The optical brightening of GS 1354-64

ATel #7637; D. M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 16 Jun 2015; 07:57 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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We report on optical imaging observations of the black hole transient GS 1354-64 (BW Cir), currently in outburst (ATel #7612, #7614, #7620). From our long-term monitoring observations with the 2-m Faulkes Telescope South at Siding Spring, Australia since 2008 (Lewis et al. 2008), the magnitudes in quiescence are V ~ 21-22, R ~ 20-21, i ~ 20-21 with substantial flickering / long-term variability (see also Casares et al. 2009).

Our first optical detection of the new outburst was on 25 May 2015 (MJD 57167.5), 7 days before the first MAXI 3-sigma 2-20 keV detection and 16 days before the Swift XRT observation that confirmed the new outburst (ATel #7612). The magnitudes on 25 May 2015 were V = 19.39 +- 0.10, R = 18.67 +- 0.06, i = 18.42 +- 0.05, already two magnitudes above the level of quiescence. Since then the source has brightened steadily to V = 18.67 +- 0.07, R = 17.98 +- 0.05, i = 17.62 +- 0.07 on 14 June 2015 (MJD 57187.0) over 19.5 days, at a rate of ~ 0.037, 0.035 and 0.041 mag/day in V, R and i. Our data were acquired with the 2-m Faulkes Telescope South and some of the Las Cumbres Observatory Global Telescope Network 1-m telescopes (at Cerro Tololo Inter-American Observatory, Chile and Siding Spring Observatory, Australia). Our (SDSS filter) i-band magnitudes on 9 and 11 June agree to within 0.3 mag with that reported by Corral-Santana et al. (ATel #7620) on 10-11 June.

At this slow rise rate, it may be a few weeks before the source reaches outburst peak (see ATel #7620 for optical peak magnitudes from previous outbursts). Most black hole transients rise quickly or are only noticed near outburst peak. We therefore encourage multi-wavelength observations of this relatively rare slow rise in the hard state.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory Global Telescope Network.

Faulkes/LCOGT monitoring page of GS 1354-64