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

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.

Simultaneous optical/X-ray study of GS 1354-64 (=BW Cir) during hard outburst: evidence for optical cyclo-synchrotron emission from the hot accretion flow

“…This gives an R magnitude of ∼17.384 for the optical counterpart of GS 1354-64 on 08 August 2015. This is very close to the R magnitude observed by Koljonen et al (2016) when compared to their nearest observation on MJD 57245. The Swift/UVOT U filter flux densities from Figure 1 of Koljonen et al (2016) are close to ∼0.055 mJy and ∼0.038 mJy during first and second observations respectively, consistent with the decrease in optical strength between two observations.…”

“…The detection of the X-ray QPO with Swift/XRT at ∼18 mHz is also consistent with the ∼30 mHz QPO detected from the NuSTAR observation taken 6 days later (on 11 July 2015; see bottom panel of Figure 3). When we extrapolate the trend observed from the QPO frequency vs time plot (as shown in Figure 1 from Koljonen et al (2016)) to 05 July 2015, we find that the frequency predicted by the extrapolation matches exactly to the observed one.…”

“…It may be noted that two observations were taken with SALT and NuSTAR during two different X-ray flux states -the first during the rising phase of the outburst on 05 July 2015 (SALT) and 11 July 2015 (NuSTAR) and the second at peak of the X-ray outburst on 08 August 2015 (SALT) and 06 August 2015 (NuS-TAR). If we compare the present outburst (also from Figure 1 in Koljonen et al (2016)) with the 1997 outburst from Brocksopp et al (2001), the optical outburst peak occurs during the rising part of the X-ray outburst while a significant decline in the optical is observed during X-ray outburst peak. Therefore, an evolution in spectral and temporal properties is expected between both observations taken with a gap of ∼1 month.…”

“…The second observation was taken on 08 August 2015 when the X-ray outburst reached the peak intensity (as observed from Figure 3 While the mean Swift/XRT count rate increases from ∼15 cts/s to ∼29 cts/s, the lightcurve presented in Figure 1 of Koljonen et al (2016) indicates decrease in optical flux.…”

“…Using the SOAR Optical Imager (SOI) at Cerro Pachon, Chile, Corral-Santana et al (2015) measured the g', r' and i' magnitudes to be ∼19.76, ∼18.49, ∼18.34 respectively on 10-11 June 2015. The detailed evolution of the 2015 outburst in GS 1354-64 at optical, UV and X-ray bands has been presented by Koljonen et al (2016). They find that the optical/UV emission is tightly correlated with the X-ray emission on time-scales of days.…”