AstroSat Observations of the Be/X-Ray Binary XTE J1946+274 During 2018 and 2021 Outbursts

We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and LAXPC instruments on the AstroSat satellite.Unlike the 1998 and 2010 outbursts,where a giant outburst was followed by several low intensity periodic outbursts,the 2018 and 2021 outbursts were single outbursts.The X-ray pulsations are detected over a broad energy band covering 0.5–80 keV from the compact object.We construct the spin evolution history of the pulsar over two decades and find that the pulsar spins-up during the outbursts but switches to spin-down state in the quiescent periods between the outbursts.Energy resolved pulse profiles generated in several bands in 0.5–80 keV show that the pulse shape varies with the energy.The energy spectrum of the pulsar is determined for the 2018 and 2021 outbursts.The best fit spectral models require presence of cyclotron resonant scattering feature at about 43 keV in the energy spectra of both the outbursts.We find indication of possible reversal in the correlation between the cyclotron line energy and luminosity which needs to be ascertained from future observations.Using the best fit spectra the X-ray luminosity of XTE J1946+274 is inferred to be 2.7×10^(37) erg s^(-1) for the 2018 observations and 2.3×10^(37) erg s^(-1) for the 2021 observations.We discuss possible mechanisms which can drive outbursts in this transient Be X-ray binary.

Crust Cooling of Soft X-Ray Transients——the Uncertainties of Shallow Heating

Crust cooling of soft X-ray transients has been observed after outbursts,but an additional shallow heating during accretion in outburst is needed to explain the quiescent light curve.However,shallow heating is significantly different between sources and even within one source between different outbursts,and the source of shallow heat is as yet unknown.Using the open source code"dStar"which solves the fully general relativistic heat diffusion equation for the crust,we investigate the effect of magnitude and depth of shallow heating on crust cooling and find that some exceptional sources(Swift J174805.3-244637,MAXI J0556-332 during outburstⅡand GRO J1750-27)in which shallow heating may be inactive could be explained by a deeper shallow heating mechanism.We compare our results with those from previous works and find that the shallow heating is model dependent.In addition,the effects of mass and radius of a neutron star on shallow heating are studied,and it is shown that the more compact the star,the less shallow heating will be needed to fit the crust cooling light curves.

Meta-analysis of electron cyclotron resonance absorption features detected in high-mass X-ray binaries

Using recent compilations of detailed X-ray observations and spectral models of exceptional quality, we record the electron cyclotron resonance absorption(ECRA) features that have been detected in 45 pulsating high-mass X-ray binaries(HMXBs) and ultraluminous X-ray(ULX) sources harboring neutron stars, although seven of these detections are still questionable and another 21 are single and/or not independently confirmed. From the comprehensive catalogs of Jaisawal & Naik and Staubert et al.and from several additional recent observations, we produce two lists of HMXB ECRA sources: a list of 17 sources in which multiple ECRA lines or single very low-energy lines are seen, in which we can reasonably assume that the lowest energy reveals the fundamental cyclotron level for each source;and a"contaminated" list of 38 sources including the 21 detections of single ECRA lines that may(not) be higherlevel harmonics. Both lists confirm a previous result that we have obtained independently by modeling the propeller lines of Magellanic HMXB pulsars: the surface dipolar magnetic fields B*of HMXB neutron stars are segregated around five distinct values with B*= 0.28 ± 0.08, 0.55 ± 0.11, 1.3 ± 0.37, 3.0 ± 0.68 and 7.9 ± 3.1, in units of TG. However, an explanation of this phenomenon is currently lacking. We have found no correlation between these B*values and the corresponding observed spin periods, spin period derivatives, orbital periods, maximum X-ray luminosities, neutron star masses or companion star masses.

Accretion by a Neutron Star Moving at a High Kick Velocity in the Supernova Ejecta

We suggest a two-dimensional time dependent analytic model to describe the accretion of matter onto a neutron star moving at a high speed across the ejecta left in the aftermath of a supernova explosion. The formation of a strange star resulting from the accretion is also addressed. The newborn neutron star is assumed to move outward at a kick velocity of Vns- 10^3 kin s^-1, and the accretion flow is treated as a dust flow. When the neutron star travels across the ejecta with high speed, it sweeps up material, and when the accreted mass has reached a critical value, the neutron star will undergo a phase transition, for instance, to become a strange star. Our results show that the accretion rate decreases in a complicated way in time, not just a power law dependence： it drops much faster than the power law derived by Colpi et al. We also found that the total accreted mass and the phase transition of the neutron star depend sensitively on the velocity of supernova ejecta.

X-Ray Properties of the Point Source Population in the Spiral Galaxy NGC 5055 (M63) with Chandra

Using Chandra ACIS S3 data we studied the X-ray properties of low-and highmass X-ray binary populations in the nearby spiral galaxy NGC 5055. A total of 43 X-ray point sources were detected within two effective radii, with 31 sources located on the disk and the rest 12 sources in the bulge. The resolved point sources dominate the X-ray emission of the galaxy, accounting for about 80% of the total counts in 0.3-10keV. From spectral fittings we calculated the 0.3-10.0keV luminosities of all the detected X-ray point sources and found that they span a wide range from a few times 10^37 erg s^-1 to over 10^39 erg s^-1. After compensating for incompleteness at the low luminosity end, we found that the corrected XLF of the bulge population is well fitted with a broken power-law with a break at 1.57-0.20^＋0.21 1038 erg s^-1, while the profile of the disk population＇s XLF agrees with a single powerlaw distribution of slope 0 9-0.06^＋0.07 The disk population is significantly richer at ≥ 2 × 10^38 erg s^- 1 than the bulge population, indicating that the disk may have undergone relatively recent, strong starbursts that significantly increased the HMXB population, although ongoing starbursts are also observed in the nuclear region. Similar XLF profiles of the bulge and disk populations were found in M81. However, in most other spiral galaxies different patterns of spatial variation of the XLF profiles from the bulge to the disk have been observed, indicating that the star formation and evolution history may be more complex than we have expected.

Accretion flow properties of XTE J1118+480 during its 2005 outburst

We study spectral and temporal properties of Galactic short orbital period transient black hole XTE J1118+480 during its 2005 outburst using archival data of RXTE PCA and HEXTE instruments in the combined energy range of 3−100 keV.Spectral analysis with the physical two component advective flow(TCAF)model allows us to understand the accretion flow properties of the source.We found that this outburst of XTE J1118+480 is an unconventional outburst as the source was only in the hard state(HS).Our spectral analysis suggests that during the entire outburst,the source was highly dominated by the low angular momentum sub-Keplerian halo rate.Since the source was active in radio throughout the outburst,we make an effort to estimate X-ray contribution of jets to total observed X-ray emissions from the spectral analysis with the TCAF model.The total X-ray intensity shows a similar nature of evolution as that of radio and jet X-ray fluxes.This allowed us to define this‘outburst’also as a jet dominated‘outburst’.Total X-ray flux is also found to subside when jet activity disappears.Our detailed spectral analysis also indicated that although the source was only in the HS during the outburst,in the late declining phase the spectrum became slightly softer due to the slow rise in the Keplerian disk rate.

Spectral analysis of new black hole candidate AT2019wey observed by NuSTAR

AT2019 wey is a new galactic X-ray binary that was first discovered as an optical transient by the Asteroid Terrestrial-impact Last Alert System(ATLAS)on December 7,2019.AT2019 wey consists of a black hole candidate as well as a low-mass companion star(M_(star)≤1.0 M_(■))and is likely to have a short orbital period(P_(orb)≤16h).Although AT2019 wey began activation in the X-ray band on March 8,2020,it did not enter the soft state during almost the entire outburst.In this study,we present a detailed spectral analysis of AT2019 wey in the low/hard state during its X-ray outburst on the basis of Nuclear Spectroscopic Telescope Array(Nu S T AR)observations.We obtain tight constraints on several of its important physical parameters by applying the state of the art relxill relativistic reflection model family.In particular,we determine that the measured inner radius of the accretion disk is most likely to have extended to the innermost stable circular orbit(ISCO)radius,i.e.,R_(in)=1.38^(+0.23)_(-0.16)R_(ISCO).Hence,assuming R_(in)=R_(ISCO),we find the spin of AT2019 wey to be a*~0.97,which is close to the extreme and an inner disk inclination angle of i~22°.Additionally,according to our adopted models,AT2019 wey tends to have a relatively high iron abundance of AFe~5A_(Fe,■)and a high disk ionization state of logξ~3.4.