AstroSat X-ray Observations of Recurrent Nova RS Oph

ATel #14882; Sandeep K. Rout, Mudit K. Srivastava, Dipankar P. K. Banerjee, Santosh Vadawale, Vishal Joshi and Vipin Kumar, Astronomy and Astrophysics Division, Physical Research Laboratory (PRL), Ahmedabad, 380009, India
on 28 Aug 2021; 15:32 UT
Credential Certification: Dipankar P.K. Banerjee (dpkb12345@gmail.com)

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We report AstroSat observations of recurrent nova RS Oph (ATel# 14834, ATel# 14838, ATel# 14840, ATel# 14844, ATel# 14845, ATel# 14846, ATel# 14848, ATel# 14849, ATel# 14850, ATel# 14851, ATel# 14852, ATel# 14855, ATel# 14857, ATel# 14858, ATel# 14860, ATel# 14863, ATel# 14864, ATel# 14866, ATel# 14868, ATel# 14872, and ATel# 14873). RS Oph was observed with LAXPC (Yadav et al. 2016, ApJ, 833, 27) and SXT instruments (Singh et al. 2016, SPIE, 99051E) on 2021-08-14.60 UTC, 2021-08-16.02 UTC, 2021-08-21.12 UTC, and 2021-08-22.18 UTC i.e. 5.67, 7.09, 12.19 and 13.25 days after the outburst (August 8.93 UT, K. Geary, VSNET alert N.26131). The exposure times for LAXPC and SXT were in the range of 7000 seconds and 4000 seconds respectively.

The LAXPC (3-15 KeV) and SXT (1-7 KeV) spectra of all the epochs were fitted with an absorbed single-temperature collisionally ionized diffused plasma. The model provides slightly unsatisfactory fits for 14 and 16 August spectra and possibly requires another low temperature component at ~ 0.1 keV (also noticed while fitting the joint NICER and NuSTAR data, Luna et al. ATel #14872/ATel# 14873). The 0.5-20 keV unabsorbed X-ray flux has dropped from 18.5 X 10^{-10} erg/s/cm^2 to 8.0 X 10^{-10} erg/s/cm^2 from 14 to 22 August. The corresponding plasma temperatures were determined to be 6.6 keV, 5.2 keV, 3.0 keV and 2.6 keV for 14, 16, 21 and 22 August respectively. The Fe line at approximately 6.5 keV is clearly visible in the 14 and 16 Aug spectra, which loses its strength on 21 and 22 August.

The trend for the drop in X-ray flux, plasma temperature as well as the evolution of the X-ray spectra are very similar to that observed in the 2006 outburst of RS Oph (Sokoloski et al. 2006, Nature, 442, 276–278). The super-soft X-ray phase of RS Oph in the 2006 outburst commenced ~26 days after the outburst (Osborne et al., 2011, ApJ, 727, 124). If we expect the same trend, this phase would begin soon. However, due to technical observing constraints of the AstroSat mission for RS Oph, it is likely that the super-soft phase will not be observable by AstroSat except during a brief window in beginning-mid October, 2021. Simultaneous ultra-violet observations (~1300-1800A FUV spectroscopy) were also attempted on 21-22 August, but data is yet to be processed.