AstroSat observation of X-ray brightening from the black hole X-ray binary V4641 Sgr
ATel #15010; Swadesh Chand (GGV, Bilaspur), G. C. Dewangan (IUCAA, Pune), Parijat Thakur (GGV, Bilaspur), V. K. Agrawal (URSC, ISRO, Bangalore), Prakash Tripathi (IUCAA, Pune) and Pragati Sahu (GGV, Bilaspur)
on 2 Nov 2021; 13:08 UT
Credential Certification: Swadesh Chand (swadesh.chand@gmail.com)
Subjects: X-ray, Black Hole, Transient
Referred to by ATel #: 16804
Following the MAXI/GSC detection of the X-ray brightening from the black hole binary V4641 Sgr (ATel#14968), we observed the source with AstroSat from October 19 at 17:52:01 UTC to October 22 at 18:13:33 UTC, 2021. The source is found to be variable over the full exposure time with average count rate of ~33 cts/s measured with LAXPC20. However, no signature of quasi-periodic oscillation (QPO) is observed in the power density spectra (PDS) derived from the LAXPC20 lightcurve in the 3-50 keV band. The time-averaged X-ray energy spectrum obtained from the LAXPC20 detector in the 3-20 keV band shows the presence of a strong iron line and reflection hump. The spectrum can be well described with an absorbed power-law and diskbb components along with a Gaussian line required for the iron line. The absorption column density (N_H) is about 0.3e+22 cm^-2, which is consistent with Pahari et al. (2015) and the power-law photon index is ~2.0. The best-fit inner disk temperature (KT_in) of ~1.24 keV indicates that the contribution from the disk strongly dominates the X-ray spectrum. The estimated value of the unabsorbed flux in the 3-20 keV band is ~4.4e-09 erg cm^-2 s^-1. Further analysis of this AstroSat observation is in progress. We thank the AstroSat team for granting the ToO observation, and ISSDC, ISRO for making the data available.
