Hysteresis of spectral evolution in the soft state of black-hole binary LMC X-3

TL;DR

This study reports the discovery of hysteresis in the spectral evolution of black-hole binary LMC X-3 during its soft state, showing spectral-luminosity disconnects similar to transient state transitions.

Contribution

It provides evidence of hysteresis effects occurring without full state transitions in a persistent black-hole binary, expanding understanding of spectral state behaviors.

Findings

Hysteresis observed between spectrum and luminosity in LMC X-3

Power-law component appears during luminosity decline

Hysteresis can occur without full state transition

Abstract

We report the discovery of hysteresis between the x-ray spectrum and luminosity of black-hole binary LMC X-3. Our observations, with the Proportional Counter Array on the Rossi X-ray Timing Explorer, took place entirely within the soft spectral state, dominated by a spectral component that was fitted well with a multicolor disk blackbody. A power-law component was seen only during times when the luminosity of the disk blackbody was declining. The x-ray luminosity at these times was comparable to that seen in transient systems (x-ray novae) when they return to the hard state at the end of an outburst. Our observations may represent partial transitions to the hard state; complete transitions have been seen in this system by Wilms et al. (2001). If they are related to the soft-to-hard transition in transients, then they demonstrate that hysteresis effects can appear without a full state transition. We discuss these observations in the context of earlier observations of hysteresis within the hard state of binaries 1E 1740.7-2942 and GRS 1758-258 and in relation to published explanations of hysteresis in transients.