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

¹ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
e-mail: wako@ast.cam.ac.uk
² ISDC Data Centre for Astrophysics, ch. d’Ecogia 16, 1290 Versoix, Switzerland
³ Geneva Observatory, Geneva University, ch. des Maillettes 51, 1290 Sauverny, Switzerland
e-mail: Thierry.Courvoisier@unige.ch

Received: 26 January 2012
Accepted: 1 March 2012

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 () 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 find that the Compton cooling timescale directly leads to the observational scaling and naturally reproduces the functional dependence on black hole mass and accretion rate (). This result simply arises from general properties of the emission mechanism and is independent of the details of any specific accretion model.