The recurrent nova RS Oph brightens in X-rays

ATel #13810; G. J. M. Luna (IAFE/Conicet), J. Sokoloski (Columbia University), K. Mukai (NASA/GSFC), P. Kuin (MSSL/UCL)
on 16 Jun 2020; 11:59 UT
Credential Certification: Gerardo Juan Manuel Luna (gjmluna@iafe.uba.ar)

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On 2020-05-22 01:32:34 UTC, Swift observed the recurrent nova with the XRT and UVOT telescopes with an exposure time of 3.1 ks. The XRT detected RS Oph at a rate of 0.013 +-0.003 c/s in the 0.3-10 keV energy range. About 40 source counts were detected, all with energies below ~3 keV. A preliminary fit with an absorbed optically thin thermal plasma model yields nH of less than 10^22 cm^-2, kT=1.5 (0.52,2.16) keV with subsolar abundances (< 0.5 solar) and unabsorbed flux of ~2e-12 ergs/cm^2/s [luminosity of ~5e32 ergs/s at a distance of 1.4 kpc (Barry et al., 2008, ASPC, 401, 52)]. RS Oph was also detected with the UVOT through the UVW1 filter with a magnitude of 12.67+-0.05 (Vega system). Before the observation reported here, RS Oph was observed with Swift/XRT a few times during the year 2014 and detected at a rate about 10 times lower than in 2020 May. The AAVSO light curve shows that on average during 2014 and through 2016, the V magnitude was 11.2, whereas since mid-2017 it has been about 0.7 magnitudes brighter. It is worth remarking that the last eruption of RS Oph occurred in February 2006, 14 years ago, and that historically the quiescent periods varied from 9 to 21 years (Schaefer et al. 2010, ApJ, 187, 275). The next eruption could therefore occur at any time. We plan to request additional Swift observations to determine whether this brightening is part of a longer trend. We thank the Swift Neil Gehrels PI, Brad Cenko, for approving the ToO, as well as the Swift planning and operations teams for their continuing support.