Astrophysics Webinar

X-ray binaries consist of a compact, degenerate stellar remnant in a gravitationally bound system with a stellar companion. The X-ray emission from accreting X-ray binaries shows strong, aperiodic variability on a wide range of time scales. We investigated the spectral and timing properties of a new black hole X-ray binary (BHXRB) MAXI J1348-630 using near-simultaneous AstroSat and NICER observations. Spectral analysis using AstroSat data identified the source in the soft and hard spectral states of BHXRBs. We detected type-C and type-A quasi-periodic oscillations (QPOs) in these observations and explored the energy-dependent behaviour of the QPO and other variability components for the first time in the 0.5–80 keV energy band. In the soft state, the power density spectra are substantially lower (by a factor >5) for the NICER (0.5–12 keV) band compared to the AstroSat/LAXPC (3–80 keV) one, confirming that the disc is significantly less variable than the Comptonization component. We modelled the energy-dependent temporal properties of MAXI J1348-630 using the single-zone stochastic propagation model to describe the mechanism responsible for the observed properties. I will discuss the broadband spectro-timing properties of MAXI J1348-630 and the mechanism responsible for the observed properties.

In the second part of the talk, I will discuss ultra-luminous X-ray sources (ULXs). ULXs are bright, non-nuclear accreting X-ray sources with an isotropic X-ray luminosity above 1039 erg/s. We explored the spectral and temporal properties of ultra-luminous X-ray source NGC 55 ULX1 using Swift and XMM-Newton observations conducted during 2013–2021. In these observations, the source flux varied by a factor of ~ 5–6, and we identified the source mainly in the soft-ultraluminous (SUL) state of ULXs. We fitted the X-ray spectra with a two thermal component model consisting of a blackbody (for the soft component) and a disc (for the hard component), and the soft component dominated in these observations. The soft component in the SUL state showed properties similar to that of ultraluminous supersoft sources, for example, an anticorrelation between the characteristic radius and temperature of the blackbody component. In addition, we observed a positive correlation between the blackbody and inner disc temperatures when the X-ray spectra fitted with the two-thermal component model. These observed properties strongly support the supercritical radiatively driven outflow from the source.