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Swift X-ray observations indicate that the 2013 outburst of GX 339-4 is probably ending

ATel #5594; Devraj Pawar (R. J. College, U. Mumbai), Diego Altamirano (U. Southampton), Gregory R. Sivakoff (U. Alberta), James Miller-Jones (ICRAR Curtin), Tomaso Belloni (INAF-OAB), Dipankar Maitra (Wheaton College, MA and U. Michigan, MI), Michelle Buxton (Yale Univ.), Jeroen Homan (MIT), Dave M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project, Univ. of South Wales and Astrophysics Research Institute, LJMU), John Tomsick (SSL/UCB), Mickael Coriat (University of Cape Town), Teo Munoz-Darias (Oxford)
on 23 Nov 2013; 09:55 UT
Credential Certification: Devraj Pawar (devrajdp@gmail.com)

Subjects: X-ray, Black Hole, Transient

A new X-ray outburst of the Black hole transient GX 339-4 was reported on August 6, 2013 (ATels #5244 and #5252). Since then Swift/XRT has monitored GX 339-4 every other day.

The first observation by Swift/XRT on August 6, (MJD 56510.14) detected the source at a flux of 1.58e-10 erg/cm^2/s, (0.3-10 keV, following Evans et al., 2009, MNRAS, 397, 1177). The flux gradually increased and reached a maximum (for this outburst) of 2.34e-9 erg/cm^2/s on September 8, i.e. 33 days after the first X-ray detection. Assuming a lower limit on the distance of 6 kpc (Hynes et al. 2004 ApJ 609, 317), the peak flux translates to a 0.3-10 keV luminosity of at least ~9.6e36 erg/s during this outburst.

In later observations the flux decreased monotonically and the source went below detection level in MAXI (2-20 keV) on November 13 (MJD 56609) and also in Swift/BAT on November 15 (MJD 56611) suggesting that GX 339-4 is continuing to fade towards quiescence. Due to sun constraints, the last Swift/XRT observation was taken on October 31, MJD 56593. We detected a flux of 6.3e-11 erg/cm^2/s (0.3-10 keV), which is much higher than the lowest flux (1.8e-13 erg/cm^2/s) reported for GX 339-4, and is probably its quiescence flux (ATel #4247). However given that the Swift/XRT flux decayed monotonically until the last observation on 31st October and that the source is no longer detected with the BAT nor MAXI, we conclude that this outburst is probably ending. Precise flux values will not be available until the Swift/XRT observing window begins on January 20, 2014.

The source remained in the hard state during this outburst (ATel #5417). Radio and X-ray observations within 0.4 MJD of each other indicate that during this outburst the radio and X-ray fluxes are consistent with the radio - X-ray relation for GX 339-4 (Corbel et al. 2013, MNRAS, 428, 2500), ATel #5285.

The optical flux of GX 339-4 peaked around September 2 (MJD 56537), with magnitudes on that date of V = 15.78, R = 15.32, i' = 15.13 (errors are <0.05 mag) from data taken with the 2-m Faulkes Telescope (FT) South (located at Siding Spring, Australia). Since then the optical flux has decreased, with an exponential decay rate in V-band of 0.026 mag/day. Due to visibility constraints the source has not been observed since October 19 (MJD 56584) when the magnitudes were V = 17.15, R = 16.31, i' = 16.12, which are still ~2.5 mag brighter than quiescent flux levels. The optical fractional rms variability has been 10-16 % in i'-band during this hard state outburst (the time resolution was 80-90 seconds).

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 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.

We thank the Swift team for scheduling observations.