AstroSat observations of IGR J17091-3624 during its ongoing outburst

ATel #15298; Pinaki Roy (IISER Mohali), Aru Beri (IISER Mohali, U. Southampton), Sudip Bhattacharyya (TIFR)
on 26 Mar 2022; 11:39 UT
Credential Certification: Aru Beri (aru.beri8@gmail.com)

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The black hole X-ray binary, IGR J17091-3624, exhibits exotic variabilities similar to the well known black hole binary source GRS 1915+105. Several outbursts from this source have been seen in the past, the last one being in 2016 (ATels #8742, #8761). Recently it was found to be in outburst from the Swift/BAT X-ray transient monitor which led to a series of follow-ups by NICER (ATels #15282, #15287) as well as by other observatories (ATels #15283, #15284, #15286, #15295, #15297). A follow-up observation was scheduled with AstroSat on 19 March 2022 09:16:41 (UT) till 22 March 2022 00:47:09 (UT) i.e. MJD 59657.4 - 59660.0 for an exposure of 100 ks.

The light curve shows a variability with count rates varying between 638 and 2090 ct/s. In the power spectra, we detect type-C QPOs with centroid QPO frequency increasing from 3.33+/-0.04 Hz to 8.14+/-0.12 Hz and its rms amplitude dropping from ~5% to ~2%. The 3-25 keV broadband spectrum continuum observed with LAXPC can be described using an absorbed disk blackbody and thermal comptonisation. The best fit (chi^2/d.o.f. = 33.8/31) yields a photon index of 2.5+/-0.1, an inner disk temperature of 1.19+/-0.04 keV and an electron temperature of 9.1+2.3/-1.3 keV. The spectral residuals indicate the presence of an iron line at 6.4 keV. The absorbed flux in 3-25 keV band is 6.7+/-0.1 E-9 erg cm^-2 s^-1. Our timing and spectral results also indicate transition from hard to soft state as observed in ATels #15287, #15295, #15297. A detailed analysis of the data is underway.