A ‘high-hard’ outburst of the black hole X-ray binary GS 1354−64
2016
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/Las Cumbres Observatory Global
Telescope Network, Small & Moderate Aperture Research Telescope System and Swift. The
outburst was found to stay in the hard X-ray state, albeit being anomalously luminous with
a peak luminosity of LX > 0.15 LEdd, 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 Comptonization model, and show
softening towards the end of the outburst. In addition, we detect a QPO in the X-ray light
curves with increasing centroid frequency during the peak and decay periods of the outburst.
The long-term optical light curves 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.
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