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

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.

4 pages, accepted for publication in Astronomy and Astrophysics, Letters to the Editor

References in corpus (4)

• The Hard X-ray Spectrum as a Probe for Black-Hole Growth in Radio-Quiet Active Galactic Nuclei
• Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate
• Testing a scale-independent method to measure the mass of black holes
• AGN's UV and X-ray luminosities in clumpy accretion flows

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• The non-linear X-ray/UV relation in active galactic nuclei: contribution of instrumental effects on the X-ray variability
• A Soft X-Ray Lag Detected in Centaurus A
• Estimating longterm power spectral densities in AGN from simulations
• Modeling of High-Frequency Variability in X-ray Binaries with Black Holes