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

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.

💡 Research Summary

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The paper presents a systematic study of the rms‑intensity diagram (RID) for the black‑hole X‑ray binary GX 339‑4, using all available Rossi X‑ray Timing Explorer (RXTE) observations from its 2002, 2004 and 2007 outbursts. For each observation the authors compute the net count‑rate in the 2–15 keV band and the root‑mean‑square (rms) amplitude of the aperiodic variability integrated over 0.1–64 Hz. Plotting rms against count‑rate yields a two‑dimensional diagram that reveals distinct tracks corresponding to the canonical spectral states of black‑hole transients: the hard state, the soft state and the intermediate (transition) states.

In the hard state the RID shows a nearly linear, positive rms‑flux relation. As the count‑rate rises, rms increases proportionally, maintaining a variability fraction of roughly 30–40 %. This behavior indicates that a single, non‑thermal component—commonly identified with a hot corona or the base of a compact jet—dominates the emission and is intrinsically highly variable. The authors argue that a single power‑law component with a power‑law shaped power‑spectral density can reproduce the observed ~40 % rms at low fluxes.

When the source moves into the soft state, the RID changes dramatically. Even at high count‑rates the rms drops below 5 %, effectively erasing the rms‑flux correlation. This low variability is interpreted as the emergence of a stable, thermal accretion‑disc component that supplies >80 % of the X‑ray flux. The disc’s emission is intrinsically steady on the examined time scales, leading to a negligible rms contribution.

The intermediate states, which bridge the hard and soft regimes, occupy a complex region of the RID. Here the rms can either plunge sharply (hard‑to‑soft transition) or rise again (soft‑to‑hard transition), producing a “flip” in the rms‑flux relation. These rapid changes are taken as signatures of a re‑configuration of the energy coupling between the corona and the disc, possibly involving the collapse or re‑formation of the jet base and the inward propagation of the disc truncation radius.

A key result of the study is that the RID provides a more direct diagnostic of state transitions than the traditional hardness‑intensity diagram (HID). While the HID relies on spectral colour changes, the RID captures the intrinsic variability amplitude, allowing the identification of transition moments that may be spectrally subtle. For instance, a sudden drop of rms from ~20 % to <5 % marks the onset of the soft state even when the hardness ratio has not yet reached its softest values.

The authors also discuss the physical implications of the observed rms levels. The ~40 % rms at low fluxes suggests that the variability originates from a single, highly stochastic process, such as magnetic reconnection events or fluctuations in the mass‑accretion rate propagating through the hot flow. In contrast, the near‑zero rms in the soft state implies that the thermal disc is a remarkably stable radiator on sub‑second time scales, supporting models where the disc is geometrically thin and radiatively efficient.

In summary, the paper demonstrates that the rms‑intensity diagram is a powerful tool for tracing accretion regimes in black‑hole transients. By linking rms variability to count‑rate, the authors provide a quantitative framework to distinguish hard, soft and intermediate states, to pinpoint state transitions, and to infer the relative contributions of the variable corona and the stable disc. The methodology is readily applicable to other transient systems and will be especially valuable for upcoming high‑throughput X‑ray missions such as NICER, eXTP and Athena, where rapid variability can be measured with unprecedented precision.