X-ray spectral study of low mass X-ray binaries

Abstract

Low Mass X-ray Binaries, one of the brightest class of X-ray sources in Galaxy, show several interesting phenomena including X-ray bursts, irregular dips, and rapid variability. In order to study such X-ray properties, we have investigated three LMXBs 1755-338, 1735-444 and Serpens X-1 (1837+049). From the spectral studies, we found that the energy spectra of these sources can be represented by two components, which are associated with two physical sites of X-ray emission, generally taken to be the neutron star surface and the inner accretion disk or the accretion disk corona. The X-ray dip source 1755-338 was observed with Ginga in 1990 and simultaneously with Ginga and ROSAT in 1991. The source was in the ``high```` luminosity state during the observation in 1990, similar to the EXOSAT observation. Regularly spaced dips and no apparent low-energy absorption during these dips are confirmed. During the observation in 1991 the source was in the ``low```` luminosity state with the flux (1-37 keV) less than 35\% of the ``high```` state non-dip level. The spectrum observed in this ``low```` state is much harder than that in the ``high```` state. Also, an iron emission line centered around 6.7 keV is found in the ``low```` state. The data imply that the accreting matter in this system might have the cosmic abundance, contrary to the earlier suggestion that the heavy-element abundances in 1755-338 are extremely low. Simultaneous Ginga and ROSAT observation of the X-ray burst source 1735-444 was made in 1991. During the observation the source was in an unusually high luminosity state and showed the general correlation between the intensity and the hardness. From the analysis of the persistent emission, we show that the source was in the banana branch in the color-color diagram. This indicates that there exist some fundamental differences between atoll sources and Z sources, rather than just a difference in the average mass accretion rate alone. We observed a burst wit...