Classification of low mass X – ray binaries with spectral analysis

Studies of X-ray binaries indicate the presence of thermal and non-thermal emission components in their spectra. The thermal component, thought to be associated with the inner parts of the accretion disk, is typically modeled by a single- or multi-color blackbody model. The non-thermal component on the other hand is usually represented as a power-law (PL) to characterize whether the spectrum is soft or hard. Recent studies of a small subset of lowmass X-ray binaries (LMXBs) in the luminosity range of ~1034 – 1037 erg/s showed that the spectra of neutron-star X-ray binaries (NSXBs) soften with decreasing luminosity while the black-hole X-ray binaries (BHXBs) in the same low-luminosity range have significantly harder spectra. This softening is characterized in terms of an anti-correlation between the spectral index (Γ) and the observed X-ray luminosity (Lx) in the 0.5 – 10 keV energy range. In this thesis, we study the behavior of four such systems in the low-luminosity region (~ 1030 – 1037 erg/s). In particular, we investigate the relationship between the spectral index and the X-ray luminosity of the following sources: 2 BHXBs (2S 0921-63, 4U 1957+11), and 2 NSXBs (4U 1608-52, MAXI J1807+132). We deploy data from the NICER, SWIFT, and XMM − Newton archives. Our analysis suggests that the generic correlation survives for these individual sources in their long-term behavior.