MR. NAGENDRA KUMAR

Neutron star low-mass X-ray binaries (NS LMXBs) exhibit X-ray variability of quasi periodic nature at millisecond timescales (in frequency domain, in terms as kHz quasi periodic oscillation QPO frequencies) which occurred frequently in lifetime when source have specific spectral characteristic. The main component of X-ray spectrum is explained usually by thermal Comptonization (TC) process and its (TC) parameters are related to kHz QPOs. We have described a thermal Comptonization model for kHz QPOs in NS LMXB which also constrained the size of the Comptonizing medium L with comparing the energy dependency of kHz QPO. L has been estimated correspondingly kHz QPOs evolution for source 4U 1608 52 (transient source) with considering two degenerate spectral model. Time lag associated to TC process is 'hard lag' but the time-lag corresponding to kHz QPO is soft lag. To explain this in TC process, we assume additionally that the some fraction (¿) of Comptonized photons impinge back to the seed photon source and heated it. To estimate the ¿ for different geometry, we have set up a Monte Carlo (MC) experiment (Expt) for TC process. In addition, the hard/soft lag of TC process is inspected with same consideration (as one explain the hard lag, i.e. high energetic photons, on average, experience more scattering than low energetic photon) in this MC Expt computation.