Variability of X-ray binaries from an oscillating hot corona

TL;DR

This paper models the spectral and timing variability of X-ray binaries by analyzing an oscillating hot corona influenced by magneto-acoustic waves, revealing how boundary conditions and external radius affect observed X-ray variability features.

Contribution

It introduces a new analytical and simulation-based model of an oscillating hot corona, linking wave dynamics to X-ray variability in binaries, with implications for interpreting observed spectral features.

Findings

Corona acts as a low band-pass filter for variability.

Resonant peaks in power spectral density depend on boundary conditions.

Variability increases with energy and is inversely related to external radius.

Abstract

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 in for different boundary conditions. The resulting X-ray luminosity, through thermal comptonization of embedded soft photons, is then computed, first 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: (1) the corona plays the role of a low band-pass medium, its response to a white noise excitation being a at 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 rj. (4) Computed rms and f-spectra exhibit an overall increase of the variability with energy. Comparison with observed variability features, especially in the hard intermediate states of X-ray binaries are discussed.