Swift X-ray observations confirm outburst of GX 339-4

ATel #5252; Devraj Pawar (R. J. College, U. Mumbai), Diego Altamirano (UvA), Gregory R. Sivakoff (U. Alberta), James Miller-Jones (ICRAR Curtin), Tomaso Belloni (INAF), Dipankar Maitra (Wheaton College, MA and U. Michigan, MI) Michelle Buxton (Yale Univ.), Jeroen Homan (MIT), Dave Russell (IAC, Tenerife), Fraser Lewis (Faulkes Telescope Project, Univ. of South Wales), John Tomsick (SSL/UCB), Mickael Coriat (University of Cape Town)
on 6 Aug 2013; 16:38 UT
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Credential Certification: Diego Altamirano (d.altamirano@uva.nl)

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On August 2, 2013, Buxton et al. (Atel #5244) reported optical and near-infrared flaring of the canonical black hole X-ray binary (BHB) GX 339-4. An increase in the optical and infrared band usually indicates the onset of an X-ray outburst in BHBs.

We requested X-ray observations of GX 339-4 with Swift/XRT and the first observation was performed on August 6, 2013 (03:23:59 UTC, MJD = 56510.1417) in PC mode (ObsID: 00032490012, total of ~1.1 ks). Inspection of the image clearly shows a bright source with coordinates consistent with those of GX 339-4.

Following Evans et al. (2009, MNRAS, 397, 1177) we extracted an X-ray spectrum and find GX 339-4 at a 0.5-10 keV count rate of 2.3 cts/sec. The spectrum is well fit in the energy range 0.3 to 10 keV with an absorbed power law with photon index of 1.37+/-0.20 (unabsorbed flux of 1.58e-10 erg/cm2/s in 0.3 to 10 keV) for neutral Hydrogen column density (4.6+/-0.9)e21/cm2 (using phabs and "wilm" abundances in XSPEC). Assuming lower limit of the distance at 6 kpc (Hynes et al. 2004 ApJ 609, 317) we estimate a 2-10 keV luminosity of at least ~6.8e35 ergs/s.

Given that GX 339-4 is well above its quiescent flux level of ~5e-13 erg/cm2/s (ATel #3383) our Swift observation confirms that GX 339-4 is in outburst. Furthermore, the luminosity and X-ray spectrum are consistent with GX 339-4 being in the low-hard state. At this time, we can not yet predict whether GX 339-4 will undergo a full outburst with state transitions (e.g., 2010-2011: ATel #2577, ATel #3117) or a failed outburst (e.g., 2012: ATel #4247).

GX 339-4 will be observed daily by Swift for ~1 ks observations at least for the next 10 days. We strongly encourage observations at other wavelengths to follow the rise of the outburst.

Coordinated announcements of multi-wavelength observations of this outburst can be found at

http://www.astro.virginia.edu/xrb_jets/status.2013.html.

Please contact sivakoff@ualberta.ca to include any observations you are scheduled to perform.

We thank the Swift team for scheduling our observations.