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

We present results from simultaneous optical [South African Large Telescope (SALT)] and X-ray (Swift and INTEGRAL) observations of GS 1354-64/BW Cir during the 2015 hard state outburst. During the rising phase, optical/X-ray time series shows a strong anti-correlation with X-ray photons lagging optical. Optical and X-ray power spectra show quasi-periodic oscillations (QPOs) at a frequency of similar to 18 mHz with a confidence level of at least 99 per cent. Simultaneous fitting of Swift/XRT and INTEGRAL spectra in the range 0.5-1000.0 keV shows non-thermal, power-law-dominated (>90 per cent) spectra with a hard power-law index of 1.48 +/- 0.03, inner disc temperature of 0.12 +/- 0.01 keV and an inner disc radius of similar to 3000 km. All evidence is consistent with cyclo-synchrotron radiation in a non-thermal, hot electron cloud extending to similar to 100 Schwarzschild radii being a major physical process for the origin of optical photons. At outburst peak about one month later, when the X-ray flux rises and the optical drops, the apparent features in the optical/X-ray correlation vanish and the optical auto correlation widens. Although similar to 0.19 Hz QPO is observed from the X-ray power spectra, the optical variability is dominated by the broad-band noise, and the inner disc temperature increases. These results support a change in the dominant optical emission source between outburst rise and peak, consistent with a weakening of hot flow as the disc moves in.