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

TL;DR

This study analyzes RXTE data of GRO J1655-40 during its 2005 outburst, revealing a hysteresis pattern in the timing properties and suggesting a shift in the radiation mechanism related to the accretion state changes.

Contribution

It provides new evidence of hysteresis in the timing properties and identifies different critical luminosities during rise and decay, advancing understanding of accretion processes in black hole binaries.

Findings

Power spectrum shifts to higher frequencies with increasing luminosity.

Hysteresis observed in frequency-luminosity relationship.

Critical luminosity differs between rise and decay phases.

Abstract

We have analysed RXTE archival data to study the aperiodic variability of the black-hole binary GRO J1655-40 during the hard state of the 2005 outburst. 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.