Variability of X-ray binaries from an oscillating hot corona

The spectral and timing properties of an oscillating hot thermal corona are investigated. This oscillation is assumed to be due to a magneto-acoustic wave propagating within the corona and triggered by an external, non-specified, excitation. A cylindrical geometry is adopted and, neglecting the rotation, the wave equation is solved for different boundary conditions. The resulting X-ray luminosity, through thermal Comptonization of embedded soft photons, is then computed analytically, assuming linear dependence between the local pressure disturbance and the radiative modulation. These calculations are also compared to Monte Carlo simulations. The main results of this study are as follows. (1) The corona plays the role of a low bandpass medium, its response to a white noise excitation being a flat-top noise power spectral density (PSD) at low frequencies and a red noise at high frequency. (2) Resonant peaks are present in the PSD. Their powers depend on the boundary conditions chosen and, more specifically, on the impedance adaptation with the external medium at the corona inner boundary. (3) The flat-top noise level and break as well as the resonant peak frequencies are inversely proportional to the external radius r_j. (4) Computed rms and f-spectra exhibit an overall increase in the variability with energy. Comparison with observed variability features, especially in the hard-intermediate states of X-ray binaries, are discussed.