The physical origin of the X-ray power spectral density break timescale in accreting black holes

TL;DR

This paper proposes that the break timescale in the X-ray power spectral density of accreting black holes originates from the electron cooling timescale in the Comptonisation process, explaining the observed scaling with black hole mass and accretion rate.

Contribution

It introduces a physical explanation linking the PSD break timescale to electron cooling, providing a universal derivation independent of specific accretion models.

Findings

The PSD break timescale scales with black hole mass and accretion rate as observed.

The electron cooling timescale naturally reproduces the empirical variability scaling.

The result is derived from general emission properties, not detailed accretion physics.

Abstract

X-ray variability of active galactic nuclei (AGN) and black hole binaries can be analysed by means of the power spectral density (PSD). The break observed in the power spectrum defines a characteristic variability timescale of the accreting system. The empirical variability scaling that relates characteristic timescale, black hole mass, and accretion rate ($T_B \propto M_{BH}^{2.1}/\dot{M}^{0.98}$) extends from supermassive black holes in AGN down to stellar-mass black holes in binary systems. We suggest that the PSD break timescale is associated with the cooling timescale of electrons in the Comptonisation process at the origin of the observed hard X-ray emission. We obtain that the Compton cooling timescale directly leads to the observational scaling and naturally reproduces the functional dependence on black hole mass and accretion rate ($t_C \propto M_{BH}^{2}/\dot{M}$). This result simply arises from general properties of the emission mechanism and is independent of the details of any specific accretion model.