NGC 5907 ULX1.

In November 2019, MAXI detected an X-ray outburst from the known Be X-ray binary system RX J0209.6-7427 located in the outer wing of the Small Magellanic Cloud. We followed the outburst of the system with NICER which led to the discovery of X-ray pulsations with a period of 9.3 s. We analyzed simultaneous X-ray data obtained with NuSTAR and NICER allowing us to characterize the spectrum and provide an accurate estimate of its bolometric luminosity. During the outburst the maximum broadband X-ray luminosity of the system reached 1-2×1039 erg/s, thus exceeding by about one order of magnitude the Eddington limit for a typical 1.4 M⊙ mass neutron star (NS). Monitoring observations with Fermi/GBM and NICER allowed us to study the spin evolution of the NS and compare it with standard accretion torque models. We found that the NS magnetic field should be of the order of 3×1012 G. We conclude that RX J0209.6-7427 exhibited one of the brightest outbursts observed from a Be X-ray binary pulsar in the Magellanic Clouds, reaching similar luminosity level to the 2016 outburst of SMC X-3. Despite the super-Eddington luminosity of RX J0209.6-7427, the NS appears to have only a moderate magnetic field strength.

Ultra luminous X-ray pulsars (ULXP) are fascinating objects, whose X-ray emission greatly exceeds the Eddington limit for a solar mass object. Given the coherent pulsations we now know that these systems host accreting magnetized Neutron Stars (NS), thus challenging our understanding of accretion theory. Moreover several of these systems show super-orbital variability where the observed flux change by factor more than 10. Key questions about the nature of these systems are; is there is beaming involved that enhances the derived isotropic Luminosity? What is the magnetic field of the NS in ULXPs and how this compares to the typical X-ray pulsars? and what is the nature of the super-orbital modulation? The study of individual ULXPS can help us answer these key questions. Here I will present observational constrains on the properties of NGC 300 ULX1. Through a year long X-ray monitoring we discovered that even when the X-ray flux of the system decreased by a factor of 20 the spin-up of the NS continues at a constant rate denoting constant mass accretion onto the NS. Moreover, I will discuss the changes in the observed flux in the context of a precessing disc and outflows. In addition, I will discuss the properties of newly confirmed ULXP M51 ULX7, I will show that outflows are not a necessary requirement to account for super orbital variability, and will discuss alternative mechanisms.

We present the results of cross-correlation analysis between the Fermi-LAT gamma-ray and SMARTS optical/infrared light curves of bright 8 blazars monitored in 2008-2017. For the temporal correlation analysis of unevenly sampled variability data, we use the Discrete Correlation Function (DCF), and created an empirical bootstrapping method to assess the significance of the DCF amplitude for each blazar. The DCFs between gamma-ray and optical/infrared light curves with one week binning time scale suggest that 6 of the 8 blazars show a significant peak at zero lag at or above 3 sigma level. That is consistent with the leptonic model in which optical/infrared photons are produced by synchrotron radiation of relativistic electrons and gamma rays are produced by inverse Compton scattering of ambient photons by the synchrotron-emitting electrons. However, the DCFs with one day binning time scale suggest that among 8 blazars, only one blazar — 3C 454.3 — still has a significant peak at zero lag. The other 7 blazars tend to show much smaller peaks than those with a weekly time bin. In addition, for a given blazar, strong changes of the DCFs from one epoch to the next are shown by the analyses of time periods of one or two years. These results complicate the simplest understanding of blazar emission mechanisms. We discuss possible physical explanations.

Swift J0243.6+6124 is a Be/X-ray binary and the first known ultraluminous X-ray pulsar in our Galaxy that reached a peak luminosity Lx > 1039 erg s-1 during its 2017-2018 outburst. The proximity of this system allows for the study of super-Eddington accretion as an analog of distant ultraluminous X-ray sources. We used data from the Neil Gehrels Swift X-ray Telescope to investigate the evolution of the spectral and temporal properties of this system, looking for characteristic transitions that could reveal changes in the accretion regime with Lx. A first transition is found in the hardness-intensity diagram at Lx ~ 7 x 1036 erg s-1. The system exhibits a harder-when-brighter trend that changes to a softer-when-brighter trend. This transition is typical in Galactic BeXRB pulsars, and is often used as a proxy of the magnetic field strength of the neutron star. A second transition is indicated by changes in fractional variability and spectral hardness at a critical luminosity Lcrit ~ 3 x 1038 erg s-1. Pulsations exhibit single peak behavior and change to double peak following the transition. Associating these transitions with the formation and evolution of the accretion column can help us derive constraints on the magnetic field of the neutron star, and gain insights on super-Eddington accretion.

1RXS J050526.3-684628 is a super soft source in the Large Magellanic Cloud(LMC). A recent study reported the discovery of coherent pulsations,and identified the system as an accreting White Dwarf (WD). Moreover, thestudy of historic X-ray and optical properties of J050526 have revealedit to be a remarkably long-lived post-nova SSS (~30 year duration),reaching its peak flux around 2013. Nevertheless, the lack of multipledeep X-ray observations hampers the study of the system and prohibitsfurther testing of theoretical models. We request a 60 ks XMM-Newtonobservation to study the evolution of its spectral and temporal propertiesduring the decay of its flux.

Following the SRG/eROSITA discovery of a strong outburst from the LMC Be/X-ray binary RX J0529.8-6556 (Haberl et al., ATEL #13828) we triggered NICER ToO observations to search for pulsations and obtain high-quality spectra.

The Be X-ray binary pulsar 1A 0535+262 has recently been observed in outburst with Swift/BAT and MAXI (ATel #14157, #14173). Since then the pulsar has been rapidly evolving in X-rays.

Aims: Spectral and temporal analysis of the NuSTAR observation Galactic Be-XRB Swift J1845.7-0037. during its recent outburst. Methods: For the spectral analysis we use both phenomenological and physics-based models. We employ an often used empirical model to identify the main characteristics of the spectral shape in relation to nominal spectral characteristics of X-ray pulsars. Additionally, we used the latest version of Bulk amp; Thermal comptonization model (BW), to assess the validity of the spectral components required by the empirical model and to investigate the origin of the hard X-ray emission. We also analyzed the source light-curve, studying the pulse shape at different energy ranges and tracking the spectral evolution with pulse phase by using the model independent hardness ratio (HR). Results: We find that while both the empirical and physical (BW) spectral models can produce good spectral fits, the BW model returns physically plausible best-fit values for the source parameters and does not require any additional spectral components to the non-thermal, accretion column emission. The BW model also yielded an estimation of the neutron star magnetic field placing it in the 10^12G range. Conclusions: Our results, show that the spectral and temporal characteristics of the source emission are consistent with the scattering processes expected for radiation dominated shocks within the accretion column of highly magnetized accreting neutron stars. We further indicate that physically-derived spectral models such as BW, can be used to tentatively infer fundamental source parameters, in the absence of more direct observational signatures.

In the course of the first all-sky survey (eRASS1), the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission discovered a strong outburst from RX J0529.8-6556 in the Large Magellanic Cloud (LMC).

In the course of the first all-sky survey (eRASS1), the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission started scanning the Small Magellanic Cloud (SMC).

Supersoft X-ray sources (SSS) have been identified as white dwarfs accreting from binary companions and undergoing nuclear burning of the accreted material on their surface. Although expected to be a relatively numerous population from both binary evolution models and their identification as type Ia supernova progenitor candidates, given the very soft spectrum of SSSs relatively few are known. Here we report on the X-ray and optical properties of 1RXS J050526.3-684628, a previously unidentified accreting nuclear-burning white dwarf located in the Large Magellanic Cloud (LMC). XMM-Newton observations enabled us to study its X-ray spectrum and measure for the first time short-period oscillations of ~170 s. By analysing newly obtained X-ray data by eROSITA, together with Swift observations and archival ROSAT data, we have followed its long-term evolution over the last 3 decades. We identify 1RXS J050526.3-684628 as a slowly evolving post-nova SSS undergoing residual surface nuclear burning, which finally reached its peak in 2013 and is now declining. Though long expected on theoretical grounds, such long-lived residual-burning objects had not yet been found. By comparison with existing models, we find that the effective temperature and luminosity evolution are consistent with an ~0.7 M⊙ carbon-oxygen white dwarf accreting ${\sim} 10^{-9}~\rm {M}_{\odot }$ yr-1. Our results suggest that there may be many more undiscovered SSSs and 'missed' novae awaiting dedicated deep X-ray searches in the LMC and elsewhere.

Following the detection of recent X-ray activity from the Be X-ray binary pulsar 1A 0535+262 on 2020 November 6 (ATel #14157), the Neutron Star Interior Composition Explorer (NICER) has observed the system at multiple epochs.

We present the results of the first dedicated observation of the young X-ray pulsar SXP 1062 in the broad X-ray energy band obtained during its 2019 outburst with the NuSTAR and XMM-Newton observatories. The analysis of the pulse-phase averaged and phase-resolved spectra in the energy band from 0.5 to 70 keV did not reveal any evidence for the presence of a cyclotron line. The spin period of the pulsar was found to have decreased to 979.48 ± 0.06 s implying a ∼10% reduction compared to the last measured period during the monitoring campaign conducted about five years ago, and is puzzling considering that the system apparently has not shown major outbursts ever since. The switch of the pulsar to the spin-up regime supports the common assumption that torques acting on the accreting neutron star are nearly balanced and thus SXP 1062 likely also spins with a period close to the equilibrium value for this system. The current monitoring of the source also revealed a sharp drop in its soft X-ray flux right after the outburst, which is in drastic contrast to the behavior during the previous outburst when the pulsar remained observable for years with only a minor flux decrease after the end of the outburst. This unexpected off state of the source lasted for at most 20 days after which SXP 1062 returned to the level observed during previous campaigns. We discuss this and other findings in context of the modern models of accretion onto strongly magnetized neutron stars.

We present the results obtained from the analysis of high-mass X-ray binary pulsar 4U 1909+07 using NuSTAR and Astrosat observations in July 2015 and 2017, respectively. X-ray pulsations at ≈604 s are clearly detected in our study. Based on the long-term spin-frequency evolution, the source is found to spun-up in the last 17 yr. We observed a strongly energy-dependent pulse profile that evolved from a complex broad structure in soft X-rays into a profile with a narrow emission peak followed by a plateau in energy ranges above 20 keV. This behaviour ensured a positive correlation between the energy and pulse fraction. The pulse profile morphology and its energy evolution are almost similar during both the observations, suggesting a persistent emission geometry of the pulsar over time. The broad-band energy spectrum of the pulsar is approximated by an absorbed high-energy exponential cut-off power-law model with iron emission lines. In contrast to the previous report, we found no statistical evidence for the presence of cyclotron absorption features in the X-ray spectra. We performed phase-resolved spectroscopy using data from the NuSTAR observation. Our results showed a clear signature of absorbing material at certain pulse phases of the pulsar. These findings are discussed in terms of stellar wind distribution and its effect on the beam geometry of this wind-fed accreting neutron star. We also reviewed the subsonic quasi-spherical accretion theory and its implication on the magnetic field of 4U 1909+07 depending on the global spin-up rate.

Recent modeling of Neutron Star Interior Composition Explorer (NICER) observations of the millisecond pulsar PSR J0030+0451 suggests that the magnetic field of the pulsar is non-dipolar. We construct a magnetic field configuration where foot points of the open field lines closely resemble the hotspot configuration from NICER observations. Using this magnetic field as input, we perform force-free simulations of the magnetosphere of PSR J0030+0451, showing the three-dimensional structure of its plasma-filled magnetosphere. Making simple and physically motivated assumptions about the emitting regions, we are able to construct the multiwavelength lightcurves that qualitatively agree with the corresponding observations. The agreement suggests that multipole magnetic structures are the key to modeling this type of pulsar, and can be used to constrain the magnetic inclination angle and the location of radio emission.

In 2019 November, MAXI detected an X-ray outburst from the known Be X-ray binary system RX J0209.6-7427 located in the outer wing of the Small Magellanic Cloud. We followed the outburst of the system with NICER, which led to the discovery of X-ray pulsations with a period of 9.3 s. We analysed simultaneous X-ray data obtained with NuSTAR and NICER, allowing us to characterize the spectrum and provide an accurate estimate of its bolometric luminosity. During the outburst, the maximum broad-band X-ray luminosity of the system reached (1-2) × 1039 erg s-1, thus exceeding by about one order of magnitude the Eddington limit for a typical 1.4 M⊙ mass neutron star (NS). Monitoring observations with Fermi/GBM and NICER allowed us to study the spin evolution of the NS and compare it with standard accretion torque models. We found that the NS magnetic field should be of the order of 3 × 1012 G. We conclude that RX J0209.6-7427 exhibited one of the brightest outbursts observed from a Be X-ray binary pulsar in the Magellanic Clouds, reaching similar luminosity level to the 2016 outburst of SMC X-3. Despite the super-Eddington luminosity of RX J0209.6-7427, the NS appears to have only a moderate magnetic field strength.

In this work, we explore the applicability of standard theoretical models of accretion to the observed properties of M51 ULX-7. The spin-up rate and observed X-ray luminosity are evidence of a neutron star with a surface magnetic field of 2-7 × 1013 G, rotating near equilibrium. Analysis of the X-ray light curve of the system (Swift/XRT data) reveals the presence of a ∼39 d superorbital period. We argue that the superorbital periodicity is due to disc precession, and that material is accreted on to the neutron star at a constant rate throughout it. Moreover, by attributing this modulation to the free precession of the neutron star we estimate a surface magnetic field strength of 3-4 × 1013 G. The agreement of these two independent estimates provide strong constraints on the surface polar magnetic field strength of the NS.

STROBE-X is a probe-class mission concept, selected for study by NASA, for X-ray spectral timing of compact objects across the mass scale. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility, enabling a broad portfolio of high-priority astrophysics.

In October 2018, Swift announced the discovery of a new Galactic X-ray transient, Swift J1858. Just before Sun-angle constraints rendered the system unobservable, follow-up observations revealed extreme flaring activity, of a kind that has so far only been seen in the famous black hole X-ray binary (BHXRB) V404 Cyg during its 2015 eruption and in V4641 Sgr. The peculiar behaviour of these sources is thought to be a consequence of super-Eddington accretion regime. After several months of unusual strong and rapid flaring in its high-luminosity state, Swift J1858 is currently exhibiting impressive optical P-Cygni profiles, suggesting the pres- ence of a dense and cool wind from the outer accretion disk. The dominant spectroscopic signatures of such winds are actually expected to lie in the far-ultraviolet region, but they are usually inaccessible in black-hole X-ray binaries, due to interstellar reddening. Given its low extinction, Swift J1858 provides us with a rare chance to study the accretion disk wind in the crucial ultraviolet band - an opportunity that was missed in the other two systems. Building on an ongoing multi-wavelength campaign (X-rays: NICER; optical: GTC; radio: VLA & AMI), we therefore request far- and near-UV time-resolved spectroscopic observations of this system with HST/STIS+COS in order to (a) study its extreme accretion disk wind; (b) test proposed wind driving mechanisms; (c) characterize its UV variability properties and determine the origin of these variations; (d) construct the broad-band SED of the outer accretion disk that dominates the UV flux; and (e) determine the extinction towards the system in order to constrain the mass accretion rate.

Ultra luminous X-ray Pulsars (ULXPs) are bright binary systems that host a Neutron Star (NS) and emit radiation in excess of the Eddington Limit expected for isotropic accretion. We have studies the spectral and spin properties of the ULXP NGC300 ULX1 through archival data, and have shown that its spin evolution from ~126 s down to 16 s is consistent with almost constant accretion between 2014 and 2018. Moreover, based on the 2018 Swift/XRT and NICER monitoring campaigns of the system we have concluded that even during an 100 d period where the observed flux drops by a factor of 20, the spin-up rate and thus the mass accretion rate remained almost constant. This can be explained only by invoking extreme X-ray absorption or obscuration due to extreme outflows from the accretion disk, or disk precession. Finally, an intriguing consequence is that assuming constant spin-up rate a NS spin reversal should have occurred around 2012.

Ultra luminous X-ray (ULX) sources are among the most intriguing binary systems, that have long been thought to host the elusive intermediate mass black holes. Remarkably, within the last years there has been undisputed evidence that at least a few of these systems are powered by accreting neutron stars (NS) that are rotating with spin periods close to 1 s. In light of these recent discoveries and recently introduced models placing neutron stars as the engines of ULXs, we revisit the spectra of eighteen well-known ULXs, in search of indications that favor or reject this hypothesis. We find that the notable (>6keV) spectral curvature observed in most ULXs, is commensurate with the Wien tail of a hot (T>1keV) multicolor black-body component and confirm that a double thermal model (comprised of a ”cool” and ”hot” thermal component) with the addition of a faint non-thermal tail describes all ULX spectra in our list. More importantly, we offer a new physical interpretation for the dual thermal spectrum, where it is the result of accretion onto high magnetized NSs rather than black holes, in agreement with theoretical predictions. We estimate the magnetic-field strength and demonstrate that it correlates strongly with the source luminosity and the temperature of the hot component. We also discuss the application of our model on the most recent pulsating ULX "NGC 300 ULX1", casting doubts on the claimed presence of a cyclotron scattering feature in its spectrum. Our findings offer an additional and compelling argument in favor of NSs as prime candidates for powering ULXs, as has been also postulated by theory.

NGC 300 ULX1 is the fourth to be discovered in the class of the ultra-luminous X-ray pulsars. Pulsations from NGC 300 ULX1 were discovered during simultaneous XMM-Newton / NuSTAR observations in Dec. 2016. The period decreased from 31.71 s to 31.54 s within a few days, with a spin-up rate of -5.56×10-7 s s-1, likely one of the largest ever observed from an accreting neutron star. Archival Swift and NICER observations revealed that the period decreased exponentially from 45 s to 17.5 s over 2.3 years. The pulses are highly modulated with a pulsed fraction strongly increasing with energy and reaching nearly 80% at energies above 10 keV. The X-ray spectrum is described by a power-law and a disk black-body model, leading to a 0.3-30 keV unabsorbed luminosity of 4.7×1039 erg s-1. The spectrum from an archival XMM-Newton observation of 2010 can be explained by the same model, however, with much higher absorption. This suggests, that the intrinsic luminosity did not change much since that epoch. NGC 300 ULX1 shares many properties with supergiant high mass X-ray binaries, however, at an extreme accretion rate.

MAXI J0206-749 is a hard X-ray transient detected by MAXI/GSC (ATel #13300) in the direction of the Small Magellanic Cloud (SMC) wing. Follow-up Swift/XRT observations (ATel #13303) identified MAXI J0206-749 with the HMXB RX J0209.6-7427.

We report on a NuSTAR observation of the Be X-ray binary 4U 1901+03 taken on 2019 April 11.95 (MJD 58584.95; ObsID 90502307004). It was performed during the decline of the source's current outburst, which began around 2019-02-08 (ATel #12498, GCN #23882).

We present a source catalog from the first deep hard X-ray (E > 10 keV) survey of the Small Magellanic Cloud (SMC), the Nuclear Spectroscopic Telescope Array (NuSTAR) Legacy Survey of the SMC. We observed three fields, for a total exposure time of 1 Ms, along the bar of this nearby star-forming galaxy. Fields were chosen for their young stellar and accreting binary populations. We detected 10 sources above a 3σ significance level (4-25 keV) and obtained upper limits on an additional 40 sources. We reached a 3σ limiting luminosity in the 4-25 keV band of ∼1035 erg s-1, allowing us to probe fainter X-ray binary (XRB) populations than has been possible with other extragalactic NuSTAR surveys. We used hard X-ray colors and luminosities to constrain the compact-object type, exploiting the spectral differences between accreting black holes and neutron stars at E > 10 keV. Several of our sources demonstrate variability consistent with previously observed behavior. We confirmed pulsations for seven pulsars in our 3σ sample. We present the first detection of pulsations from a Be-XRB, SXP 305 (CXO J005215.4-73191), with an X-ray pulse period of 305.69 ± 0.16 s and a likely orbital period of ∼1160-1180 days. Bright sources (≳5 × 1036 erg s-1) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower-luminosity sources (≲5 × 1036 erg s-1) have X-ray colors and luminosities consistent with multiple classifications. We raise questions about possible spectral differences at low luminosity between SMC pulsars and the Galactic pulsars used to create the diagnostic diagrams.

Aims: We investigate accretion models for the newly discovered pulsating ultraluminous X-ray source (ULX) NGC 300 ULX1. Methods: We analyzed broadband XMM-Newton and NuSTAR observations of NGC 300 ULX1, performing phase-averaged and phase-resolved spectroscopy. Using the Bayesian framework, we compared two physically motivated models for the source spectrum: Non-thermal accretion column emission modeled by a power law with a high-energy exponential roll-off (AC model), and multicolor thermal emission from an optically thick accretion envelope plus a hard power-law tail (MCAE model). The AC model is an often used phenomenological model for the emission of X-ray pulsars, while the MCAE model has recently been proposed for the emission of the optically thick accretion envelope that is expected to form in ultraluminous (LX > 1039 erg s-1), highly magnetized accreting neutron stars. We combined the findings of our Bayesian analysis with qualitative physical considerations to evaluate the suitability of each model. Results: The low-energy part (< 2 keV) of the source spectrum is dominated by non-pulsating, multicolor thermal emission. The (pulsating) high-energy continuum is more ambiguous. If modeled with the AC model, a residual structure is detected that can be modeled using a broad Gaussian absorption line centered at ∼12 keV. However, the same residuals can be successfully modeled using the MCAE model, without the need for the absorption-like feature. Model comparison using the Bayesian approach strongly indicates that the MCAE model without the absorption line is the preferred model. Conclusions: The spectro-temporal characteristics of NGC 300 ULX1 are consistent with previously reported traits for X-ray pulsars and (pulsating) ULXs. All models considered strongly indicate the presence of an accretion disk that is truncated at a large distance from the central object, as has recently been suggested for a large portion of both pulsating and non-pulsating ULXs. The hard, pulsed emission is not described by a smooth spectral continuum. If modeled by a broad Gaussian absorption line, the fit residuals can be interpreted as a cyclotron scattering feature (CRSF) compatible with a ∼1012 G magnetic field. However, the MCAE model can successfully describe the spectral and temporal characteristics of the source emission, without the need for an additional absorption feature, and it yields physically meaningful parameter values. Therefore strong doubts are cast on the presence of a CRSF in NGC 300 ULX1.

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.

SXP 15.3 (RX J0052.1-7319) is a Be X-ray binary pulsar located in the Small Magellanic Cloud. The source was classified as a transient X-ray binary candidate based on ROSAT observations in the 1990's, and pulsations at 15.3 s were subsequently discovered from an outburst in 1996 from the ROSAT data. The source has never been studied in an outburst or a bright state ever since. Following reports of an outburst in November 2017, we triggered a Target of Opportunity observation of SXP 15.3 with AstroSat. We report here the first broadband spectral and timing studies of the source, when the source was accreting near the Eddington limit of 10^38 erg/s. We discuss the energy dependence of the pulse profiles and the broadband spectrum in context of accretion onto magnetized neutron stars accreting near its Eddington limit.

Long period pulsars (P> 1000 s) constitute a sub-population ofhigh-mass X-ray binaries. To date, only a few of these rare systems,which occupy the tail of the spin period distribution of X-ray pulsars,have been discovered. Nevertheless, their study offers unique insightsinto evolutionary scenarios of the high-mass X-ray binary population. Wepropose four XMM-Newton observations aiming at improving our understandingof long period pulsars. The proposed observations will be performedto two systems located in the LMC with known spin periods for studyingtheir spin evolution, and two new candidate long period pulsars in orderto increase the number of known systems.

NICER observed the new transient Swift J005139.2-721704 located in the Small Magellanic Cloud (SMC), and discovered on 2018 Nov. 9 (ATel #12209). The source position identified using Swift XRT as RA=00:51:39.2 and DEC=-72:17:03.6 (with an uncertainty of 1.4 & Prime;, ATel #12209).

We report on the temporal analysis of NICER and Fermi/GBM observations of the new transient Swift J005139.2-721704 located in the SMC (ATel #12209, #12219) that resulted in its identification with the known X-ray pulsar XTE J0052-723 (SXP 4.78).

Following the discovery of the newly discovered ULX pulsar in NGC 300 (ATel #11158) we searched the available X-ray data for the evolution of the spin period of the neutron star and the X-ray luminosity.

Swift J1858.6-0814: Localization and variability of the optical counterpart G. Vasilopoulos (Yale), C. Bailyn (Yale), J. Milburn (Caltech) Following the detection of the new galactic transient source Swift J1858.6-0814 (Krimm et al. ATEL #12151) we performed photometric follow up observations of its optical counterpart using the WASP instrument on the Hale 200" Telescope at Palomar Observatory.

The Large Magellanic Cloud (LMC) currently hosts around 23 high-mass X-ray binaries (HMXBs) of which most are Be/X-ray binaries. The LMC XMM-Newton survey provided follow-up observations of previously known X-ray sources that were likely HMXBs, as well as identifying new HMXB candidates. In total, 19 candidate HMXBs were selected based on their X-ray hardness ratios. In this paper we present red and blue optical spectroscopy, obtained with Southern African Large Telescope and the South African Astronomical Observatory 1.9-m telescope, plus a timing analysis of the long-term optical light curves from OGLE to confirm the nature of these candidates. We find that nine of the candidates are new Be/X-ray binaries, substantially increasing the LMC Be/X-ray binary population. Furthermore, we present the optical properties of these new systems, both individually and as a group of all the BeXBs identified by the XMM-Newton survey of the LMC.

We report the results of AstroSat and NuSTAR observations of the Be/X-ray binary pulsar SXP 15.3 in the Small Magellanic Cloud during its outburst in late 2017, when the source reached a luminosity level of ∼1038 erg s-1, close to the Eddington limit. The unprecedented broad-band coverage of the source allowed us to perform timing and spectral analysis between 3 and 80 keV. The pulse profile exhibits a significant energy dependence, and morphs from a double-peaked profile to a single broad pulse at energies >15 keV. This can be explained by a spectral hardening during an intensity dip seen between the two peaks of the pulse profile. We detect a Cyclotron Resonance Scattering Feature at ∼5 keV in the X-ray spectrum, independent of the choice of the continuum model. This indicates a magnetic field strength of 6 × 1011 G for the neutron star.

Aims: We study the temporal and spectral characteristics of SMC X-3 during its recent (2016) outburst to probe accretion onto highly magnetized neutron stars (NSs) at the Eddington limit. Methods: We obtained XMM-Newton observations of SMC X-3 and combined them with long-term observations by Swift. We performed a detailed analysis of the temporal and spectral behavior of the source, as well as its short- and long-term evolution. We have also constructed a simple toy-model (based on robust theoretical predictions) in order to gain insight into the complex emission pattern of SMC X-3. Results: We confirm the pulse period of the system that has been derived by previous works and note that the pulse has a complex three-peak shape. We find that the pulsed emission is dominated by hard photons, while at energies below ~1 keV, the emission does not pulsate. We furthermore find that the shape of the pulse profile and the short- and long-term evolution of the source light-curve can be explained by invoking a combination of a "fan" and a "polar" beam. The results of our temporal study are supported by our spectroscopic analysis, which reveals a two-component emission, comprised of a hard power law and a soft thermal component. We find that the latter produces the bulk of the non-pulsating emission and is most likely the result of reprocessing the primary hard emission by optically thick material that partly obscures the central source. We also detect strong emission lines from highly ionized metals. The strength of the emission lines strongly depends on the phase. Conclusions: Our findings are in agreement with previous works. The energy and temporal evolution as well as the shape of the pulse profile and the long-term spectra evolution of the source are consistent with the expected emission pattern of the accretion column in the super-critical regime, while the large reprocessing region is consistent with the analysis of previously studied X-ray pulsars observed at high accretion rates. This reprocessing region is consistent with recently proposed theoretical and observational works that suggested that highly magnetized NSs occupy a considerable fraction of ultraluminous X-ray sources.

NGC 300 ULX1 is a newly identified ultra-luminous X-ray pulsar. The system is associated with the supernova impostor SN 2010da that was later classified as a possible supergiant Be X-ray binary. In this work we report on the spin period evolution of the neutron star based on all the currently available X-ray observations of the system. We argue that the X-ray luminosity of the system has remained almost constant since 2010, at a level above ten times the Eddington limit. Moreover, we find evidence that the spin period of the neutron star evolved from ∼126 s down to ∼18 s within a period of about 4 years. We explain this unprecedented spin evolution in terms of the standard accretion torque theory. An intriguing consequence for NGC 300 ULX1 is that a neutron star spin reversal should have occurred a few years after the SN 2010da event.

B[e] supergiants are luminous evolved massive stars. The mass-loss during this phase creates a complex circumstellar environment with atomic, molecular, and dusty regions usually found in rings or disc-like structures. For a better comprehension of the mechanisms behind the formation of these rings, detailed knowledge about their structure and dynamics is essential. To address that, we obtained high-resolution optical and near-infrared (near-IR) spectra for eight selected Galactic B[e] supergiants, for which CO emission has been detected. Assuming Keplerian rotation for the disc, we combine the kinematics obtained from the CO bands in the near-IR with those obtained by fitting the forbidden emission [O I] λ5577, [O I] λλ6300,6363, and [Ca II] λλ7291,7323 lines in the optical to probe the disc structure. We find that the emission originates from multiple ring structures around all B[e] supergiants, with each one of them displaying a unique combination of rings regardless of whether the object is part of a binary system. The confirmed binaries display spectroscopic variations of their line intensities and profiles as well as photometric variability, whereas the ring structures around the single stars are stable.

The supernova impostor SN 2010da located in the nearby galaxy NGC 300, later identified as a likely supergiant B[e] high-mass X-ray binary, was simultaneously observed by NuSTAR and XMM-Newton between 2016 December 16 and 20, over a total time span of ∼310 ks. We report the discovery of a strong periodic modulation in the X-ray flux with a pulse period of 31.6 s and a very rapid spin-up, and confirm therefore that the compact object is a neutron star. We find that the spin period is changing from 31.71 s to 31.54 s over that period, with a spin-up rate of -5.56 × 10-7 s s-1, likely the largest ever observed from an accreting neutron star. The spectrum is described by a power-law and a disc blackbody model, leading to a 0.3-30 keV unabsorbed luminosity of 4.7 × 1039 erg s-1. Applying our best-fitting model successfully to the spectra of an XMM-Newton observation from 2010, suggests that the lower fluxes of NGC 300 ULX1 reported from observations around that time are caused by a large amount of absorption, while the intrinsic luminosity was similar as seen in 2016. A more constant luminosity level is also consistent with the long-term pulse period evolution approaching an equilibrium value asymptotically. We conclude that the source is another candidate for the new class of ultraluminous X-ray pulsars.

Long period pulsars (P> 1000 s) constitute a sub-population ofhigh-mass X-ray binaries. To date, only a few of these rare systems,which occupy the tail of the spin period distribution of X-ray pulsars,have been discovered. Nevertheless, their study offers unique insightsinto evolutionary scenarios of the high-mass X-ray binary population. Wepropose four XMM-Newton observations aiming at improving our understandingof long period pulsars. The proposed observations will be performedto two systems located in the Large Magellanic Cloud with known spinperiods for studying their spin evolution, and two new candidate longperiod pulsars in order to increase the number of known systems.

Nearby galaxies are well suited for investigating X-ray source populations in environments different to our own Galaxy. Sources in these galaxies have well determined distances and are less absorbed than sources in the Galactic plane. The Large and the Small Magellanic Clouds (MC) are the nearest gas-rich star-forming galaxies and their gravitational interactions are believed to have tidally triggered recent bursts of star formation. Here, we will focus on the X-ray spectral and temporal properties of three Be/X-ray binary pulsars located in the MC that have been observed in the recent years with XMM-Newton during super-Eddington outbursts. Phase-resolved spectral analysis has revealed the presence of a non-pulsating soft component. By analysing multiple observations, corresponding to different luminosity levels, we argue that this component could not originate from the surface of a traditional thin disk, but most probably this emission is a result of reprocessed emission from material located near the NS magnetospheric radius. Interestingly, we find that the temperature of this component does not change much with the luminosity of the system, in contrast to its size that increases with increasing luminosity. We argue that this indicates the formation and expansion of an envelope around the magnetosphere of the NS.

In light of recent discoveries of pulsating ULXs and recently introduced models placing neutron stars as the central engines of ULXs, we revisit the spectra of seventeen ULXs, in search of indications that favor this hypothesis. To this end we examined the spectra from XMM-Newton observations of all seventeen sources in our sample. For six sources, these were complimented with spectra from public NuSTAR observations. We demonstrate that the notable ({>}6 keV) spectral curvature observed in most ULXs, is most likely due to thermal emission, with T{>} 1keV. More importantly, we find that a double thermal model (comprised of a 'cool' and 'hot' thermal component) - often associated with emission from neutron star X-ray binaries - describes all ULX spectra in our list. We propose that the dual thermal spectrum is the result of accretion onto highly magnetized NSs, as predicted in recent theoretical models (Mushtukov et al. 2017). We further argue that this finding offers an additional and compelling argument in favor of neutron stars as prime candidates for powering ULXs, as has been recently suggested (King & Lasota 2016; King et al. 2017). In my talk I will discuss the implications of our interpretation along with its merits and shortcomings.

NGC 300 ULX-1 is a newly identified ULX pulsar. The system has shown a extraordinary spin up rate within the last year, when it spun-up from 31 sec to 20 sec. We request 2x10 ks chandra observations separated by 2-4 days in order to accurately measure the spin up rate of the pulsar.

Swift_J010745.0-722740 is a BeXRB candidate located in the SMC (ATel #5778) with no X-ray pulsations detected so far. Its companion exhibits strong (0.3-0.4 mag) outbursts, with a recurring time of 1180 d (ATel #5781, #5778).

The transient IGR J01217-7257 in the Small Magellanic Cloud was found to be in a new outburst during INTEGRAL observations. We triggered an XMM-Newton target of opportunity observation near outburst maximum, which lead to the discovery of X-ray pulsations with a period of 2.165 s. This period is very similar to that detected from XTE J0119-731, suggesting that both sources are identical. The pulse profile obtained from the EPIC-pn instrument is complex and highly energy dependent. Pulse-phase spectroscopy reveals variations in the spectral slope correlated with the changes in flux during the pulse, with the harder X-ray spectrum at pulse maximum and softer during minimum. Analysis of XMM-Newton reflection grating spectra reveals the presence of emission lines that suggest the presence of ionized material around the neutron star. By monitoring the system during its outburst with Swift/XRT we detected a possible transition from the accretor to the propeller stage.

We report on the X-ray spectral and temporal properties of the Be/X-ray binary system XMMU J004855.5-734946 located in the Small Magellanic Cloud. The system was monitored by Swift/XRT during a moderate outburst in 2016 July, while an unanticipated Chandra target of opportunity observation was triggered when the luminosity of the system was greater than 1036 erg s-1 allowing a detailed study of X-ray properties of the systems. Specifically, its X-ray spectrum, as observed during the outburst, is well modelled by an absorbed power law (Γ = 0.58). Timing analysis of the collected photon events revealed coherent X-ray pulsations with a period of ∼15.64 s, thus confirming XMMU J004855.5-734946 as a high-mass X-ray binary pulsar. By analysing archival XMM-Newton observations, we determined the long-term spin period evolution of the neutron star, showing that the compact star has spun-up by \dot{P}∼ -0.0028 s yr^{-1}. By modelling the X-ray pulsed emission as detected by Chandra, we set constraints on the inclination of the magnetic and rotation axis of the neutron star, as well to its compactness (I.e. (M/M⊙)/(R/km) = 0.095 ± 0.007).

Context. The Exploring the X-ray Transient and variable Sky (EXTraS) project searches for coherent signals in the X-ray archival data of XMM-Newton. Aims: XMM-Newton performed more than 400 pointed observations in the region of the Large Magellanic Cloud (LMC). We inspected the results of the EXTraS period search to systematically look for new X-ray pulsators in our neighbour galaxy. Methods: We analysed the XMM-Newton observations of two sources from the 3XMM catalogue which show significant signals for coherent pulsations. Results: 3XMM J051259.8-682640 was detected as a source with a hard X-ray spectrum in two XMM-Newton observations, revealing a periodic modulation of the X-ray flux with 956 s. As optical counterpart we identify an early-type star with Hα emission. The OGLE I-band light curve exhibits a regular pattern with three brightness dips which mark a period of 1350 d. The X-ray spectrum of 3XMM J051034.6-670353 is dominated by a super-soft blackbody-like emission component (kT 70 eV) which is modulated by nearly 100% with a period of 1418 s. From GROND observations we suggest a star with r' = 20.9 mag as a possible counterpart of the X-ray source. Conclusions: 3XMM J051259.8-682640 is confirmed as a new Be/X-ray binary pulsar in the LMC. We discuss the long-term optical period as the likely orbital period which would be the longest known from a high-mass X-ray binary. The spectral and temporal properties of the super-soft source 3XMM J051034.6-670353 are very similar to those of RX J0806.3+1527 and RX J1914.4+2456 suggesting that it belongs to the class of double-degenerate polars and is located in our Galaxy rather than in the LMC.

Aims: In light of recent discoveries of pulsating ultraluminous X-ray sources (ULXs) and recently introduced theoretical schemes that propose neutron stars (NSs) as the central engines of ULXs, we revisit the spectra of eighteen well known ULXs, in search of indications that favour this newly emerging hypothesis. Methods: We examine the spectra from high-quality XMM-Newton and NuSTAR observations. We use a combination of elementary black body and multicolour disk black body (MCD) models, to diagnose the predictions of classic and novel theoretical models of accretion onto NSs. We re-interpret the well established spectral characteristics of ULXs in terms of accretion onto lowly or highly magnetised NSs, and explore the resulting parameter space for consistency. Results: We confirm the previously noted presence of the low-energy (≲6 keV) spectral rollover and argue that it could be interpreted as due to thermal emission. The spectra are well described by a double thermal model consisting of a "hot" (≳1 keV) and a "cool" (≲0.7 keV) multicolour black body (MCB). Under the assumption that the "cool" MCD emission originates in a disk truncated at the neutron star magnetosphere, we find that all ULXs in our sample are consistent with accretion onto a highly magnetised (B ≳ 1012 G) neutron star. We note a strong correlation between the strength of the magnetic field, the temperature of the "hot" thermal component and the total unabsorbed luminosity. Examination of the NuSTAR data supports this interpretation and also confirms the presence of a weak, high-energy (≳15 keV) tail, most likely the result of modification of the MCB emission by inverse Compton scattering. We also note that the apparent high-energy tail, may simply be the result of mismodelling of MCB emission with an atypical temperature (T) versus radius (r) gradient, using a standard MCD model with a fixed gradient of T r-0.75. Conclusions: We have offered a new and robust physical interpretation for the dual-thermal spectra of ULXs. We find that the best-fit derived parameters of our model, are in excellent agreement with recent theoretical predictions that favour super-critically accreting NSs as the engines of a large fraction of ULXs. Nevertheless, the considerable degeneracy between models and the lack of unequivocal evidence cannot rule out other equally plausible interpretations. Deeper broadband observations and time-resolved spectroscopy are warranted to further explore this newly emerging framework.

The Magellanic Clouds (MCs) offer an ideal laboratory for the study of the SNR population in star-forming galaxies, since they are relatively nearby and free of large absorption. Both the LMC and SMC have been targeted by large XMM-Newton surveys, which, combined with archival observations, provide the best dataset to systematically study the X-ray emission of their numerous SNRs (∼ 60 in the LMC, ∼ 20 in the SMC). In this talk, I will highlight the results from this homogeneous analysis, which allows for the first time meaningful comparisons of temperature, chemical composition, and luminosity of SNRs in the MCs. The SNRs can be used as probes of their host galaxies: We measured chemical abundances in the hot phase of the LMC, and constrained the ratio of core-collapse to type Ia SN rates. The X-ray luminosity function of SNRs in the MCs are compared to those in other Local Group galaxies with different metallicities and star formation properties. Finally, we present a new population of evolved type Ia SNRs that was discovered recently in the MCs via their iron-rich X-ray emission.

The Magellanic Clouds harbour a large sample of Be/X-ray binariesat a moderate and well known distance with low Galactic foregroundabsorption. However, the transient nature of Be/X-ray binaries complicatesobservations in X-rays. Serendipitous detections of bright outburstsprovide rare chances for further investigation. We propose three triggeredXMM-Newton observations of new or unexplored systems to continue thebuild up of a large statistical sample of these sources.

We request a 50 ks observation of a pulsating (P~170 s), accretingwhite dwarf binary located in the Large Magellanic Cloud. The systemwas serendipitously detected off-axis during analysis of an archivalXMM-Newton observation (2013) that was severely contaminated by highbackground. Such objects are thought to be candidates for the progenitorsof type Ia supernovae, or may alternatively undergo accretion-inducedcollapse to form the neutron stars in some millisecond pulsars. Thenature of their short period oscillations remains unknown. The on-axisXMM-Newton observation proposed below would deliver a much improvedspectrum, allowing precise characterisation of the system.

The processing of all available XMM-Newton data in the LMC region, and those of the VLP survey in particular, was done with the data reduction pipeline developed in our research group over several years. Various non-X-ray data were used to supplement the XMM-Newton observations. They allow us to assess e.g. the relation between the population of SNRs and large scale structure of the LMC, or to evaluate doubtful candidates in the sample compilation. We compiled a sample of 59 definite SNRs, cleaned of misclassified objects and doubtful candidates. (2 data files).

We present results from the latest outburst of the Be/X-ray binary system SMC X-2, which in late 2015 entered it's first X-ray outburst since 2000. SMC X-2 was first discovered in 1977 by the SAS-3 satellite, and hosts a 2.37s period pulsar. Regular, almost daily, Swift observations of SMC X-2 were performed during the entirety of the latest outburst, from first detection by MAXI to it’s rapid turn off and return back to quiescence. These observations have allowed us to measure with the flux, spectral and temporal properties of SMC X-2. Timing analysis of observation by the Swift X-ray telescope allowed us to track the evolution of the pulsar spin period, and in addition modeling of the orbital parameters of the system by measuring changes in the pulsar spin period due to Doppler effects. In addition we report on an observation of SMC X-2 taken with NuSTAR, which allowed both to better measure the continuum fit above 10 keV, and to perform a sensitive measure of the pulse profile and period of the source.

PSR J1119-6127 is a rotationally-powered (RP) pulsar whose pulsations are detected in radio, X-rays and gamma-rays. It is a high magnetic field neutron star, with an inferred dipole field strength of about 4 & sdot;1013 G. On July 27 it exhibited a strong X-ray burst, detected by Swift/BAT (ATel #9274) and Fermi/GBM (GCN Circular #19736).

XMMU J004855.5-734946 is a candidate BeXRB system in the Small Magellanic Cloud (SMC, Haberl & Sturm 2016, A & A, 586, 81). The system was detected in a recent 45 s Swift/XRT observation on 2016-06-24 as part of a routine Swift/XRT survey of the SMC (ATel #9197).

We report on the results of a ∼40-d multi-wavelength monitoring of the Be X-ray binary system IGR J05007-7047 (LXP 38.55). During that period the system was monitored in the X-rays using the Swift telescope and in the optical with multiple instruments. When the X-ray luminosity exceeded 1036 erg s-1 we triggered an XMM-Newton ToO observation. Timing analysis of the photon events collected during the XMM-Newton observation reveals coherent X-ray pulsations with a period of 38.551(3) s (1σ), making it the 17th known high-mass X-ray binary pulsar in the LMC. During the outburst, the X-ray spectrum is fitted best with a model composed of an absorbed power law (Γ = 0.63) plus a high-temperature blackbody (kT ∼2 keV) component. By analysing ∼12 yr of available OGLE optical data we derived a 30.776(5) d optical period, confirming the previously reported X-ray period of the system as its orbital period. During our X-ray monitoring the system showed limited optical variability while its IR flux varied in phase with the X-ray luminosity, which implies the presence of a disc-like component adding cooler light to the spectral energy distribution of the system.

Aims: We present a comprehensive X-ray study of the population of supernova remnants (SNRs) in the Large Magellanic Cloud (LMC). Using primarily XMM-Newton observations, we conduct a systematic spectral analysis of LMC SNRs to gain new insight into their evolution and the interplay with their host galaxy. Methods: We combined all the archival XMM-Newton observations of the LMC with those of our Very Large Programme LMC survey. We produced X-ray images and spectra of 51 SNRs, out of a list of 59 objects compiled from the literature and augmented with newly found objects. Using a careful modelling of the background, we consistently analysed all the X-ray spectra and measure temperatures, luminosities, and chemical compositions. The locations of SNRs are compared to the distributions of stars, cold gas, and warm gas in the LMC, and we investigated the connection between the SNRs and their local environment, characterised by various star formation histories. We tentatively typed all LMC SNRs, in order to constrain the ratio of core-collapse to type Ia SN rates in the LMC. We also compared the column densities derived from X-ray spectra to H I maps, thus probing the three-dimensional structure of the LMC. Results: This work provides the first homogeneous catalogue of the X-ray spectral properties of SNRs in the LMC. It offers a complete census of LMC remnants whose X-ray emission exhibits Fe K lines (13% of the sample), or reveals the contribution from hot supernova ejecta (39%), which both give clues to the progenitor types. The abundances of O, Ne, Mg, Si, and Fe in the hot phase of the LMC interstellar medium are found to be between 0.2 and 0.5 times the solar values with a lower abundance ratio [α/Fe] than in the Milky Way. The current ratio of core-collapse to type Ia SN rates in the LMC is constrained to NCC/NIa=1.35(-0.24+0.11), which is lower than in local SN surveys and galaxy clusters. Our comparison of the X-ray luminosity functions of SNRs in Local Group galaxies (LMC, SMC, M31, and M33) reveals an intriguing excess of bright objects in the LMC. Finally, we confirm that 30 Doradus and the LMC Bar are offset from the main disc of the LMC to the far and near sides, respectively. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

Spin periods of Be/X-ray binary (BeXRB) pulsars are important for probing their formation channels and the connection between orbital period and pulsar spin in XRBs (Knigge+ 2011). From the 120 BeXRBs in the SMC only 50% have known spin periods, with the majority of them measured during outburst with ToOs. From the remaining systems only 6 have a known orbital period. XMMU J004855.5-734946 is a BeXRB in the SMC (Haberl & Sturm 2016, XMM Lx (0.03-1)x10^35 erg/s) with a 36.43d orbital period (Atel#9198), but with no X-ray pulsations detected so far. On 06/24 a 37s Swift/XRT observation detected the system at Lx 10^37 erg/s (Atel#9197) following a marginal detection on 06/16 (Lx<5x10^36 erg/s) confirming the onset of a large outburst. A follow-up 1ks Swift ToO on 06/29 measures an Lx = 8.3x10^36 erg/s. This is a unique opportunity to measure the spin period of one of the few BeXRBs in the SMC with known orbital period but unknown spin period.

We report first results from our XMM-Newton observation of IGR J01217-7257 performed on November 7, 2015. This transient in the Small Magellanic Cloud (Coe et al. 2014, ATel #5806) was found to be in a new outburst during Integral observations end of October 2015 (Coe et al. 2015, ATel #8246).

V404 Cyg is a known black hole Low mass X-ray binary with a late G-type companion, having a ~6.5 d orbital period. On June 15 18:32 UT Swift Burst Alert Telescope (BAT) was triggered due to the high X-ray activity of the system (Barthelmy et al.

We present new optical spectroscopy of 20 candidate counterparts of 17 X-ray sources in the direction of the M31 disc. By comparing the X-ray catalogue from the XMM-Newton survey of M31 with star catalogues from the Local Group Galaxy Survey, we chose counterpart candidates based on optical colour and X-ray hardness. We have discovered 17 counterpart candidates with spectra containing stellar features. Eight of these are early-type stars of O or B type in M31, with hard X-ray spectra, making them good high-mass X-ray binary (HMXB) candidates. Three of these eight exhibit emission lines, which we consider to be the strongest HMXB candidates. In addition, our spectra reveal two likely Galactic cataclysmic variables, one foreground M star, two probable low-mass X-ray binaries related to M31 globular clusters, one emission-line region with an embedded Wolf-Rayet star and one newly discovered supernova remnant. Finally, two of the sources have stellar spectra with no features indicative of association with an X-ray source.

We report an outburst of Swift J0513.4-6547, a 27.28 s pulsar Be/X-ray binary in the Large Magellanic Cloud, which was discovered by Krimm et al. (2009, ATel #2011) and has an orbital period of 27 d (ATel #5511).

We report on the evolution of the current X-ray outburst of the LMC Be/X-ray binary pulsar RX J0520.5-6932 (see ATel #5673) from our Swift/XRT monitoring. Since the start of the outburst (2013 Dec 28) the luminosity of the source has continued to rise to a maximum of 1.91×1038erg s-1 (0.3-10 keV band), which is close to or at the Eddington limit for a neutron star.

A series of observations performed with Swift/XRT (Target ID 33042) in the SMC detected a variable X-ray source within the field of view. The source appeared in observations performed on and after 2014 Jan 06, while it remained undetected in the period 2013 Dec 13 - 24. The XRT J2000 position is RA = 01:07:45.00, DEC = -72:27:40.9, (90% error radius of 4.0"). This position matches [MCS2008] 206, detected with Chandra on 2006 Feb 10.

We identify a new candidate for a Be/X-ray binary in the XMM-Newton slew survey and archival Swift observations that is located in the transition region of the Wing of the Small Magellanic Cloud and the Magellanic Bridge. We investigated and classified this source with follow-up XMM-Newton and optical observations. We model the X-ray spectra and search for periodicities and variability in the X-ray observations and the Optical Gravitational Lensing Experiment I-band light curve. The optical counterpart has been classified spectroscopically, with data obtained at the South African Astronomical Observatory 1.9 m telescope, and photometrically, with data obtained using the Gamma-ray Burst Optical Near-ir Detector at the MPG 2.2 m telescope. The X-ray spectrum is typical of a high-mass X-ray binary with an accreting neutron star. We detect X-ray pulsations, which reveal a neutron-star spin period of Ps = (264.516 ± 0.014) s. The source likely shows a persistent X-ray luminosity of a few 1035 erg s-1 and in addition type-I outbursts that indicate an orbital period of ∼146 d. A periodicity of 0.867 d, found in the optical light curve, can be explained by non-radial pulsations of the Be star. We identify the optical counterpart and classify it as a B1-2II-IVe star. This confirms SXP 265 as a new Be/X-ray binary pulsar originating in the tidal structure between the Magellanic Clouds.

Aims: We observed RX J0520.5-6932 in the X-rays and studied the optical light curve of its counterpart to verify it as a Be/X-ray binary. Methods: We performed an XMM-Newton anticipated target-of-opportunity observation in January 2013 during an X-ray outburst of the source in order to search for pulsations and derive its spectral properties. We monitored the source with Swift to follow the evolution of the outburst and to look for further outbursts to verify the regular pattern seen in the optical light curve with a period of ~24.4 d. Results: The XMM-Newton EPIC light curves show coherent X-ray pulsations with a period of 8.035331(15) s (1σ). The X-ray spectrum can be modelled by an absorbed power law with photon index of ~0.8, an additional black-body component with temperature of ~0.25 keV, and an Fe K line. Phase-resolved X-ray spectroscopy reveals that the spectrum varies with pulse phase. We confirm the identification of the optical counterpart within the error circle of XMM-Newton at an angular distance of ~0.8'', which is an O9Ve star with known Hα emission. By analysing the combined data from three OGLE phases we derived an optical period of 24.43 d. Conclusions: The X-ray pulsations and long-term variability, as well as the properties of the optical counterpart, confirm that RX J0520.5-6932 is a Be/X-ray binary pulsar in the Large Magellanic Cloud. Based on the X-ray monitoring of the source, we conclude that the event in January 2013 was a moderately bright type-I X-ray outburst, with a peak luminosity of 1.79 × 1036 erg s-1. Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA); with Swift, a NASA mission with international participation.

We observed a newly discovered X-ray source int X-rays and in the optical to confirm its nature as a high mass X-ray binary. We analysed XMM-Newton and Swift X-ray data, along with optical observations with the ESO Faint Object Spectrograph, to investigate the spectral and temporal characteristics of the source. The XMM-Newton data show coherent X-ray pulsations while the spectra can be modelled with a combination of a power law plus a black body component. We performed optical spectroscopy from which we classify the companion star as a B0-1.5Ve star. The X-ray pulsations, the long-term x-ray variability and the properties of the optical counterpart confirms the the x-ray source as a new Be/X-ray binary pulsar in the LMC.

Nearby galaxies are well suited for investigating X-ray source populations in different environments than in our own Galaxy. Moreover, sources in these galaxies have well determined distances and are less absorbed than sources in the galactic plane. The Large (LMC) and the Small (SMC) Magellanic Clouds (MC) are the nearest gas-rich star-forming galaxies and their gravitational interactions are believed to have tidally triggered recent bursts of star formation. The XMM-Newton large program for the SMC, together with archival observations covers an area of 5.5 square degrees and has already produced significant results. The XMM-Newton large program for the LMC has just been completed and has covered an even bigger area of about 10 square degrees. Both surveys reach a limiting luminosity of 1032 erg/s and provide a unique data set for X-ray source population studies. The two surveys have allowed us to derive hardness ratios for the point sources and conduct spectral classification. For the brightest sources, we performed spectral and timing analysis. By complementing these results with surveys at other wavelengths we have managed to extend our understanding of the nature of individual sources as well as providing complete data-sets of X-ray source populations (X-ray Binaries, supersoft sources, supernova remnants, background active galactic nuclei and foreground galactic sources). From the classification of the sources we have constructed luminosity functions which will allow as to compare X-ray populations in the different environments that the MCs provide. Here, we present an overview of these two surveys together with the highlights of the most interesting sources that they have produced so far (e.g. Be/X-ray binaries).

In a monitoring observation of the central region of the Andromeda Galaxy (M 31) with the Ultra-violet/Optical Telescope (UVOT) on board the Swift satellite (ObsID 00035336102, starting 2013-05-27.15 UT), we detected a new UV transient (UVW1 filter, 181-321 nm). The source is located at RA 00h 42m 55.62s, Dec +41d 14' 12.3" (± 0.5", J2000, 90% confidence level). The following table lists the Swift ObsID, the MJD at the beginning of the exposure, UVW1 magnitudes (Vega system), and 1σ statistical uncertainties.

We report a new high-mass X-ray binary (HMXB), found in a Swift observation performed on 2013 October 11 as part of the monitoring of the nova LMC2012 (ObsId: 00049549004). The Swift/XRT count rate was 0.023±0.008 cts s-1, corresponding to a flux of 1.6×10-12 erg cm-2 s-1 (0.3-10 keV) and an unabsorbed luminosity of 5.1×1035 erg s-1 for LMC distance (50 kpc).

A Swift observation performed on 2013 January 13 as part of the LMC UV survey (PI: S. Immler) detected the high-mass X-ray binary candidate RX J0520.5-6932 in a moderately bright X-ray outburst. Assuming a power-law spectrum with a photon index of 0.9 and absorption of 1021 cm-2, we derive a 0.2-12 keV flux of 1.67 × 10-12 erg s-1 cm-2. This flux is ~25 times higher than measured during a recent XMM-Newton observation (see below).

A series of observations performed with Swift/XRT (Target ID 33042) in the SMC detected a relatively bright X-ray source at the edge of the field of view. The source is consistent with XMMU J010429.4-723136. It is classified as a high-mass X-ray binary (HMXB) candidate in the XMM-Newton point-source catalogue of the SMC (Sturm et al.

RX J0520.5-6932 is a recently confirmed Be/X-ray binary system in the LMC (see ATel #4748).

Aims: We observed the newly discovered X-ray source Swift J053041.9-665426in the X-ray and optical regime to confirm its proposed nature as a high mass X-ray binary. Methods: We obtained XMM-Newton and Swift X-ray data, along with optical observations with the ESO Faint Object Spectrograph, to investigate the spectral and temporal characteristics of Swift J053041.9-665426. Results: The XMM-Newton data show coherent X-ray pulsations with a period of 28.77521(10) s (1σ). The X-ray spectrum can be modelled by an absorbed power law with photon index within the range 0.76 to 0.87. The addition of a black body component increases the quality of the fit but also leads to strong dependences of the photon index, black-body temperature and absorption column density. We identified the only optical counterpart within the error circle of XMM-Newton at an angular distance of ~0.8'', which is 2MASS J05304215-6654303. We performed optical spectroscopy from which we classify the companion as a B0-1.5Ve star. Conclusions: The X-ray pulsations and long-term variability, as well as the properties of the optical counterpart, confirm that Swift J053041.9-665426 is a new Be/X-ray binary pulsar in the Large Magellanic Cloud. Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA).