Early Swift-XRT observations of the 2021 eruption of RS Oph

ATel #14848; K. L. Page, J. P. Osborne (U. Leicester) & E. Aydi (MSU)
on 11 Aug 2021; 16:57 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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The recurrent nova RS Oph was announced to have re-erupted on 2021 August 8 (K. Geary; vsnet-alert 26131), and has subsequently been detected at high energies by Fermi-LAT (ATels #14834 and #14845), H.E.S.S. (ATel #14844) and MAXI/GSC (ATel #14846).

Swift began observing RS Oph on 2021 August 9.9, 1.3 days after the first hint of an optical brightening on 2021 Aug. 8.57 (taken from AAVSO data). At this time, a bright, hard X-ray source was seen, at a count rate of 3.63 +/- 0.06 count s^-1 (0.3-10 keV; single pixel events only). A second observation, on day 2.2, showed the count rate to have increased to 6.85 +/- 0.08 count s^-1. Each spectrum can be parameterised by an absorbed, single-temperature optically thin component. While the temperature does not change noticeably between the two observations (kT = 5.8 +0.9/-0.7 and 6.1 +1.0/-0.8 keV, respectively), the total absorbing column is found to decrease substantially, from (12.2 +0.8/-0.7) x 10²² cm^-2 during the first observation, to (5.9 +/- 0.3) x 10²² cm^-2 during the second. For comparison, the ISM value in the direction of RS Oph is 2.4 x 10²¹ cm^-2. The unabsorbed 0.3-10 keV flux increased from (1.32 +/- 0.09) x 10^-9 to (1.5 +/- 0.07) x 10^-9 erg cm^-2 s^-1 between the two observations.

The declining absorption is thought to be caused by the progression of the ejecta shock expanding through the wind from the red giant companion (Sokoloski et al., 2006 Nature, 422, 276; Bode et al., 2006, ApJ, 652, 629). These recent Swift observations were obtained slightly earlier than those following the 2006 event.

Due to the current optical brightness of the nova, no Swift/UVOT data can currently be collected.

Swift monitoring continues with a daily cadence. We thank the Swift PI for approving these observations, and the Swift MOC team for implementing them.