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.

Large-scale Dynamics of Line-driven Winds with the Re-radiation Effect

Previous simulations studying winds only focus on the line force due to photons from central active galactic nuclei.What properties of the winds will be when including the re-radiation force due to the scattered and reprocessed photons(i.e.,the re-radiation effect)?We perform simulations to study the large-scale dynamics of accretion disk winds driven by radiation line force and re-radiation force.For the fiducial run,we find that the re-radiation force drives stronger outflows during the early stages.When the flows get into the steadiness,the UV radiation due to spectral lines dominates total radiation and the re-radiation effect could be negligible.The opening angle of winds narrows as the initial gas density increases.The larger the gas density is,the stronger the re-radiation effect will be.For MBH=10^(6)M_(⊙),ε=0.3,the outflows do become much stronger with the re-radiation effect and the winds still cannot escape from gravitational potential.We find that the detection probability of ultra-fast outflows and the properties of the winds are both consistent with the observations.

Detection of thermonuclear X-ray bursts and dips from the X-ray binary 4U 1323-62 with AstroSat/LAXPC

Using data from the Large Area X-ray Proportional Counter(LAXPC)on the AstroSat satellite,we observed Type-1 thermonuclear X-ray bursts from the low mass X-ray binary neutron star 4 U 1323-62.Observations of 4 U 1323-62 that were carried out during the performance verification phase of the AstroSat satellite indicated six thermonuclear X-ray bursts in a total effective exposure of^49.5 ks for about two consecutive days.Recurrence time of the detected thermonuclear bursts is in accordance with the orbital period of the source,~9400 s.Moreover,the light curve of 4 U 1323-62 revealed the presence of two dips.We present the results from time-resolved spectroscopy performed during all of the six X-ray bursts and also report the detection of a known low frequency quasi-periodic oscillation(LFQPO)at^1 Hz from the source.However,any evidence of kilohertz QPO was not found.We have shown the burst profile at different energy ranges.Assuming a distance of 10 kpc,we observed a mean flux of^1.8×10^-9 erg cm^2 s^-1.The radius of the blackbody is found to be highly consistent with the blackbody temperature and the blackbody flux of the bursts.