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 (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.

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.

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.

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.