Speaker
Description
The twin-peak quasi-periodic oscillations (QPOs) observed in neutron star (NS) X-ray binaries, represent a possible tool for probing strong gravity and dense matter equation of state. Nevertheless, the QPO modulation mechanism remains an unresolved puzzle, especially because the QPO amplitudes are sometimes as high as 30% rms. Oscillations of an inner accretion torus can modulate the accretion rate and induce a NS boundary layer (BL) variable luminosity. Using relativistic ray tracing, we investigate light curves originating from variable BL and oscillating toroidal structures of accreted fluid. We identify the scenario which allows for high QPO amplitudes and discuss implications for a variety of QPO models. In the case of simulation including torus exhibiting radial and vertical oscillations, we observe frequency peak corresponding to the vertical oscillations, which has not been identified in previous hydrodynamical simulations of accretion. We also show that when oscillations disrupt the torus stability, the resulting product strongly imprints the Keplerian frequency in the observed light curve.