# 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

## TL;DR

This study presents simultaneous optical and X-ray observations of GS 1354-64 during a 2015 outburst, providing evidence that optical emission is primarily due to cyclo-synchrotron radiation from a hot electron cloud in the accretion flow.

## Contribution

It offers the first simultaneous optical/X-ray analysis of GS 1354-64 during outburst, revealing the physical origin of optical photons as cyclo-synchrotron emission from the hot accretion flow.

## Key findings

- Optical and X-ray show anti-correlation with X lagging optical.
- Detection of ~18 mHz QPOs in both optical and X-ray spectra.
- Spectral analysis indicates a non-thermal, power-law dominated emission consistent with cyclo-synchrotron radiation.

## Abstract

We present results from simultaneous optical (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 show a strong anti-correlation with X-ray photons lagging optical. Optical and X-ray power spectra show quasi-periodic oscillations at a frequency of ~18 mHz with a confidence level of at least 99%. Simultaneous fitting of Swift/XRT and INTEGRAL spectra in the range 0.5-1000 keV shows non-thermal, power-law dominated (> 90%) spectra with a hard power-law index of 1.48 +/- 0.03, inner disc temperature of 0.12 +/- 0.01 keV and inner disc radius of ~3000 km. All evidence is consistent with cyclo-synchrotron radiation in a non-thermal, hot electron cloud extending 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 ~0.19 Hz QPO is observed from the X-ray power spectra, the optical variability is dominated by the broadband 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.