Fast variability as a tracer of accretion regimes in black hole transients

TL;DR

This paper introduces the rms-intensity diagram as a tool to distinguish accretion regimes in black hole transients, revealing state-specific variability relations and transitions.

Contribution

It demonstrates that the rms-intensity diagram effectively traces different accretion states and their transitions in black hole X-ray binaries.

Findings

Rms-flux relation varies across different accretion states.

Transitions cause significant changes in variability patterns.

A single variability component explains ~40% of low-rate fluctuations.

Abstract

We present the rms-intensity diagram for black hole transients. Using observations taken with the Rossi X-ray timing explorer we study the relation between the root mean square (rms) amplitude of the variability and the net count-rate during the 2002, 2004 and 2007 outbursts of the black hole X-ray binary GX 339-4. We find that the rms-flux relation previously observed during the hard state in X-ray binaries does not hold for the other states, when different relations apply. These relations can be used as a good tracer of the different accretion regimes. We identify the hard, soft and intermediate states in the rms-intensity diagram. Transitions between the different states are seen to produce marked changes in the rms-flux relation. We find that one single component is required to explain the ~ 40 per cent variability observed at low count rates, whereas no or very low variability is associated to the accretion-disc thermal component.