A "high-hard" outburst of the black hole X-ray binary GS 1354-64

TL;DR

This study details the 2015 outburst of the black hole binary GS 1354-64, revealing it was an unusually luminous hard state event with correlated optical, UV, and X-ray emissions, and detecting a QPO with evolving frequency.

Contribution

It provides the first detailed multi-wavelength analysis of an exceptionally luminous hard state outburst in GS 1354-64, highlighting the role of disc irradiation and evaporation processes.

Findings

Outburst reached a peak luminosity > 0.15 L_Edd, possibly the most luminous in a hard state.

Optical/UV emission tightly correlated with X-ray, indicating disc irradiation or jet contribution.

Detected a QPO with increasing frequency during the outburst.

Abstract

We study in detail the evolution of the 2015 outburst of GS 1354-64 (BW Cir) at optical, UV and X-ray wavelengths using Faulkes Telescope South/LCOGT, SMARTS and Swift. The outburst was found to stay in the hard X-ray state, albeit being anomalously luminous with a peak luminosity of L$_{X} >$ 0.15 L$_{Edd}$, which could be the most luminous hard state observed in a black hole X-ray binary. We found that the optical/UV emission is tightly correlated with the X-ray emission, consistent with accretion disc irradiation and/or a jet producing the optical emission. The X-ray spectra can be fitted well with a Comptonisation model, and show softening towards the end of the outburst. In addition, we detect a QPO in the X-ray lightcurves with increasing centroid frequency during the peak and decay periods of the outburst. The long-term optical lightcurves during quiescence show a statistically significant, slow rise of the source brightness over the 7 years prior to the 2015 outburst. This behaviour as well as the outburst evolution at all wavelengths studied can be explained by the disc instability model with irradiation and disc evaporation/condensation.