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Unusual optical and X-ray flaring activity in GX 339-4

ATel #1586; D. M. Russell (Univ. of Amsterdam), D. Altamirano (Univ. of Amsterdam), F. Lewis (Las Cumbres Observatory Global Telescope Network (LCOGTN)/Open Univ.), P. Roche (LCOGTN/Cardiff Univ.), C. B. Markwardt (CRESST/GSFC/UMD), R. P. Fender (Univ. of Southampton)
on 23 Jun 2008; 19:21 UT
Credential Certification: Diego Altamirano (diego@science.uva.nl)

Subjects: Optical, X-ray, Black Hole, Transient

Referred to by ATel #: 1588, 1945, 1962, 2270, 2459, 10797

Since September 2007 we have been monitoring the optical counterpart (V, R and i-bands) of the black hole X-ray binary GX 339-4 with the Faulkes Telescope South situated at Siding Spring in Australia. The source has continued to decline from its 2006-7 outburst (ATel #968) until May 2008, in which a period of strong variability began. At its faintest flux level, on 2008-04-29 (MJD 54585.51) the magnitudes were V = 18.39 +- 0.06; R = 18.10 +- 0.03; i = 17.22 +- 0.03. Between then and 2008-05-29 strong flares of amplitudes up to 0.7 mag have been observed on timescales of days, and the mean flux is increasing. The flaring is most prominent in the i-band (i.e. at lower frequencies) suggesting its origin may be synchrotron emission from the jets, which is known to dominate the infrared flux during hard X-ray states (Homan et al. 2005, ApJ, 624, 295). Flaring of this amplitude on timescles of days is very unusual for GX 339-4 except during state transitions at higher flux levels (ATel #1027). Inspection of the RXTE PCA Galactic bulge scan light curve of GX 339-4 also reveals short-timescale flares of up to ~ 5 mCrab (50 counts/sec/5PCU) in the 2-10 keV range. The flares appear on timescales of days or less; most of the PCA scans do not detect the source with 3-sigma confidence. The amplitude of the variability appears to be one order of magnitude or more, which (as far as we are aware) is stronger than previous periods of both outburst and low-flux levels. There are no 3-sigma detections of the source from the RXTE ASM 1-day averages (2-12 keV) during this flaring period, implying the luminosity generally remains below ~ 3 x 10^36 erg/sec (assuming a distance of 8 kpc). There are also no detections in hard X-rays (15-50 keV) from BAT on board SWIFT. To test whether the optical and X-ray flaring behaviours are correlated, we are monitoring the source with SWIFT (UVOT+XRT) for the next 4 weeks. The first UVOT V-band magnitude obtained is V = 17.10 +- 0.18 (on 2008-06-20; MJD 54637.35); one magnitude brighter than 3 weeks prior to this date; V = 18.07 +- 0.05; 17.59 +- 0.02; i = 16.56 +- 0.02 (on 2008-05-29; MJD 54615.40). It is possible that GX 339-4 is beginning a new outburst. Our light curves are shown here http://staff.science.uva.nl/~davidr/faulkes/gx339.html and will be updated regularly during the next few weeks. Observations at radio frequencies are encouraged. The Faulkes observations are part of an ongoing monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0712.2751). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). FL acknowledges support from the Dill Faulkes Educational Trust.

GX 339-4 optical light curves