Recent activity of the black hole X-ray binary IGR J17091-3624 as observed with the SWIFT/XRT : spectral hardening following the sharp drop in X-ray intensity

ATel #4282; Mayukh Pahari (TIFR, India), Sudip Bhattacharyya (TIFR, India) and J S Yadav (TIFR, India)
on 27 Jul 2012; 10:41 UT
Credential Certification: Mayukh Pahari (mp@tifr.res.in)

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The last X-ray activity of the transient, Galactic, black hole X-ray binary IGR J17091-3624 was reported by Altamirano et al. (Atel #3913), where, using SWIFT/XRT data, they showed the source count rate of 15 +/- 2 cts/s on 31st January, 2012. Using INTEGRAL and IBIS detector, Drave et al. (Atel# 3916) detected the source activity in the 18-40 keV and 40-100 keV energy range respectively.

Here, we are briefly summarizing the source activity using all observations from February 2012 onwards by the SWIFT/XRT. We define the Hardness as the ratio of count rate between 2.15-10.0 keV and 0.10-2.15 keV. From February 4, 2012 to March 02, 2012, the source showed continuous decrease in background-subtracted X-ray flux from 11.8 +/- 1.2 cts/s to 4.6 +/- 0.4 cts/s in the energy range 0.5-10.0 keV. However, no significant change in the average hardness of the source has been observed during this period. From March 02, 2012 to June 20, 2012, no significant variation in the X-ray flux was observed, but the average hardness of the source was increased by a factor of ~ 1.9. Spectral hardening was first observed on May 14, 2012 which becomes more significant on May 30, 2012 and onwards. Finally, on July 12, 2012 the source activity decreased significantly and the source probably faded away below the detectable level of the SWIFT/XRT. However, monitoring the source activity in the hard X-ray band may provide valuable information about the source state.

We fit energy spectra of few observations with absorbed powerlaw model. The NH value was used 1.20 x 10^{-22} cm^{-2}. Initially, the photon power-law index was found to be ~2.62 +/- 0.06 on February 8, 2012. Later it increased to ~2.79 +/- 0.07 on February 28, 2012. Observations during high X-ray flux show that addition of a disk blackbody model component improves the fit with the significance of 4.3-sigma. Due to poor signal-to-noise ratio of low flux state observations, we are unable to carry out energy spectral analysis. An increase in low-frequency noise power in the power density spectrum is observed during the spectral hardening of the source.

Multiwavelength observations may be important in detecting the quiescent state of the source. We thank the SWIFT team for continuous monitoring of the source.