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MAXI J1659-152 is a 2.4 h orbital period X-ray dipping transient black hole candidate. We present spectroscopy of its $I\approx23$ quiescent counterpart where we detect H$\alpha$ emission with full-width at half-maximum (FWHM) of $3200 \pm 300$ km s$^{-1}$. Applying the correlation between the H$\alpha$ FWHM and radial velocity semi-amplitude of the donor star for quiescent X-ray transients, we derive $K_2 = 750 \pm 80$ km s$^{-1}$. The orbital period and $K_2$ lead to a mass function of $4.4\pm1.4~M{_\odot}$ (1$\sigma$). The donor to compact object mass ratio and binary inclination are likely in the range $q={M_2}/{M_1} = 0.02-0.07$ and $i=70{^\circ}-80{^\circ}$. These constraints imply a 68% confidence level interval for the compact object mass of $3.3 \lesssim M_1(M_\odot) \lesssim 7.5$, confirming its black hole nature. These quasi-dynamical limits are compared to mass estimates from modelling of X-ray data and any discrepancies are discussed. We review the properties of optical spectroscopy and time-series photometry collected during the 2010-2011 outburst. We interpret the apparent modulations found soon after the onset of high-accretion activity and during the 2011 rebrightening event as originating in the accretion disc. These have signatures consistent with superhumps, with the 2011 modulation having a fractional period excess $< 0.6\%$ (3$\sigma$). We propose that direct irradiation of the donor by the central X-ray source was not possible due to its occultation by the disc outer regions. We argue that disc shielding significantly weakens the donor star contribution to the optical variability in systems with $q\lesssim0.07$, including neutron star ultra-compact X-ray binaries., Comment: 8 pages, 1 figure, accepted for publication in MNRAS

В работе представлен краткий обзор исследований EXO 040830-7134.7 с момента открытия рентгеновского транзиента. Анализируются данные его наблюдений в рентгеновском и оптическом диапазонах спектра. В результате обработки кривых блеска и путем визуального контроля обнаружено более 30 значительных вспышек. Зафиксирована максимальная энергия вспышки 1034 эрг. Публикация проиллюстрирована изображениями с наиболее характерными вспышками., This paper provides a brief overview of the research on EXO 040830-7134.7 since the discoveryof the X-ray transient. The data of its observations in the X-ray and optical ranges of the spectrum areanalyzed. As a result of processing the light curves and by means of visual control, more than 30 significantflares have been detected. The maximum flare energy is recorded at 1034 erg. Images with the most typicalflares are presented in this publication., Известия Крымской астрофизической обсерватории, Выпуск 118 (2) 2022, Pages 28-33

SSM onboard AstroSat is designed to monitor X-ray sky in the energy range 2.5–10 keV to detect and locate X-ray sources in outburst. SSM with its three almost identical 1D-proportional counters mounted on a rotating platform, scans the sky in step and stare mode of operation. It observes the X-ray sky and generates light curves for X-ray sources detected. Here, we discuss the positional calibration to carry out imaging with SSM. Onboard calibration of SSM has been carried out with Crab, the standard X-ray source. SSM observations of Crab are compared with that of MAXI on ISS for cross calibration of the instrument.

We report on the X-ray properties of the new transient Swift J0840.7$-$3516, discovered with Swift/BAT in 2020 February, using extensive data of Swift, MAXI, NICER, and NuSTAR. The source flux increased for $\sim 10^3$ s after the discovery, decayed rapidly over $\sim$ 5 orders of magnitude in 5 days, and then remained almost constant over 9 months. Large-amplitude short-term variations on time scales of 1--$10^4$ s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law shaped spectrum with a photon index of $\sim 1.0$ extending up to $\sim 30$ keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became $\sim 2$ at the end of the decay. A spectral absorption feature at 3--4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star., 15 pages, 10 figures, 3 tables, Accepted for publication in ApJ

We report on the results of an XMM-Newton observation of the Supergiant Fast X-ray Transient (SFXT) IGR J08408-4503 performed in June 2020. The source is composed by a compact object (likely a neutron star) orbiting around an O8.5Ib-II(f)p star, LM Vel. The X-ray light curve shows a very low level of emission, punctuated by a single, faint flare. Analysis of spectra measured during the flare and during quiescence is performed. The quiescent state shows a continuum spectrum well deconvolved to three spectral models: two components are from a collisionally-ionized plasma (with temperatures kT1=0.24 keV and kT2=0.76 keV), together with a power law model (photon index of 2.55), dominating above 2 keV. The X-ray flux emitted at this lowest level is 3.2$\times10^{-13}$ erg/cm2/s (0.5-10 keV, corrected for the interstellar absorption), implying an X-ray luminosity of 1.85$\times10^{32}$ erg/s (at 2.2 kpc). The two temperature collisionally-ionized plasma is intrinsic to the stellar wind of the donor star, while the power law can be interpreted as emission due to residual, low level accretion onto the compact object. The X-ray luminosity contributed by the power law component only, in the lowest state, is (4.8$\pm{1.4})\times10^{31}$ erg/s, the lowest quiescent luminosity detected from the compact object in an SFXT. Thanks to this very faint X-ray state caught by XMM-Newton, X-ray emission from the wind of the donor star LM Vel could be well-established and studied in detail for the first time, as well as a very low level of accretion onto the compact object. The residual accretion rate onto the compact object in IGR J08408-4503 can be interpreted as the Bohm diffusion of (possibly magnetized) plasma entering the neutron star magnetosphere at low Bondi capture rates from the supergiant donor wind at the quasi-spherical radiation-driven settling accretion stage., Comment: 13 pages, 6 figures, 5 tables; accepted for publication in Astronomy & Astrophysics

We present results for the first observed outburst from the transient X-ray binary source MAXI J0637-430. This study is based on eight observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and six observations from the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT) collected from 2019 November 19 to 2020 April 26 as the 3-79 keV source flux declined from 8.2e-10 to 1.4e-12 erg/cm^2/s. We see the source transition from a soft state with a strong disk-blackbody component to a hard state dominated by a power-law or thermal Comptonization component. NuSTAR provides the first reported coverage of MAXI J0637-430 above 10 keV, and these broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test whether blackbody emission from the plunging region could explain the excess emission. As an alternative, we test a reflection model that includes a physical Comptonization continuum. Finally, we also test a spectral component based on reflection of a blackbody illumination spectrum, which can be interpreted as a simple approximation to the reflection produced by returning disk radiation due to the bending of light by the strong gravity of the black hole. We discuss the physical implications of each scenario and demonstrate the value of constraining the source distance., Comment: 17 pages, 10 figures, accepted to The Astrophysical Journal (ApJ)

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. We studied the X-ray properties of the supergiant HMXB XMMU J053108.3-690923 in the Large Magellanic Cloud to understand its nature. We performed a detailed temporal and spectral analysis of the eROSITA and XMM-Newton data of XMMU J053108.3-690923. 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., 11 pages, 10 figures, Accepted for publication in A&A on 05/11/2020

We present the discovery of a luminous X-ray transient, serendipitously detected by Swift's X-ray Telescope (XRT) on 2020 February 5, located in the nucleus of the galaxy SDSS J143359.16+400636.0 at z=0.099 (luminosity distance $D_{\rm L}=456$ Mpc). The transient was observed to reach a peak luminosity of $\sim10^{44}$ erg s$^{-1}$ in the 0.3--10 keV X-ray band, which was $\sim20$ times more than the peak optical/UV luminosity. Optical, UV, and X-ray lightcurves from the Zwicky Transient Facility (ZTF) and Swift show a decline in flux from the source consistent with $t^{-5/3}$, and observations with NuSTAR and Chandra show a soft X-ray spectrum with photon index $\Gamma=2.9\pm0.1$. The X-ray/UV properties are inconsistent with well known AGN properties and have more in common with known X-ray tidal disruption events (TDE), leading us to conclude that it was likely a TDE. The broadband spectral energy distribution (SED) can be described well by a disk blackbody model with an inner disk temperature of $7.3^{+0.3}_{-0.8}\times10^{5}$ K, with a large fraction ($>40$%) of the disk emission up-scattered into the X-ray band. An optical spectrum taken with Keck/LRIS after the X-ray detection reveals LINER line ratios in the host galaxy, suggesting low-level accretion on to the supermassive black hole prior to the event, but no broad lines or other indications of a TDE were seen. The stellar velocity dispersion implies the mass of the supermassive black hole powering the event is log($M_{\rm BH}$/$M_{\odot}$)$=7.41\pm0.41$, and we estimate that at peak the Eddington fraction of this event was $\sim$50%. This likely TDE was not identified by wide-field optical surveys, nor optical spectroscopy, indicating that more events like this would be missed without wide-field UV or X-ray surveys., Comment: Accepted for publication by ApJ. Accepted version now replaces initial submission

We studied the outburst evolution and timing properties of the recently discovered X-ray transient MAXI J1348-630 as observed with NICER. We produced the fundamental diagrams commonly used to trace the spectral evolution, and power density spectra to study the fast X-ray variability. The main outburst evolution of MAXI J1348-630 is similar to that commonly observed in black hole transients. The source evolved from the hard state, through hard- and soft-intermediate states, into the soft state in the outburst rise, and back to the hard state in reverse during the outburst decay. At the end of the outburst, MAXI J1348-630 underwent two reflares with peak fluxes ~1 and ~2 orders of magnitude fainter than the main outburst, respectively. During the reflares, the source remained in the hard state only, without undergoing any state transitions, which is similar to the so-called "failed outbursts". Different types of quasi-periodic oscillations (QPOs) are observed at different phases of the outburst. Based on our spectral-timing results, we conclude that MAXI J1348-630 is a black hole candidate., 12 pages, 8 figures, 1 table, accepted for publication in MNRAS

In 2017, the Be/X-ray transient 4U 0115+63 exhibited a new type-II outburst that was two times fainter than its 2015 giant outburst (in the Swift/BAT count rates). Despite this difference between the two bright events, the source displayed similar X-ray behaviour after these periods. Once the outbursts ceased, the source did not transit towards quiescence directly, but was detected about a factor of 10 above its known quiescent level. It eventually decayed back to quiescence over time scales of months. In this paper we present the results of our Swift monitoring campaign, and an XMM-Newton observation of 4U 0115+63 during the decay of the 2017 type-II outburst, and its subsequent low-luminosity behaviour. We discuss the possible origin of the decaying source emission at this low-level luminosity, which has now been shown as a recurrent phenomenon, in the framework of the two proposed scenarios to explain this faint state: cooling from an accretion-heated neutron-star crust or continuous low-level accretion. In addition, we compare the outcome of our study with the results we obtained from the 2015/2016 monitoring campaign on this source., Comment: 12 pages, 5 figures, 3 tables. Accepted, Astronomy & Astrophysics

We study the X-ray transient Swift J1745-26, using observations obtained from its outburst in September 2012, up to its decay towards quiescence in March 2013. We obtained optical and infrared observations, through override programme at ESO/VLT with FORS2 and ISAAC instruments, and archival optical (VLT/VIRCAM), radio and X-ray (Swift) observations, to build the lightcurve and the broad-band Spectral Energy Distribution (SED) of Swift J1745-26. We show that, during its outburst and also during its decay towards quiescence, Swift J1745-26 SED can be adjusted, from infrared up to X-rays, by the sum of both a viscous irradiated multi-colour blackbody emitted by an accretion disk, and a synchrotron powerlaw at high energy. In the radio domain, the SED arises from synchrotron emission from the jet. While our SED fitting confirms that the source remained in the low/hard state during its outburst, we determine an X-ray spectral break at frequency 3.1 < nu_break < 3.4 x 10^14 Hz, and a radio spectral break at 10^12 Hz < nu_break < 10^13 Hz. We also show that the system is compatible with an absorption Av of ~7.69 magnitudes, lies within a distance interval of D~[2.6-4.8] kpc, with an upper limit of orbital period P_orb = 11.3 hours, and that the companion star is a late-type star of spectral type in the range K0 - M0 V, confirming that the system is a low-mass X-ray binary. We finally plot the position of Swift J1745-26 on an optical-infrared - X-ray luminosity diagram: its localization on this diagram is consistent with the source staying in the low-hard state during outburst and decay phases., A&A in press, 8 pages, 6 figures, replaced with final version in press

We report on the results of a NuSTAR observation of the Supergiant Fast X-ray Transient pulsar IGRJ11215-5952 during the peak of its outburst in June 2017. IGRJ11215-5952 is the only SFXT undergoing strictly periodic outbursts, every 165 days. NuSTAR caught several X-ray flares, spanning a dynamic range of 100, and detected X-ray pulsations at 187.0 s, consistent with previous measurements. The spectrum from the whole observation is well described by an absorbed power-law (with a photon index of 1.4) modified, above 7 keV, by a cutoff with an e-folding energy of 24 keV. A weak emission line is present at 6.4 keV, consistent with Kalpha emission from cold iron in the supergiant wind. The time-averaged flux is 1.5E-10 erg/cm2/s (3-78 keV, corrected for the absorption), translating into an average luminosity of about 9E35 erg/s (1-100 keV, assuming a distance of 6.5 kpc). The NuSTAR observation allowed us to perform the most sensitive search for cyclotron resonant scattering features in the hard X-ray spectrum, resulting in no significant detection in any of the different spectral extractions adopted (time-averaged, temporally-selected, spin-phase-resolved and intensity-selected spectra). The pulse profile showed an evolution with both the energy (3-12 keV energy range compared with 12-78 keV band) and the X-ray flux: a double peaked profile was evident at higher fluxes (and in both energy bands), while a single peaked, sinusoidal profile was present at the lowest intensity state achieved within the NuSTAR observations (in both energy bands). The intensity-selected analysis allowed us to observe an anti-correlation of the pulsed fraction with the X-ray luminosity. The pulse profile evolution can be explained by X-ray photon scattering in the accreting matter above magnetic poles of a neutron star at the quasi-spherical settling accretion stage., Accepted for publication in Astronomy & Astrophysics (Received 3 April 2020 / Accepted 17 April 2020). 14 pages, 5 Tables, 9 Figures

We present the discovery of an optical accretion disk wind in the X-ray transient Swift J1858.6-0814. Our 90-spectrum data set, taken with the 10.4 m Gran Telescopio Canarias telescope over eight different epochs and across five months, reveals the presence of conspicuous P-Cyg profiles in He i at 5876 A and Hα. These features are detected throughout the entire campaign, albeit their intensity and main observational properties are observed to vary on timescales as short as 5 minutes. In particular, we observe significant variations in the wind velocity, between a few hundreds and ∼2400 km s; -1. In agreement with previous reports, our observations are characterized by the presence of frequent flares, although the relation between the continuum flux variability and the presence/absence of wind features is not evident. The reported high activity of the system at radio waves indicates that the optical wind of Swift J1858.6-0814 is contemporaneous with the radio jet, as is the case for the handful of X-ray binary transients that have shown so far optical P-Cyg profiles. Finally, we compare our results with those of other sources showing optical accretion disk winds, with emphasis on V404 Cyg and V4641 Sgr, since they also display strong and variable optical wind features as well as similar flaring behavior., Facultad de Ciencias Astronómicas y Geofísicas, Instituto Argentino de Radioastronomía

Modern soft X-ray observatories can yield unique insights into time domain astrophysics, and a huge amount of information is stored - and largely unexploited - in data archives. Like a treasure-hunt, the EXTraS project harvested the hitherto unexplored temporal domain information buried in the serendipitous data collected by the European Photon Imaging Camera instrument onboard the XMM- Newton satellite in 20 years of observations. The result is a vast catalogue, describing the temporal behaviour of hundreds of thousands of X-ray sources. But the catalogue is just a starting point because it has to be, in its turn, further analysed. During the project an education activity has been defined and run in several workshops for high school students in Italy, Germany and UK. The final goal is to engage the students, and in perspective citizen scientists, to go through the whole validation process: they look into the data and try to discover new sources, or to characterize already known sources. This paper describes how the EXTraS science gateway is used to accomplish these tasks and highlights the first discovery, a flaring X-ray source in the globular cluster NGC 6540., Comment: Accepted for publication in Future Generation Computing Systems, Special issue on Science gateways 2018

We report results from the analysis of XMM-Newton and INTEGRAL data of IGR J16479-4514. The unpublished XMM-Newton observation, performed in 2012, occurred during the source eclipse. No point-like X-ray emission was detected from the source, conversely extended X-ray emission was clearly detected up to a size distance compatible with a dust scattering halo produced by the source X-ray emission before being eclipsed by its companion donor star. The diffuse emission of the dust-scattering halo could be observed without any contamination from the central point X-ray source, compared to a previous XMM-Newton observation published in 2008. Our comprehensive analysis of the 2012 unpublished spectrum of the diffuse emission as well as of the 2008 re-analysed spectra extracted from three adjacent time intervals and different extraction regions (optimized for point-like and extended emission) allowed us to clearly disentangle the scattering halo spectrum from the residual point-like emission during the 2008 eclipse. Moreover, the point-like emission detected in 2008 could be separated into two components attributed to the direct emission from the source and to scattering in the stellar wind, respectively. From archival unpublished INTEGRAL data, we identified a very strong (3$\times$10$^{-8}$ erg cm$^{-2}$ s$^{-1}$) and fast (25 minutes duration) flare which was classified as giant hard X-ray flare since the measured peak-luminosity is 7$\times$10$^{37}$ erg s$^{-1}$. Giant X-ray flares from SFXTs are very rare, to date only one has been reported from a different source. We propose a physical scenario to explain the origin in the case of IGR J16479-4514., Accepted for publication on The Astrophysical Journal (received 20-Apr-2020, accepted 27-Jul-2020)

We focus on two repeating fast radio bursts (FRBs) recently detected by the CHIME/FRB experiment in 2018--2019 (Source 1: 180916.J0158+65, and Source 2: 181030.J1054+73). These sources have low excess dispersion measures (DMs) ($ < 100 \rm \, pc \, cm^{-3}$ and $ < 20 \rm \, pc \, cm^{-3}$, respectively), implying relatively small maximal distances. They were repeatedly observed by AGILE in the MeV--GeV energy range. We do not detect prompt emission simultaneously with these repeating events. This search is particularly significant for the submillisecond and millisecond integrations obtainable by AGILE. The sources are constrained to emit a MeV-fluence in the millisecond range below $F'_{MeV} = 10^{-8} \, \rm erg \, cm^{-2}$ corresponding to an isotropic energy near $E_{MeV,UL} \simeq 2 \times 10^{46}\,$erg for a distance of 150 Mpc (applicable to Source 1). We also searched for $\gamma$-ray emission for time intervals up to 100 days, obtaining 3$\,\sigma$ upper limits (ULs) for the average isotropic luminosity above 50 MeV, $L_{\gamma,UL} \simeq \,$(5-10)$\,\times 10^{43} \rm \, erg \, s^{-1}$. For a source distance near 100 kpc (possibly applicable to Source 2), our ULs imply $E_{MeV,UL}\simeq10^{40} \rm erg$, and $L_{\gamma,UL} \simeq \,$2$\,\times 10^{37} \rm \, erg \, s^{-1}$. Our results are significant in constraining the high-energy emission of underlying sources such as magnetars, or other phenomena related to extragalactic compact objects, and show the prompt emission to be lower than the peak of the 2004 magnetar outburst of SGR 1806-20 for source distances less than about 100 Mpc., Comment: to be published in ApJL

KV UMa (XTE J1118+480) is an X-ray binary that is known to undergo outbursts in 2000 and 2005. This paper presents the discovery of a large outburst starting in 1927 on the archival photographic plates and an analysis of the long-term optical activity of this system. We used the photographic data from DASCH (Digital Access to a Sky Century @ Harvard). We placed the 1927 outburst in the context of the observed outbursts of KV UMa. We show that it is a double event, with a precursor similar to the one of the outbursts in 2000. We find a big difference between the 1927 and 2000 outbursts as regards the length of the gap between the precursor and the main outburst. It is more than 250 d in 1927, whereas it is about 20 d in 2000, although the brightnesses of all peaks are mutually comparable. We also show that the individual optical outbursts of KV UMa differ from each other by the duration of the stage of a slow decline of brightness (sometimes roughly a plateau). This determines the length of the entire main outburst. Both the peak magnitude and the brightness of the outburst when the slow decline transitions to a steep final decaying branch plausibly reproduce in all three outbursts. In the interpretation, the short duration of the precursor is caused by the fact that only the thermal-viscous instability operated in the accretion disk while also the tidal instability of the disk contributed in the subsequent main outburst.

Scanning Sky Monitor (SSM) onboard AstroSat is an Xray sky monitor in the soft X-ray band designed with a large field of view to detect and locate transient X-ray sources and alert the astronomical community about interesting phenomena in the X-ray sky. SSM comprises position sensitive proportional counters with 1D coded mask for imaging. There are three detector units mounted on a platform capable of rotation which helps covering about 50% of the sky in one full rotation. This paper discusses the elaborate details of the instrument and few immediate results from the instrument after launch.

We present the X-ray timing and spectral analysis of the new Galactic X-ray transient Swift J1658.2-4242 observed with LAXPC and SXT instruments onboard {\it Astrosat}. We detect prominent C-type quasi-periodic oscillations (QPOs) of frequencies varying from $\sim 1.5$ Hz to $\sim 6.6$ Hz along with distinct 2nd harmonics and sub-harmonics. The QPO detected at $\sim 1.56$ Hz drifts to a higher centroid frequency of $\sim 1.74$ in the course of the observation, while the QPO detected at $\sim 6.6$\,Hz disappeared during hard flarings. The fractional rms at the QPO and the sub-harmonic frequencies increases with photon energy, while at the 2nd harmonic frequencies the rms seems to be constant. In addition, we have observed soft time lag at QPO and sub-harmonic frequencies up to a time scale of $\sim 35$ ms, however, at the 2nd harmonic frequencies there is weak/zero time lag. We attempt spectral modeling of the broadband data in the 0.7--25 keV band using the doubly absorbed disk plus thermal Comptonization model. Based on the spectral and timing properties, we identified the source to be in the hard intermediate state of black hole X-ray binaries. To quantitatively fit the energy and frequency-dependent fractional rms and time lag, we use a single zone fluctuation propagation model and discuss our results in the context of that model., Accepted for Publication in ApJ, 13 Pages, 14 Figures, 3 Tables

Supergiant fast X-ray transients (SFXTs) are X-ray binary systems with a supergiant companion and likely a neutron star, which show a fast ($\sim 10^3$ s) and high variability with a dynamic range up to $10^{5-6}$. Given their extreme properties, they are considered among the most valuable laboratories to test accretion models. Recently, the orbital parameters of a member of this class, IGR J08408-4503, were obtained from optical observations. We used this information, together with X-ray observations from previous publications and new results from X-ray and optical data collected by INTEGRAL and presented in this work, to study the accretion mechanisms at work in IGR J08408-4503. We found that the high eccentricity of the compact object orbit and the large size of the donor star imply Roche lobe overflow (RLO) around the periastron. It is also likely that a fraction of the outer layers of the photosphere of the donor star are lost from the Lagrangian point $L_2$ during the periastron passages. On the basis of these findings, we discuss the flaring variability of IGR J08408-4503 assuming the presence of an accretion disc. We point out that IGR J08408-4503 may not be the only SFXT with an accretion disc fueled by RLO. These findings open a new scenario for accretion mechanisms in SFXTs, since most of them have so far been based on the assumption of spherically symmetric accretion., Accepted for publication in Astronomy & Astrophysics

The Be/X-ray transient GRO J1750-27 exhibited a type-II (giant) outburst in 2015. After the source transited to quiescence, we triggered our multi-year Chandra monitoring programme to study its quiescent behaviour. The programme was designed to follow the cooling of a potentially heated neutron-star crust due to accretion of matter during the preceding outburst, similar to what we potentially have observed before in two other Be/X-ray transients, namely 4U 0115+63 and V 0332+53. However, unlike for these other two systems, we do not find any strong evidence that the neutron-star crust in GRO J1750-27 was indeed heated during the accretion phase. We detected the source at a rather low X-ray luminosity (~10^33 erg/s) during only three of our five observations. When the source was not detected it had very low-luminosity upper limits (, 13 pages, 6 figures, 5 tables. Accepted for A&A

Time-resolved SALT spectra of the short-period, dipping X-ray transient, Swift J1357.2-0933, during its 2017 outburst has revealed broad Balmer and HeII4686 absorption features, blue-shifted by ~600 km/s. Remarkably these features are also variable on the ~500s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (>~30,000K) outflowing wind seen at very high inclination, and draw comparisons with other accretion disc corona sources. We argue against previous distance estimates of 1.5 kpc and favour a value >~6 kpc, implying an X-ray luminosity L_X>~4x10^{36} erg/s$. Hence it is not a very faint X-ray transient. Our preliminary 1D Monte-Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic L_X, a column density N_H>~10^{24} cm^{-2} and a low covering factor for the wind. Our study shows that Swift J1357.2-0933 is truly remarkable amongst the cohort of luminous, galactic X-ray binaries, showing the first example of He II absorption, the first (and only) variable dip period and is possibly the first black hole 'accretion disc corona' candidate., MNRAS Letters in press. Updated to match published version

We report on a \textit{NuSTAR} and \textit{Swift}/XRT observation of the newly discovered X-ray transient MAXI J1820+070. \textit{Swift}/XRT and \textit{NuSTAR} have concurrently observed the newly detected source on 14 March 2018. We have simultaneously fitted the broad-band spectra obtained from \textit{Swift}/XRT and \textit{NuSTAR}. The observed joint spectra in the energy range 0.6--78.0 keV are well modeled with a weak disk black-body emission, dominant thermal Comptonization and relativistic reflection fraction. We have detected a fluorescent Iron-K$\alpha$ line relativistically broadened, and a Compton hump at $\sim$ 30 keV. We constrain the inner disk radius as well as the disk inclination angle and their values are found to be 4.1$^{+0.8}_{-0.6}$ R$_{ISCO}$ (where R$_{ISCO}\equiv$ radius of the innermost stable circular orbit) or 5.1$^{+1.0}_{-0.7}$ r$_{g}$ (where r$_{g}\equiv$ gravitational radius) and 29.8$^{+3.0}_{-2.7}$ $^\circ$ respectively. The best fit broad-band spectra suggest that the source was in the hard state and evolving. The source emission is best described by weak thermal emission along with strong thermal Comptonization from a relatively cold, optically thick, geometrically thin and ionized accretion disk. X-ray spectral modeling helps us to understand the accretion and ejection properties in the vicinity of the compact object., Comment: 07 pages, 05 figures and 02 tables, Accepted for publication by Monthly Notices of the Royal Astronomical Society

The supersoft X-ray and optical transient ASASSN-16oh has been interpreted by Maccarone et al. (2019) as having being induced by an accretion event on a massive white dwarf, resembling a dwarf nova super-outburst. These authors argued that the supersoft X-ray spectrum had a different origin than in an atmosphere heated by shell nuclear burning, because no mass was ejected. We find instead that the event's timescale and other characteristics are typical of non-mass ejecting thermonuclear runaways, as already predicted by Shara et al. (1977) and the extensive grid of nova models by Yaron et al. (2005). We suggest that the low X-ray and bolometric luminosity in comparison to the predictions of the models of nuclear burning are due to an optically thick accretion disk, hiding most of the white dwarf surface. If this is the case, we calculated that the optical transient can be explained as a non-ejective thermonuclear event on a WD of $\simeq$1.1M$_\odot$ accreting at the rate of $\simeq3.5{-}5{\times}10^{-7}$M$_\odot$yr$^{-1}$. We make predictions that should prove whether the nature of the transient event was due to thermonuclear burning or to accretion; observational proof should be obtained in the next few years, because a new outburst should occur within $\simeq$10-15 years of the event., 7 pages, 2 figures

Binary neutron star (NS) mergers may result in remnants of supra-massive or even stable NS, which have been supported indirectly by observed X-ray plateau of some gamma-ray bursts (GRBs) afterglow. Recently, Xue et al. (2019) discovered a X-ray transient CDF-S XT2 that is powered by a magnetar from merger of double NS via X-ray plateau and following stepper phase. However, the decay slope after the plateau emission is a little bit larger than the theoretical value of spin-down in electromagnetic (EM) dominated by losing its rotation energy. In this paper, we assume that the feature of X-ray emission is caused by a supra-massive magnetar central engine for surviving thousands of seconds to collapse black hole. Within this scenario, we present the comparisons of the X-ray plateau luminosity, break time, and the parameters of magnetar between CDF-S XT2 and other short GRBs with internal plateau samples. By adopting the collapse time to constrain the equation of state (EOS), we find that three EOSs (GM1, DD2, and DDME2) are consistent with the observational data. On the other hand, if the most released rotation energy of magnetar is dominated by GW radiation, we also constrain the upper limit of ellipticity of NS for given EOS, and it is range in $[0.32-1.3]\times 10^{-3}$. Its GW signal can not be detected by aLIGO or even for more sensitive Einstein Telescope in the future., 13 pages, 5 figures,1 table. Accepted for publication by Research in Astronomy and Astrophysics

Neutron star-neutron star mergers are known to be associated with short gamma-ray bursts. If the neutron star equation of state is sufficiently stiff, at least some of such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field (i.e., a magnetar). Such a magnetar signature may have been observed as the X-ray plateau following a good fraction (up to 50%) of short gamma-ray bursts, and it has been expected that one may observe short gamma-ray burst-less X-ray transients powered by double neutron star mergers. A fast X-ray transient (CDF-S XT1) was recently found to be associated with a faint host galaxy whose redshift is unknown. Its X-ray and host-galaxy properties allow several possibleexplanations including a short gamma-ray burst seen off axis, a low-luminosity gamma-ray burst at high redshift, or a tidal disruption event involving an intermediate mass black hole and a white dwarf. Here we report a second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift z = 0.738. The light curve is fully consistent with being powered by a millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its star-forming host galaxy with a moderate offset from the galaxy center, as short bursts often do. The estimated event rate density of similar X-ray transients, when corrected to the local value, is consistent with the double neutron star merger rate density inferred from the detection of GW170817., Comment: 29 pages, 4 figures, 3 tables, published in Nature on 11 April 2019

Very recently \citet{XueYQ2019} reported an important detection of the X-ray transient, CDF-S XT2, whose light curve is analogous to X-ray plateau features of gamma-ray burst afterglows. They suggested that this transient is powered by a remnant stable magnetar from a binary neutron star merger since several pieces of evidence (host galaxy, location, and event rate) all point toward such an assumption. In this paper, we revisit this scenario and confirm that this X-ray emission can be well explained by the internal gradual magnetic dissipation process in an ultra-relativistic wind of the newborn magnetar. We show that both the light curve and spectral evolution of CDF-S XT2 can be well fitted by such a model. Furthermore, we can probe some key properties of the central magnetar, such as its initial spin period, surface magnetic field strength and wind saturation Lorentz factor., 5 pages, 3 figures, ApJL accepted

We report on observations of black hole Swift J1357.2-0933, a member of the modest population of very faint X-ray transients. This source has previously shown intense dips in the optical lightcurve, a phenomena that has been linked to the existence of a "unique toroidal structure" in the inner region of the disc, seen at a high inclination. Our observations, carried out by the Neil Gehrels Swift and NuSTAR X-ray observatories, do not show the presence of intense dips in the optical light curves. We find that the X-ray light curves do not show any features that would straightforwardly support an edge-on configuration or high inclination configuration of the orbit. This is similar to what was seen in the X-ray observations of the source during its 2011 outburst. Moreover, the broadband spectra were well described with an absorbed power-law model without any signatures of the cut-off at energies above 10 keV, or any reflection from the disc or the putative torus. Thus, the X-ray data do not support the unique obscuring torus scenario proposed for J1357. We also performed a multi-wavelength study using the data of X-ray telescope and Ultraviolet/Optical Telescope aboard Swift, taken during the 4.5 months duration of the 2017 outburst. This is consistent with what was previously inferred for this source. We found a correlation between the simultaneous X-ray and ultraviolet/optical data and our study suggests that most of the reprocessed flux must be coming out in the ultraviolet., 13 pages, Accepted for publication in MNRAS

IGR J17503-2636 is a hard X-ray transient discovered by INTEGRAL on 2018 August 11. This was the first ever reported X-ray emission from this source. Following the discovery, follow-up observations were carried out with Swift, Chandra, NICER, and NuSTAR. We report in this paper the analysis and results obtained from all these X-ray data. Based on the fast variability in the X-ray domain, the spectral energy distribution in the 0.5-80 keV energy range, and the reported association with a highly reddened OB supergiant at ~10 kpc, we conclude that IGR J17503-2636 is most likely a relatively faint new member of the supergiant fast X-ray transients. Spectral analysis of the NuSTAR data revealed a broad feature in addition to the typical power-law with exponential roll-over at high energy. This can be modeled either in emission or as a cyclotron scattering feature in absorption. If confirmed by future observations, this feature would indicate that IGR J17503-2636 hosts a strongly magnetized neutron star with B~2e12 G., Submitted to A&A, revised version after referee's comments

We discovered and studied an ultraluminous X-ray source (CXOU J203451.1+601043) that appeared in the spiral galaxy NGC 6946 at some point between 2008 February and 2012 May, and has remained at luminosities $\approx$2-4 $\times 10^{39}$ erg s$^{-1}$ in all observations since then. Our spectral modelling shows that the source is generally soft, but with spectral variability from epoch to epoch. Using standard empirical categories of the ultraluminous regimes, we find that CXOU J203451.1+601043 was consistent with a broadened disk state in 2012, but was in a transitional state approaching the super-soft regime in 2016, with substantial down-scattering of the hard photons (similar, for example, to the ultraluminous X-ray source in NGC 55). It has since hardened again in 2018-2019 without any significant luminosity change. The most outstanding property of CXOU J203451.1+601043 is a strong emission line at an energy of of $(0.66 \pm 0.01)$ keV, with equivalent width of $\approx$100 eV, and de-absorbed line luminosity of $\approx$2 $\times 10^{38}$ erg s$^{-1}$, seen when the continuum spectrum was softest. We identify the line as OVIII Ly$\alpha$ (rest frame energy of 0.654 keV); we interpret it as a strong indicator of a massive outflow. Our finding supports the connection between two independent observational signatures of the wind in super-Eddington sources: a lower temperature of the Comptonized component, and the presence of emission lines in the soft X-ray band. We speculate that the donor star is oxygen-rich: a CO or O-Ne-Mg white dwarf in an ultracompact binary. If that is the case, the transient behaviour of CXOU J203451.1+601043 raises intriguing theoretical questions., Comment: Accepted for publication in ApJ. 22 pages, 4 tables, 9 figures

We present quasi-simultaneous radio and X-ray observations of the black hole X-ray binary GRS 1739--278 of its 2015-2016 mini-outbursts, i.e. between 2015 June 10 and 2016 October 31, with the X-ray-to-radio time interval being less than one day. The monitor champaign was run by \swift\, in the X-rays and by JVLA in the radio (at both 5 GHz and 8 GHz). We find the brightest radio emission is actually achieved during the soft sate, where radio spectrum is unexpectedly flat with a spectral index $\alpha\approx -0.2$ (flux $F_\nu\propto \nu^\alpha$). For the radio emission in hard state, we also find a large diversity in the spectral index, i.e. a majority of radio spectra are optically thick with $-0.5 < \alpha < 0.5$, while a few are optically thin with $\alpha < -1$. We also investigate the correlation between the luminosities in radio (monochromatic at 5 GHz, $L_R$) and 1-10 keV X-rays ($L_X$) during the hard state. We find this source does not follow the standard correlation whose $p\approx 0.6$ (in the form of $L_R\propto L_X^p$), but instead belongs to the "outlier" category that may follow a hybrid correlation. For more than orders of magnitude variation in the X-ray luminosity this source exhibits a flat correlation with $p\approx 0.16$. Both the slope and the corresponding luminosity range agree well with those in H1743--322, the prototype of the hybrid correlation., Comment: 11 pages, 5 figures and 1 table. ApJ accepted

We present a new Chandra observation (performed in July 2016) of the neutron star X-ray transient IGR J17480-2446, located in the globular cluster Terzan 5. We study the continued cooling of the neutron star crust in this system that was heated during the 2010 outburst of the source. This new observation was performed two years after the last observation of IGR J17480-2446, hence, significantly extending the cooling baseline. We reanalysed all available Chandra observations of the source (but excluding observations during which one of the known transients in Terzan 5 was in outburst) and fitted the obtained cooling curve with our cooling code NSCool, which allows for much improved modelling than what was previously performed for the source. The data and our fit models indicate that the crust was still cooling ~5.5 years after the outburst ended. The neutron star crust has likely not reached crust-core thermal equilibrium yet, and further cooling is predicted (which can be confirmed with additional Chandra observations in >5 years). Intriguingly, we find indications that the thermal conductivity might be relatively low in part of the crust compared to what has been inferred for other crust-cooling sources and tentatively suggest that this layer might be located around the neutron drip. The reason for this difference is unclear, but might be related to the fact that IGR J17480-2446 harbours a relatively slowly rotating neutron star (with a spin of 11 Hz) that has a relatively strong inferred surface magnetic field ($10^{9-10}$ Gauss) compared to what is known or typically assumed for other cooling sources., 17 pages, 10 figures, 4 tables, accepted for publication in MNRAS

We studied the transient neutron-star low-mass X-ray binary GRS 1747-312, located in the globular cluster Terzan 6, in its quiescent state after its outburst in August 2004, using an archival XMM-Newton observation. A source was detected in this cluster and its X-ray spectrum can be fitted with the combination of a soft, neutron-star atmosphere model and a hard, power-law model. Both contributed roughly equally to the observed 0.5-10 keV luminosity (~4.8X10^33 erg/s). This type of X-ray spectrum is typically observed for quiescent neutron-star X-ray transients that are perhaps accreting in quiescence at very low rates. Therefore, if this X-ray source is the quiescent counterpart of GRS 1747-312, then this source is also accreting at low levels in-between outbursts. Since source confusion a likely problem in globular clusters, it is quite possible that part, if not all, of the emission we observed is not related to GRS 1747-312, and is instead associated with another source or conglomeration of sources in the cluster. Currently, it is not possible to determine exactly which part of the emission truly originates from GRS 1747-312, and a Chandra observation (when no source is in outburst in Terzan 6) is needed to be conclusive. Assuming that the detected emission is due to GRS 1747-312, we discuss the observed results in the context of what is known about other quiescent systems. We also investigated the thermal evolution of the neutron star in GRS 1747-312, and inferred that GRS 1747-312 can be considered a typical quiescent system under our assumptions., 9 pages, 3 figures, 3 tables. Accepted by MNRAS - March 15, 2018

We present the results of combined INTEGRAL and XMM-Newton observations of the supergiant fast X-ray transient (SFXT) IGR J17354$-$3255. Three XMM-Newton observations of lengths 33.4 ks, 32.5 ks and 21.9 ks were undertaken, the first an initial pointing to identify the correct source in the field of view and the latter two performed around periastron. Simultaneous INTEGRAL observations across $\sim66\%$ of the orbital cycle were analysed but the source was neither detected by IBIS/ISGRI nor by JEM-X. The XMM-Newton light curves display a range of moderately bright X-ray activity but there are no particularly strong flares or outbursts in any of the three observations. We show that the spectral shape measured by XMM-Newton can be fitted by a consistent model throughout the observation, suggesting that the observed flux variations are driven by obscuration from a wind of varying density rather than changes in accretion mode. The simultaneous INTEGRAL data rule out simple extrapolation of the simple powerlaw model beyond the XMM-Newton energy range., Comment: 13 pages, 9 figures, This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society Published by Oxford University Press

The black hole candidate MAXI J1535-571 is a recently discovered X-ray transient. We report on the monitoring observations of Swift Gamma-Ray Burst Mission during outburst. The source transits from the hard state to the intermediate state, and reaches the soft state; near the end of the outburst, it returns to the hard state passing through the low intermediate state, following a typical Q-shaped loop in the hardness-intensity diagram. During the high intermediate state, detailed spectral analyses using a multi-temperature disk model reveal that the disk luminosity is flatter than the inner disk temperature to the fourth power, and the disk temperature profile varies as the disk radius raised to the power of -0.5, consistent with the behavior of a slim disk, implying that the disk structure has been modified due to the high luminosity of ~ 10^{39} erg/s. Meanwhile, the column density increases with the Eddington ratio, suggesting that the outflow matters are driven by radiation pressure due to the high accretion rate., Comment: 13 pages, 9 figures, 2 tables. Accepted for publication in MNRAS

The unique capabilities of Swift that make it ideal for discovery and follow-up of Gamma-Ray bursts also makes it the idea mission for discovery and monitoring of X-ray Transients in the Milky Way and the Large and Small Magellanic Clouds. The Burst Alert Telescope allows for detection of new transient outbursts, the automated follow-up capabilities of Swift allow for rapid observation and localization of the new transient in X-rays and optical/UV bands, and Swift's rapid slewing capabilities allows for low-overhead short observations to be obtained, opening up the possibility of regular, sensitive, long term monitoring of transient outbursts that are not possible with other currently operational X-ray missions. In this paper I describe the methods of discovery of X-ray transients utilizing Swift's BAT and also collaboration with the MAXI telescope. I also detail two examples of X-ray transient science enabled by Swift: Swift discovery and monitoring observations of MAXI J1659-152, a Black Hole candidate Low Mass X-ray Binary in the Galactic Halo, which has the shortest known orbital period of any such system; and Swift monitoring of IGR J00569-7226, an edge on Be/X-ray binary that displayed a outburst in 2013 and 2014, and which monitoring by Swift allowed for detection of dips, eclipses and the determination of the orbital parameters, utilizing a measurement of doppler shifts in the pulsar period., 8 pages, 7 figures. Review paper for "Swift 10 Years of Discovery" conference. Accepted for Publication in the Journal of High Energy Astrophysics

When neutron stars reside in transient X-ray binaries, their crustal layers become heated during accretion outbursts and subsequently cool in quiescence. Observing and modeling this thermal response has yielded valuable insight into the physics of neutron star crusts. However, one unresolved problem is the evidence in several neutron stars for an extra energy source, located at shallow depth in the crust, that is not accounted for by standard heating models. Its origin remains puzzling, and it is currently unclear whether this additional heating occurs in all neutron stars, and if the magnitude is always the same. Here, we report on Chandra observations that cover two years after the 2012 outburst of the transient neutron star X-ray binary Swift J174805.3-244637 in the globular cluster Terzan 5. The temperature of the neutron star was elevated during the first two months following its ~8 week accretion episode, but had decayed to the pre-outburst level within ~100 days. Interpreting this as rapid cooling of the accretion-heated crust, we model the observed temperature curve with a thermal evolution code. We find that there is no need to invoke shallow heating for this neutron star, although an extra energy release up to ~1.4 MeV/nucleon is allowed by the current data (2-sigma confidence). We also present two new data points on the crust cooling curve of the 11-Hz X-ray pulsar IGR J17480-2446 in Terzan 5, which was active in 2010. The temperature of this neutron star remained significantly above its pre-outburst level, but we detect no temperature change since the previous measurements of 2013 February. This is consistent with the slower cooling expected several years post-outburst., 11 pages, 6 figures, 5 tables, MNRAS in press. Minor (textual) changes to match published version

We present an optical and infrared (IR) study of IC 10 X-2, a high-mass X-ray binary in the galaxy IC 10. Previous optical and X-ray studies suggest X-2 is a Supergiant Fast X-ray Transient: a large-amplitude (factor of $\sim$ 100), short-duration (hours to weeks) X-ray outburst on 2010 May 21. We analyze R- and g-band light curves of X-2 from the intermediate Palomar Transient Factory taken between 2013 July 15 and 2017 Feb 14 show high-amplitude ($\gtrsim$ 1 mag), short-duration ($\lesssim8$ d) flares and dips ($\gtrsim$ 0.5 mag). Near-IR spectroscopy of X-2 from Palomar/TripleSpec show He I, Paschen-$\gamma$, and Paschen-$\beta$ emission lines with similar shapes and amplitudes as those of luminous blue variables (LBVs) and LBV candidates (LBVc). Mid-IR colors and magnitudes from Spitzer/IRAC photometry of X-2 resemble those of known LBV/LBVcs. We suggest that the stellar companion in X-2 is an LBV/LBVc and discuss possible origins of the optical flares. Dips in the optical light curve are indicative of eclipses from optically thick clumps formed in the winds of the stellar counterpart. Given the constraints on the flare duration ($0.02 - 0.8$ d) and the time between flares ($15.1\pm7.8$ d), we estimate the clump volume filling factor in the stellar winds, $f_V$, to be $0.01 < f_V < 0.71$, which overlaps with values measured from massive star winds. In X-2, we interpret the origin of the optical flares as the accretion of clumps formed in the winds of an LBV/LBVc onto the compact object., Comment: 15 pages, 4 figures. Submitted to ApJ on Sep 26 2017

We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the \textit{Nuclear Spectroscopic Telescope Array} (\textit{NuSTAR}) satellite. While \textit{NuSTAR} was designed as an astrophysics mission, it can observe the Sun above 2~keV with unprecedented sensitivity due to its pioneering use of focusing optics. \textit{NuSTAR} first observed quiet Sun regions on 2014 November 1, although out-of-view active regions contributed a notable amount of background in the form of single-bounce (unfocused) X-rays. We conducted a search for quiet Sun transient brightenings on time scales of 100 s and set upper limits on emission in two energy bands. We set 2.5--4~keV limits on brightenings with time scales of 100 s, expressed as the temperature T and emission measure EM of a thermal plasma. We also set 10--20~keV limits on brightenings with time scales of 30, 60, and 100 s, expressed as model-independent photon fluxes. The limits in both bands are well below previous HXR microflare detections, though not low enough to detect events of equivalent T and EM as quiet Sun brightenings seen in soft X-ray observations. We expect future observations during solar minimum to increase the \textit{NuSTAR} sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background., 11 pages, 7 figures; accepted for publication in The Astrophysical Journal

We monitored the neutron star low-mass X-ray binary SAX J1750.8-2900 after the end of its 2015/2016 outburst using the X-ray Telescope (XRT) aboard Swift to detect possible post-outburst 'rebrightenings', similar to those seen after its 2008 outburst. We did not detect any rebrightening behaviour, suggesting that the physical mechanism behind the rebrightening events is not always active after each outburst of the source. Any model attempting to explain these rebrightenings should thus be able to reproduce the different outburst profiles of the source at different times. Surprisingly, our Swift/XRT observations were unable to detect the source, contrary to previous Swift/XRT observations in quiescence. We determined a temperature upper limit of $\leq$ 106 eV, much colder than the post 2008 outburst value of $\sim$ 145 eV. We also report on an archival Chandra observation of the source after its 2011 outburst and found a temperature of $\sim$ 126 eV. These different temperatures, including the non-detection very close after the end of the 2015/2016 outburst, are difficult to explain in any model assuming we observe the cooling emission from a neutron star core or an accretion-heated crust. We discuss our observations in the context of a change in envelope (the outer $\sim$ 100 m of the crust) composition and (possibly in combination with) a cooling crust. Both hypotheses cannot explain our results unless potentially unrealistic assumptions are made. Irrespective of what causes the temperature variability, it is clear that the neutron star in SAX J1750.8-2900 may not be as hot as previously assumed., Accepted for publication in MNRAS

We report on the detection and follow-up multi-wavelength observations of the new X-ray transient MAXI J1807+132 with the MAXI/GSC, Swift, and ground-based optical telescopes. The source was first recognized with the MAXI/GSC on 2017 March 13. About a week later, it reached the maximum intensity ($\sim$10 mCrab in 2-10 keV), and then gradually faded in $\sim$10 days by more than one order of magnitude. Time-averaged Swift/XRT spectra in the decaying phase can be described by a blackbody with a relatively low temperature (0.1-0.5 keV), plus a hard power-law component with a photon index of $\sim$2. These spectral properties are similar to those of neutron star low-mass X-ray binaries (LMXBs) in their dim periods. The blackbody temperature and the radius of the emission region varied in a complex manner as the source became dimmer. The source was detected in the optical wavelength on March 27-31 as well. The optical flux decreased monotonically as the X-ray flux decayed. The correlation between the X-ray and optical fluxes is found to be consistent with those of known neutron star LMXBs, supporting the idea that the source is likely to be a transient neutron star LMXB., Comment: 13 pages, 11 figures, accepted for publication in ApJ

We present the results of our long-term photometric and spectroscopic observations at the Russian-Turkish RTT-150 telescope for the optical counterpart to one of the best-known sources, representatives of the class of fast X-ray transients, IGR J17544-2619. Based on our optical data, we have determined for the first time the orbital and physical parameters of the binary system by the methods of Doppler spectroscopy. We have computed theoretical spectra of the optical counterpart by applying non-LTE corrections for selected lines and obtained the parameters of the stellar atmosphere and the optical star ($T_{\rm eff}=33\,000$ K, $\log\,g=3.85$, $R=9.5\ R_{\odot},$ and $M=23\ M_{\odot}$). The latter suggest that the optical star is not a supergiant as has been thought previously., 13 pages, 11 figures, 4 tables

The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in October 2015. The source did not decay to its quiescent state but settled in a meta-stable plateau state (a factor ~10 brighter than quiescence) in which its luminosity slowly decayed. We used XMM-Newton to observe the system during this phase and we found that its spectrum can be well described using a black-body model with a small emitting radius. This suggests emission from hot spots on the surface, which is confirmed by the detection of pulsations. In addition, we obtained a relatively long (~7.9 ksec) Swift/XRT observation ~35 days after our XMM-Newton one. We found that the source luminosity was significantly higher and, although the spectrum could be fitted with a black-body model the temperature was higher and the emitting radius smaller. Several weeks later the system started a sequence of type-I accretion outbursts. In between those outbursts, the source was marginally detected with a luminosity consistent with its quiescent level. We discuss our results in the context of the three proposed scenarios (accretion down to the magnestospheric boundary, direct accretion onto neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase., Comment: 8 pages, 4 figures, 2 tables, accepted for publication in MNRAS