The rapid extreme fading of GX 339-4

ATel #4247; Thomas J. Maccarone (University of Southampton), David M. Russell (IAC), Fraser LewisUniv. of Glamorgan, Faulkes Telescope Project
on 9 Jul 2012; 21:16 UT
Credential Certification: Tom Maccarone (tjm@phys.soton.ac.uk)

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We report on a rapid fading of GX 339-4 to its lowest ever observed X-ray flux level. In response to the recent report of the start of an outburst of GX 339-4 (ATel #4162), we submitted a Swift TOO to check its X-ray flux. The observations were performed on July 8, 2012, from 14:00:59 to 15:53:55 UT. The live on-source exposure time of the observations was 2193 seconds. To our surprise, the source was not significantly detected, with a count rate of 0.0025+-0.0017 cts/sec from 0.3-10 keV, using Phil Evans' standard pipeline (Evans et al. 2009, MNRAS, 397, 1177). Assuming a Gamma=1.7 power law, foreground absorption of 4*10^21 cm^-2, and that the best estimate count rate corresponds to a detection, the unabsorbed flux of GX 339-4 would be 1.8*10^-13 ergs/sec/cm^2 in the 0.3-10 keV band, a factor of two below the previously faintest reported level for the source (ATel #196). Given the uncertainty on the flux, the source cannot be conclusively stated to be at its faintest level ever, but it is certainly at a flux level near the previous faintest levels, if not below them. Fortunately, we note that the source is currently rising in the optical; the previous very faint observation was made in late September, when the source was observable only for a short amount of time at high airmass. Thus, this quiescent episode represents an ideal chance to attempt ellipsoidal variability measurements of GX 339-4. The UVOT magnitude was 21.75+-0.36, but some minor contamination from the wings of the PSF of a nearby star is likely, so this value should be taken as an upper limit. This magnitude corresponds to 7.4+-2.5 mJy. Continued monitoring with the Faulkes Telescope South also shows a rapid fade in the optical flux of GX 339-4, by almost 2 magnitudes in 3 days. On July 4 (MJD 56112.6) and July 7 (MJD 56115.4) the magnitudes were V = 17.64, R = 16.69, i'= 16.63; and V = 19.16, R = 18.48, i'= 18.31, respectively (errors are <= 0.06 mag). The magnitudes on July 7 are consistent with the usual low luminosity state of GX 339-4. We do not have an obvious explanation for why the decay time of an outburst would be so much more rapid than the slow rise it had been undergoing previously, but note that it is not particularly common behavior for an X-ray transient. The Faulkes Telescope observations are part of an on-going monitoring campaign of about 30 low-mass X-ray binaries (Lewis et al. 2008). The Faulkes Telescope South is maintained and operated by Las Cumbres Observatory Global Telescope Network. FL acknowledges support from the Dill Faulkes Educational Trust.