Shocked plasma in X-ray grating spectra of RS Oph

ATel #14906; M. Orio (INAF-Padova and University of Wisconsin), E. Behar (Technion), J. Drake (Center for Astrophysics), J. Mikolajewska (Nicolaus Copernicus Astronomical Center), J. U. Ness (ESA-ESAC), N. Ospina (INFN and Padova University)
on 10 Sep 2021; 18:18 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Marina Orio (orio@astro.wisc.edu)

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We report on X-ray grating observations of RS Oph done on 2021 August 27 with the Chandra High Energy X-ray Gratings (HETG) and ACIS camera, and on 2021 August 30 with the XMM-Newton Reflection Grating Spectrographs (RGS). The first was obtained in a continuous exposure of 29 kiloseconds, and the second in a continuous exposure of 54 kiloseconds. The dates correspond to approximately day 20 and day 23 after the optical maximum. There were no flares and no large count rate variations in the 0.4-10 keV of the HETG exposure on the first date, neither on the second date in the energy range 0.35-2.5 KeV of the RGS or of the XMM-Newton EPIC cameras (0.15-12 keV for EPIC-pn and 0.3-12 keV for the EPIC MOS). Both grating spectra showed strong emission lines and no luminous supersoft component, which was also not present in the XMM-Newton EPIC cameras on August 30. On August 27 we measured H-like and He-like emission lines of calcium, argon, magnesium and neon and other lines of silicon and iron (Fe XXVI, Fe XXV, Fe XVIII, Fe XVII). The sensitivity of the setup with the ACIS camera, however, is quite reduced above about 20 Å (0.62 keV), so we do not rule out emission lines at higher wavelengths. On August 30 we also detected H-like and He-like lines of aluminum, oxygen and nitrogen, but almost no emission at wavelengths above 30 Å (0.41 keV). The August 27 HETG spectrum can be fitted with two components of collisionally ionized plasma in thermal equilibrium with plasma temperature of 2.2 keV and 0.7 keV respectively, and absorbing column density of 8.7 x 10²¹ cm^-2. The August 30 RGS spectrum indicates cooling, and probably decreased column density (best fit with 7.4 x .10²¹ cm^-2). It requires at least three components to be fitted, with a highest temperature of only about 0.8 keV. Like in the early X-ray grating spectra of 2006, in the HETG spectrum of August 27 asymmetric line profiles indicate that the redshifted material moving away in the line of sight is increasingly more absorbed than the material moving towards us. The spectra have clear similarities with X-ray grating spectra obtained in the 2006 outburst on days 14 and 26 after the optical maximum, although on day 26 a large soft flare was observed during the exposure. A new XMM-Newton exposure is currently scheduled on 2021 September 15.