Speaker
Description
The presence of a stellar-mass black hole, an accretion disk, jets, and intense X-ray emissions renders LMC X-1 and Cygnus X-1 unique sources for study. This study presents our findings from broad-band spectral analyses in which we tested the common spectral models. Their performance was evaluated based on spectral fitting. The data was collected by the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. Further, we assessed how the spectral parameters change with the photon index. To finalize our findings the relativistic accretion disk model (kerrbb) is applied and, spin parameters, and accretion rates for both sources are estimated.
To study the aptness of models the study focused on the transitional phase of Cygnus X-1, particularly, from a low-hard state (LHS), transitioning to a high-soft state (HSS). LMC X-1, was observed twice in its high soft state (HSS) by AstroSat. Since Cygnus X-1 is a bright source, an annular source region with an inner radius of approximately 5′ and an outer radius of 15’ is excluded for data extraction to avoid pile-up. As LMC X-1 is not as bright as Cygnus X-1, a circular region with an outer radius of 10’ has been constructed. The extracted cleaned event files generate energy spectra with XSELECT V2.4d. SXT, LAXPC spectra, light curve, and hardness ratio curves are extracted over the SXT CCD image of the source.
The spectral analysis of Cygnus X-1 and LMC X-1 involved data fitting and modeling using XSPEC version 12.10.1. For Cygnus X-1, spectral fitting was conducted in the energy ranges of 0.7 to 6.0 keV for SXT and 4.0 to 30.0 keV for LAXPC, incorporating a uniform systematic error of 3%. For Cygnus X-1, various models were applied across different orbits to account for spectral features and instrumental effects. These included gaussian + powerlaw, nthComp, and diskbb + nthComp, with adjustments made for temperature parameters and gain corrections. The softest state of Cygnus X-1 utilized the model diskbb + nthComp with additional smedge components. In contrast, attempts to use the physical Gaussian model for LMC X-1 failed to yield meaningful results. Instead, diskbb + powerlaw model was employed, with the addition of a smedge. Spin parameters and accretion rates were determined for both sources, refining models to achieve physically plausible fits. Cygnus X-1's model comprised gaussian+kerrbb, replacing diskbb with the kerrbb relativistic thin disk model. For LMC X-1, the model simpl*kerrbb + powerlaw was utilized, with parameters adjusted to achieve satisfactory fits. The resulting reduced chi-squared (χ²) values indicated the goodness of fit for the models applied to each source.
The graphical analyses conducted using MATLAB unveils that the hydrogen column density decreases until a photon index value of 2.22 is reached, followed by a subsequent proportional increase. The behaviour observed for seed photon temperatures (kTee) exhibits an inverse pattern compared to nH. The accretion disk temperature (Tin) displays an overall increase across the examined range of photon index.
