AstroSat observations of GRS 1915+105 during the recent re-brightening phase

ATel #14811; Ravishankar B. T. (U R Rao Satellite Centre, Bengaluru), Tilak Katoch (Tata Institute of Fundamental Research, Mumbai), Athulya M. P. (Dayananda Sagar University, Bengaluru), Radhika D. (Dayananda Sagar University, Bengaluru), Vivek K. Agrawal (U R Rao Satellite Centre, Bengaluru), Anuj Nandi (U R Rao Satellite Centre, Bengaluru)
on 26 Jul 2021; 15:48 UT
Credential Certification: D. Radhika (radhikad_isac@yahoo.in)

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The enigmatic black hole X-ray binary GRS 1915+105 which had been in activity for more than two decades ever since its discovery in 1992, had stopped showing significant variability for more than two years since March 2019. Even during the longest low flux phase, the source has been displaying sudden flaring activities, with the most recent such activity begun since 2021-07-05 UTC and was reported with detection of a Low Frequency Quasi-periodic Oscillation (LFQPO) at 0.17 +/- 0.01 Hz by NICER (ATel #14792, Neilsen et al. 2021). The MAXI flux of the source had shown a gradual increase from 0.2 to 0.4 photons/cm^2/s since 2021-07-05 UTC. We proposed for a follow-up observation with AstroSat during the period of 2021-07-22 18:39:17 UTC (MJD 59417.77) to 2021-07-24 15:31:47 UTC (MJD 59419.64). At the beginning of the AstroSat observations, the nearest MAXI flux in 2.5 - 10 keV band available is on 2021-07-22 17:46:16 UTC (MJD 59417.74) at 0.15 +/- 0.07 photons/cm^2/s (~ 61.6 mCrab). During the AstroSat observations, from the available MAXI data points, we noticed that the flux values have increased to 0.22 +/- 0.02 photons/cm^2/s (~ 88 mCrab) on 2021-07-23, and 0.32 +/- 0.02 photons/cm^2/s (~ 129.17 mCrab) on 2021-07-24. The preliminary analysis of the AstroSat/LAXPC observations have been performed. We noticed significant variation in the power density spectrum (PDS) during the 2 days of AstroSat observation period. On MJD 59417.77, the PDS obtained for 3 - 80 keV energy range and in 0.002 - 10 Hz frequency range, revealed a strong LFQPO of 0.16 Hz (Q-factor of 2.8, amplitude of 8.7%, significance of 10.5), with the total rms variability of 12.5%. While, during MJD 59419.39, we found significant variation in the nature of the PDS, with the QPO frequency increased to 0.186 Hz (Q-factor = 3.27, amplitude = 19.6%, significance = 10.6) and the total rms variability increased to 31.3%. The source energy spectra in 3 - 25 keV band, comprised of a thermal multi-colour disc black body (modelled with diskbb) and a Comptonised component (modelled with nthcomp). During the first observation on MJD 59417.77, the spectral fit resulted in a disc temperature (Tin) of 1.09 (-0.03,+0.02) keV, photon index (Gamma) of 2.13 +/- 0.03 with the electron temperature (kTe) at 3.64 +/- 0.09 keV. The source unabsorbed flux in 3 - 25 keV band was calculated to be of 2.45e-9 erg/cm^2/s. Two days later (MJD 59419.39), the spectral parameters varied to Tin = 0.98 +/- 0.08 keV, Gamma = 2.94 +/- 0.03 and kTe = 11.5 +/- 1 keV. The unabsorbed source flux was seen to have increased to 3.58e-9 erg/cm^2/s. A fluorescent emission (Fe-line) varying in the energy range 6.46 - 6.67 keV during the observation period was also evident in the energy spectra. All the errors are quoted at 90% confidence interval. Even though the obscuration signature was not observed in the LAXPC spectra, it will be further explored considering broad-band spectral data using SXT and LAXPC of AstroSat. The preliminary results of AstroSat observations can be found at https://www.issdc.gov.in/docs/as1/ATel_GRS1915_plots_v2.pdf. Considering the gradual increase in source flux and variation of the spectral and temporal characteristics, we encourage multi-wavelength observations to monitor for any further changes in the source behaviour. We thank the AstroSat Time Allocation Committee, Satellite operations team, and ISSDC team for the quick response to our ToO proposal and data release.

Preliminary results from AstroSat