AstroSat view of a quasi-regular large amplitude variability during the current outburst in IGR J17091-3624
ATel #15343; Pinaki Roy (IISER Mohali)
on 22 Apr 2022; 08:24 UT
Credential Certification: Pinaki Roy (pinaki.roy1989@gmail.com)
Subjects: X-ray, Binary, Black Hole
AstroSat observed the black hole X-ray binary, IGR J17091-3624, for the second time during its current outburst. Previously, this outburst was observed with NICER (ATels #15282, #15287, #15304), AstroSat (ATel #15298) and other observatories (ATels #15283, #15284, #15286, #15295, #15297). The follow-up AstroSat observation took place on 01 April 2022 06:09:07 (UT) till 2 April 2022 05:53:54 (UT) i.e. MJD 59670.26 - 59671.25 for 36.3 ks exposure. The AstroSat/LAXPC count rate evolved between 730 and 2266 ct/s. The latest AstroSat observation shows a quasi-regular large amplitude variability
around 0.1 Hz. Such a variability was earlier seen from this source (upper panel, Fig. 4, Pahari et al., 2012), and was called Class V (Court et al., 2017) and μ class (in comparison with a similar variability in GRS 1915+105; Belloni et al., 2000) variability. Similar pattern was seen in the March 28 NICER observation (ATel #15304). From the analysis of NICER data, we have found that a similar variability continued at least up to 06 April 2022.
The 3-25 keV AstroSat/LAXPC spectrum can be described by an absorbed disk blackbody and thermal comptonisation. The best fit (chi^2/d.o.f. = 34.8/31) yields a photon index of 2.4+/-0.1, an inner disk temperature of 1.38+/-0.07 keV and an electron temperature of 4.1+/-0.1 keV. An iron line at 6.4 keV has been included to improve the fit. The absorbed flux in 3-25 keV band is 7.7+/-0.1 E-9 erg cm^-2 s^-1. The timing and spectral results indicate that the source was in the intermediate state as noted in ATel #15304.
Acknowledgement: The author thanks the AstroSat team and the NICER team for scheduling the observations and making the data available.
Segments of the AstroSat light curve with 1 s binsize
