The causal connection between disc and power-law variability in hard state black hole X-ray binaries

TL;DR

This study uses X-ray timing data to show that accretion disc instabilities cause variability in black hole binaries, with different lag times indicating viscous propagation and X-ray heating effects.

Contribution

It extends spectral time-lag analysis into the soft X-ray band, revealing the causal relationship between disc and power-law variability in black hole X-ray binaries.

Findings

Disc variations lead power-law variations by tenths of a second on long time-scales.

Shorter time-scale lags are consistent with X-ray heating effects.

Variability is driven by accretion disc instabilities.

Abstract

We use the XMM-Newton EPIC-pn instrument in timing mode to extend spectral time-lag studies of hard state black hole X-ray binaries into the soft X-ray band. We show that variations of the disc blackbody emission substantially lead variations in the power-law emission, by tenths of a second on variability time-scales of seconds or longer. The large lags cannot be explained by Compton scattering but are consistent with time-delays due to viscous propagation of mass accretion fluctuations in the disc. However, on time-scales less than a second the disc lags the power-law variations by a few ms, consistent with the disc variations being dominated by X-ray heating by the power-law, with the short lag corresponding to the light-travel time between the power-law emitting region and the disc. Our results indicate that instabilities in the accretion disc are responsible for continuum variability on time-scales of seconds or longer and probably also on shorter time-scales.