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.

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.

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.

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.

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.

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. 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 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.

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.