Outburst peak of the black hole transient GS 1354-64
ATel #7887; K. I. I. Koljonen, D. M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 7 Aug 2015; 12:30 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)
Subjects: Optical, X-ray, Binary, Black Hole, Transient
Following the recent outburst activity of GS 1354-64 (BW Cir; ATel #7612, #7614, #7620, #7637, #7656), we report that this black hole transient has reached the peak of its outburst at optical (from our Faulkes/LCOGT monitoring; see ATel #7637), UV (Swift/UVOT), and X-ray wavelengths (MAXI, Swift/BAT), and the source is starting to decay in flux. The rise (~45 days: from MJD 57171 to MJD 57216) and peak (~18 days: from MJD 57216 to MJD 57234) durations correspond well to the previous outburst in 1997 (Brocksopp et al. 2001). However, this time the peak fluxes are slightly lower, with optical magnitudes:
V: 17.60 +/- 0.07
R: 16.89 +/- 0.05
i': 16.70 +/- 0.05
u-band flux density:
0.092 +/- 0.007 mJy
and mean X-ray fluxes:
MAXI 2-20 keV: 0.34 +/- 0.03 cts/cm^2/s
BAT 15-50 keV: 0.052 +/- 0.003 cts/cm^2/s
During a 0.5 hour LCOGT (1-m telescope at SAAO, Sutherland, South Africa) observation on MJD 57235, the optical fractional rms variability was measured in i'-band to be < 7% (from 15 images, at a time resolution of ~120 sec). This is lower than the optical rms of GX 339-4 in the hard state at a similar time resolution (typically >= 10%; Cadolle Bel et al. 2011). Optical, UV and X-ray fluxes rise and peak in unison indicating correlated behavior.
Based on X-ray spectral fits from the publicly available pointings of the Swift/XRT instrument, the outburst is hard. All the Windowed Timing (WT) mode spectra can be fitted with a simple absorbed power law model, excluding a soft excess that seems to originate below 0.8 keV and several emission/absorption lines. The mean column density is N_H = 0.8 +/- 0.1 E+22 cm^-2, and the power law index changes from ~1.3 during the rise, to ~1.6 in the peak indicating a slight spectral softening. The mean unabsorbed power law flux in the X-ray band 0.8-10 keV is 5.8 E-9 erg/s/cm^2 during MJD 57221-57227, which corresponds to a luminosity of 4.3 E+38 erg/s at 25 kpc (Casares et al. 2009). We note that the source remained in the hard state throughout its 1997 outburst, and it is likely to do the same in the current outburst.
Continued multiwavelength observations are encouraged during the outburst decay. 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.
Optical light curves of GS 1354-64
