Multi-wavelength study of X-ray reprocessing in Low Mass X-ray Binaries

Abstract

Neutron Star Low Mass X-ray Binary systems (NS - LMXBs) consist of a NS accreting matter from a low mass companion star. Primary X-ray photons which are emitted from the NS surface or the inner accretion disk, irradiate various components of the binary, get absorbed and are eventually re-emitted at UV/optical wavelengths. Since these point source systems are not resolvable even using telescopes with the highest angular resolution, X-ray to UV/optical reprocessing becomes a useful tool that can be used to explore such systems. These systems also exhibit a number of X-ray variability signatures like Quasi Periodic Oscillations, thermonuclear bursts, flares, etc. that often get reprocessed into the UV/optical band. Although optical studies have been carried out on LMXB sources before, due to lack of simultaneous X-ray and optical observations, the reprocessing phenomenon has remained largely unexplored. As part of this thesis, results of detailed studies of X-ray to optical reprocessing in several individual LMXBs using X-ray and optical data obtained from a number of space-based X-ray telescopes and ground-based optical telescopes are presented. We have detected a number of interesting reprocessing signatures that indicate the presence of evolving structures on the accretion disk. I further discuss disk wind emission scenarios, their possible correlation to the luminosity state in NS LMXBs and our study of the X-ray spectrum of an LMXB exhibiting disk winds. The thesis is structured in such a manner that the scientific context of the work is presented in the introductory chapters followed by chapters detailing timing and spectral analysis work on three individual LMXBs. In the concluding segment of the thesis, I discuss the possible extention of the results of this thesis work in the context of light curve modeling, inadequacies of the reprocessing phenomenon and binary parameter estimation for potential sources of Gravitational Waves.