A possible state change in the black-hole candidate IGR J17091-3624

ATel #3916; S. P. Drave (Univ. of Southampton, UK), M. Fiocchi (IASF-INAF, ROMA, Italy), V. Sguera (IASF-INAF, Bologna, Italy), A. Bazzano (IASF-INAF, Roma, Italy), A. J. Bird (Univ. of Southampton, UK), L. Sidoli (IASF-INAF, Milano, Italy), E. Kuulkers (ESA/ESAC, Spain) on behalf of the INTEGRAL Galactic Plane Scanning team.
on 7 Feb 2012; 19:38 UT
Credential Certification: Sebastian Drave (sd805@soton.ac.uk)

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The black hole candidate source IGR J17091-3624 (ATel #149, ATel #3144, ATel #3168, ATel #3667) was detected in hard X-rays during the most recent INTEGRAL Galactic Plane Scan observations. The source was observed between UTC 17:57:15 2012-02-05 and 00:38:42 2012-02-06 for an effective exposure of ~ 18.5 ks and 4.3 ks in the IBIS and JEM-X detectors respectively. The source was detected in the IBIS instrument with a flux of 3.52 +/- 0.35 counts/sec (20.1 +/- 2.0 mCrab, 10 sigma) in the 18 - 40 keV band and 1.76 +/- 0.27 counts/sec (20.1 +/- 3.1 mCrab, 6.5 sigma) in the 40 - 100 keV band. A flux of 0.535 +/- 0.08 counts/sec (~ 3.1 +/- 0.5 mCrab) was also detected in the JEM-X instrument corresponding to a significance of 6.7 sigma in the 3 - 10 keV band. Due to the short on source exposures IBIS and JEM-X spectra could not be extracted with sufficient signal to noise to allow a detailed spectral study of the source. However, using the Swift/XRT spectral parameters of Altamirano et al. (ATel #3913) we estimate the JEM-X count rate to correspond to a 2 - 10 keV flux of 1.58e-10 erg/cm2/s, representing a decrease in the soft X-ray flux by a factor of ~ 4 since the XRT observations. Until now, no significant hard X-ray detection has been reported by BAT or IBIS during recent observations. The continued decrease in soft X-ray flux (as reported in ATel #3913 and here) and significant detection at higher energies suggests that IGR J17091-3624 may be undergoing an X-ray state change. We will continue to monitor IGR J17091-3624 in up coming INTEGRAL GPS observations and encourage further follow-up at multiple wavelengths to follow the further evolution of this source.