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

We present the root mean square (rms)-intensity diagram for black hole transients. Using observations taken with the Rossi X-ray Timing Explorer, we study the relation between the 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 similar to 40 per cent variability observed at low count rates, whereas no or very low variability is associated to the accretion-disc thermal component.