The Ubiquity of the rms-flux relation in Black Hole X-ray Binaries

TL;DR

This study demonstrates that the linear rms-flux relation is a common feature across various states in black hole X-ray binaries, suggesting it is fundamental to the accretion process.

Contribution

It extends the detection of the rms-flux relation to a larger sample and range of states, highlighting its ubiquity and potential fundamental role in black hole accretion.

Findings

Rms-flux relation is observed in all high signal-to-noise observations.

The flux intercept varies significantly with source state, especially during state transitions.

Little flux dependence is found in broad-band noise shape, except in some soft-state observations.

Abstract

We have investigated the short term linear relation between the rms variability and the flux in 1,961 observations of 9 black hole X-ray binaries. The rms-flux relation for the 1-10 Hz range is ubiquitously observed in any observation with good variability signal to noise (> 3 % 1-10 Hz fractional rms). This concurs with results from a previous study of Cygnus X-1 (Gleissner et. al. 2004), and extends detection of the rms-flux relation to a wider range of states. We find a strong dependence of the flux intercept of the rms-flux relation on source state; as the source transitions from the hard state into the hard intermediate state the intercept becomes strongly positive. We find little evidence for flux dependence of the broad-band noise within the PSD shape, excepting a small subset of observations from one object in an anomalous soft-state. We speculate that the ubiquitous linear rms-flux relation in the broad band noise of this sample, representing a range of different states and objects, indicates that its formation mechanism is an essential property of the luminous accretion flow around black holes.