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Swift/BAT reports renewed activity from IGR J17091-3624

ATel #3144; H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), J. A. Kennea (PSU), C. B. Markwardt (GSFC), D. Palmer (LANL), T. Sakamoto (CRESTT/GSFC/UMBC), G. Skinner (CRESST/GSFC/UMD), M. Stamatikos (OSU/GSFC), J. Tueller (GSFC), T. Ukwatta (GSFC/GWU)
on 3 Feb 2011; 21:10 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 3148, 3150, 3159, 3167, 3168, 3203, 3225, 3229, 3230, 3232, 3266, 3299, 3913, 3916, 4773

The Swift/BAT hard X-ray transient monitor reports renewed activity in the 15-50 keV band from a location consistent with the X-ray binary system and black hole candidate IGR J17091-3624 (aka SAX J1709.1-3624). The source has increased in intensity from its normal average level of 0.0006 cts/cm^2/s (~3 mCrab) up to 0.0039 +/- 0.0016 cts/cm^2/sec (20 mcrab) on 28 Jan 2011 (MJD 55589), rising further to 0.014 /- 0.003 cts/cm^2/sec (60 mcrab) by 03 Feb 2011.

We note that there is another source, also a black hole candidate, IGR J17098-3628, located 8.5 arc minutes from IGR J17091-3624. Although the signal is significantly stronger when centroiding on IGR J17091-3624, the BAT point spread functions for the two sources overlap, so we cannot definitively determine which source is in outburst from the BAT data alone. A Swift target of opportunity observation to resolve these sources using Swift/XRT has been requested and approved.

If it is indeed IGR J17091-3624 in outburst, this would mark the second large outburst (>60 mCrab) of the source seen in the BAT since the launch of Swift in 2004, which was after the discovery outburst of the source in 2003 (Kuulkers et al, ATel #149). The 2007 outburst peaked in July 2007 (peak rate of ~70 mCrab on MJD 54296 ) and lasted approximately 40 days.

IGR J17091-3624 has been classified as a possible black hole candidate (Capitanio, F. et al, ApJ 690:1621, 2009)

BAT hard X-ray transient monitor light curve for IGR J17091-3624