Journal Article

A ‘high-hard’ outburst of the black hole X-ray binary GS 1354−64

K. I. I. Koljonen, D. M. Russell, J. M. Corral-Santana, M. Armas Padilla, T. Muñoz-Darias, F. Lewis, M. Coriat and F. E. Bauer

in Monthly Notices of the Royal Astronomical Society

Volume 460, issue 1, pages 942-955
ISSN: 0035-8711
Published online April 2016 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1093/mnras/stw1007
A ‘high-hard’ outburst of the black hole X-ray binary GS 1354−64

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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.

Keywords: accretion, accretion discs; binaries: close; X-rays: binaries; X-rays: individual: GS 1354−64; X-rays: stars

Journal Article.  10694 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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