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Swift-BAT detects a bright hard X-ray outburst from GX 339-4

ATel #968; H. A. Krimm (GSFC/USRA), L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), on behalf of the Swift-BAT team
on 20 Dec 2006; 01:16 UT
Credential Certification: Hans A. Krimm (krimm@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Binary, Black Hole

Referred to by ATel #: 1586, 2270, 2281

The Burst Alert Telescope (BAT) on the Swift satellite has detected the suspected black hole binary GX 339-4 at its brightest level since the launch of Swift in November 2004. This source was observed in a series of short exposures beginning at 1:03:57 UT on December 18, 2006 and continuing until 21;57:04 December 18. The flux (15-50 keV) during this period varied from a low of 0.034 +/- 0.010 cts/s/cm^2 at the start of the observations, to a peak of 0.053 +/- 0.010 cts/s/cm^2 at 2:40 UTC and the same flux at 20:20 UT. The peak flux corresponds to 230 mCrab. A fit to a simple power law for the sum of the observations gave a photon index of 1.71 +/- 0.19.

There is evidence of spectral softening between the two peak observations. At the 2:40 UTC, the counts hardness ratio (25-50)/(15-25) = 9.9 +/- 3.7. At 20:20 UTC, the hardness ratio is 1.7 +/- 1.0.

This is the peak (up to now) of the latest outburst from this source which began on Nov. 19, 2006, shortly after activity was from this source was reported by the RXTE team (ATel #944, Swank et al.). GX 339-4 continued to brighten, reaching ~100 mCrab by December 4, after which it was not observed for 14 days due to its proximity to the Sun.

This is the third period of activity for GX 339-4 observed by the BAT in its transient monitor, http://swift.gsfc.nasa.gov/docs/swift/results/transients/. The previous two episodes were in April 2005 and from January through April 2006. In these earlier episodes the 15-50 keV emission peaked at ~90 mCrab. Both of these episodes were also seen in the RXTE (ATel #455, Smith et al., ATel #707, Bezayiff et al.).

The source GX 339-4 is now approximately 27 degrees away from the Sun. The Sun distance is increasing.

BAT Hard X-ray Transient Monitor