A Study of the X-ray Variability and Accretion Mechanism in High Mass X-ray Binary Pulsars

Abstract

High-mass X-ray binaries are among the most luminous X-ray sources. They host an accreting compact object (more often a neutron star) and a massive (M > 10 M⊙) donor star. They are among the earliest discovered X-ray sources and are essential for various astrophysical contexts ranging from high energy astrophysics, stellar evolution, and knowledge of gravitational wave progenitors. They show variation in the X-ray intensity on all timescales ranging from seconds to days or months, which carry information about the underlying accretion mechanisms and their immediate environment. We have observed the X-ray variability in some of such sources and provided the possible physical pictures behind them.

Firstly, we have investigated a unique partial eclipse seen in IGR J16393-4643. Unlike other HMXBs, IGR J16393-4643 shows a partial eclipse with ∼25% of total intensity when observed with Swift-BAT making it the only partially eclipsing candidate in the HMXB catalog. We have presented the results from our spectroscopic study, which indicates that the low-intensity state might not be an eclipse, as previously thought, but absorption in the stellar corona.

In the second part, we studied a Be X-ray binary GRO J2058+42 that went through a Type-II outburst during March-April 2019, lasting for about 50 days. With this unique opportunity, we could analyze the broadband characteristics of the pulsar using three NuSTAR observations during the outburst and quiescent for the first time and the accretion torque characteristics of the pulsar over a range of X-ray luminosity.

Lastly, We have discussed the spin evolution of accreting persistent X-­ray pulsars. The process of transfer of the accretion torque in the wind-fed persistent pulsar is complex and does not have a clear correlation with the mass accretion rate. A possible explanation of such torque behavior can be the change in the accretion mode and geometry. In this work, we have used the pulse profiles of four wind-fed pulsars as a tool to observe any changes in the accretion geometry at different accretion torque.