Sub-second optical flaring in GX 339-4 during the 2017 outburst early rise

ATel #10820; P. Gandhi (Southampton), M. M. Kotze, D. A.H. Buckley (SAAO), J. A. Paice, D. Altamirano, P. A. Charles (Southampton), D. M. Russell (NYU Abu Dhabi), A. C. Fabian (Cambridge)
on 5 Oct 2017; 12:59 UT
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Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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The black hole X-ray binary GX 339-4 has been caught at the early stage of a new outburst. According to optical monitoring, the outburst began between 2017-08-24 and 2017-09-14, presumably in the outer disc (ATel #10797). The first Swift/XRT observation was carried out on 2017-09-29 (~UT 05:30) and showed the source to be well detected in X-rays with a spectrum characterised by a hard-state power law and a flux of 7.2(+1.4,-1.2)e-11 erg/s/cm2 (90% uncertainties) over ~0.3-10 keV (ATel #10798). By this time, the optical brightness had risen to ~2.2-2.6 mag above quiescence (e.g. V=17.4, R=16.5; ATel #10797). Subsequent XRT observations (~1 ks PC mode exposures) on 2017-10-01 and 2017-10-03 show further X-ray brightening, with flux levels of between ~(0.9-1.6)e-10 erg/s/cm2. The detection of a flat-spectrum radio source by ATCA on Sep 30 is consistent with the presence of a compact jet in the hard state (ATel #10808).

Sub-second optical and infrared flaring has been found in GX 339-4 during past hard state outbursts (e.g. Motch et al. 1982 A&A 109 L1, Gandhi et al. 2010 MNRAS 407 2166, Casella et al. 2010 MNRAS 404 L21). We thus triggered a SALT Target of Opportunity observation (PI: Buckley) with SALTICAM slot mode (O'Donoghue et al. 2006 MNRAS 372 151). The observation was carried out on Oct 01 starting at 18:06 UT for just under 2000 s through a clear fused silica filter. Observing conditions were clear. The exposure time per frame was close to 0.14 s.

A segment of the light curve is shown here. There is strong sub-second flaring, well in excess of a brighter field comparison star observed simultaneously. The peak-to-peak variability is a factor of 5 across the full light curve. A handful of the strongest sub-second flares show factor ~2 changes between adjacent time bins. The fractional r.m.s. of the source light curve is 0.22.

This is one of the earliest detections of sub-second optical variations during the rise of an outburst for GX 339-4. Most previous reports have been for observations carried out much farther along the outburst and/or decline. The sub-second optical flaring during the rise appears now to be qualitatively similar. There are, additionally, hints of quasi-periodicities (e.g. at timescales of a few hundred seconds); power spectral analysis is ongoing.

The origin of such flaring in GX 339-4 and other sources is still debated (e.g. Merloni et al. 2000 MNRAS 318 L15, Yuan et al. 2005 ApJ 620 905, Durant et al. 2009 MNRAS 392 309, Gandhi et al. 2010, Veledina et al. 2013 MNRAS 430 3196, Malzac 2014 MNRAS 443 299, Gandhi et al. 2016 MNRAS 459 554). There is likely to be a mixture of components contributing on multiple timescales. The presence of a hard-state flat-spectrum radio source (ATel #10808) could favour a compact jet origin for the fastest sub-second optical fluctuations (e.g. Gandhi et al. 2010).

The source is setting and is no longer visible with SALT, so we encourage rapid multiwavelength timing from any other instrument/observatory. Assuming that this outburst will last several months, there ought to be opportunities for planning observations during the presumptive outburst decline from early 2018 onwards (when ground-based optical visibility will also be improving). However, any opportunities to carry out further rapid timing during the outburst rise now would be particularly valuable.