Recent softening & decreased hard X-ray flux of IGR J17091-3624 as observed by INTEGRAL

ATel #15297; P. Thalhammer (Remeis-Observatory, FAU Erlangen-Nuernberg), F. Cangemi (CNRS/LPNHE), J. Rodriguez (CEA), T. Belloni (INAF/OAB), P. Laurent (CEA), P.-O. Petrucci (CNRS/IPAG), V. Grinberg (ESA/ESTEC), K. Pottschmidt (CRESST/UMBC/GSFC), E. Egron (INAF/OAC), J. Wilms (Remeis-Observatory, FAU)
on 25 Mar 2022; 17:04 UT
Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

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We report on the most recent INTEGRAL observations of the low-mass X-ray binary/black hole candidate IGR J17091-3624. The source has been shown to go into recurring outbursts every few years (Katoch et al. 2020) and its most recent increase in brightness has been detected in X-Rays (ATel #15282, 15283), as well as in the radio (ATel #15286) and at optical wavelengths (ATel #15284).

In Atel #15283, Grebenev et al. reported on the initial brightening of the source until 2022 March 8/9 as seen by the ISGRI instrument onboard INTEGRAL.

Since then, there have been further INTEGRAL observations on March 10-24th as part of the ongoing monitoring campaign of the Galactic center region. During this period the ISGRI count-rate in the 28-84 keV band decreased from its peak of 89 +/-4 mCrab on 2022 March 13th to 34 +/-6 mCrab on 2022 March 24. This decrease in the hard X-ray flux is associated with a significant reduction in the hardness ratio (HR): During the decline in flux the HR decreased from 0.0 +/-0.2 to -0.4+/-0.2, where the HR was defined as (H-S)/(H+S), where S denotes the count-rate in the 28-45 keV band and H is the 45-84 keV count-rate. These results are in agreement with the recent NICER observation on March 23, which, together with BAT data, indicated a transition from hard to intermediate state (ATel #15287).

See http://sternwarte.uni-erlangen.de/~thalhammer/J17091_mosa_lc.pdf and http://sternwarte.uni-erlangen.de/~thalhammer/J17091_hr.pdf for plots of the light curve and hardness ratio evolution of the source.

Further INTEGRAL observations have been scheduled for the upcoming days that will continue to track the outburst in the hard X-rays.