MR. NAGENDRA KUMAR

A thermal Comptonization model is described that predicts both the fractional amplitude (rms) and the time lag versus energy for Quasi-periodic Oscillation (QPO) at kHz QPO frequency for neutron star low mass X-ray binaries (LMXB). A simplified geometry is considered where a spherical seed photon source is surrounded by a Comptonizing medium. We also consider the possibility that a fraction ($\eta$) of the Comptonized photons impinge back onto the seed photon source. Two sets of spectral parameters (which correspond to the soft and hard states) are considered to explore the general picture. We find that in general a finite $\eta$ may explain the observed soft lags in these QPOs.