Evidence for a change in the radiation mechanism in the hard state of GRO J1655−40. Hysteresis in the broad-band noise components

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We have analysed archival data from the Rossi X-ray Timing Explorer (RXTE) to study the aperiodic variability of the black hole binary GRO J1655−40 during the hard state of the 2005 outburst. This work was motivated by the recent finding of a spectral change in the hard state X-ray radiation mechanism in black hole binaries. We computed the 0.008–64 Hz power spectral density during the rise and decay of the 2005 outburst, and we found that they were reasonably well modelled by the sum of two, broad Lorenztian functions in most cases (plus a narrow QPO), which correspond to three different variability components. Our aim is to study the evolution of the timing properties of the source during the outburst, by studying the correlation between the characteristics of the broad-band noise components in the power spectra and the source luminosity. Our results suggest that the whole power spectrum shifts to high (low) frequencies as the source luminosity increases (decreases), in agreement with previous studies of other black hole binaries. However, we also detect a strong ‘hysteresis’ pattern in the ‘frequency–luminosity’ plots, and show that the ‘critical’ luminosity limit, above which the timing properties of the source change, is different during the rise and the decay phase of the outburst. We discuss the general implications of these results in the context of the truncated disc model.