Probing the nature of the accretion flow in low mass X-ray binaries using the spectral-timing properties of the kiloHertz Quasi-Periodic Oscillations

Over the last two decades several efforts have been made to try and explain the energy-dependent variability observed in low mass X-ray binary (LMXB) systems. The kilohertz quasi-periodic oscillations (kHz QPOs) represent the fastest variability so far observed from neutron-star (NS) LMXBs. We present a model that reproduces the fractional rms amplitude and time lags of the kHz QPOs as a function of photon energy and QPO frequency. Our model builds upon a previously explored idea, namely that the variability is driven by a coupled oscillation of the temperature of the accretion disc and the heating rate (and hence the electron temperature) of the corona. New here is that we explore the effect of the (expected) coupled oscillation of the electron number density (optical depth and physical size) of the corona and a range of physically motivated feed-back mechanisms between the disc and the corona. We further examined the impact of the seed photon spectra, a corona with a non-uniform optical depth and electron temperature, and the physical size of the corona on the QPO properties. We compared the predictions of our model to simultaneous XMM-Newton and the Rossi X-ray Timing Explorer (RXTE) energy spectra, and RXTE power-density and time-lag spectra of the NS LMXB 4U 1636-53. The ultimate goal is to unravel the underlying mechanisms that drive the variability of these sources.