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.

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.

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.

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.

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.

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.

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.

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.

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.