The 2021 outburst of the recurrent nova RS Ophiuchi observed in X-rays by the Neil Gehrels Swift Observatory: a comparative study

Abstract

On 2021 August 8, the recurrent nova RS Ophiuchi erupted again, after an interval of 15.5 yr. Regular monitoring by the Neil Gehrels Swift Observatory began promptly, on August 9.9 (0.37 day after the optical peak), and continued until the source passed behind the Sun at the start of November, 86 days later. Observations then restarted on day 197, once RS Oph emerged from the Sun constraint. This makes RS Oph the first Galactic recurrent nova to have been monitored by Swift throughout two eruptions. Here we investigate the extensive X-ray datasets from 2006 and 2021, as well as the more limited data collected by EXOSAT in 1985. The hard X-rays arising from shock interactions between the nova ejecta and red giant wind are similar following the last two eruptions. In contrast, the early supersoft source (SSS) in 2021 was both less variable and significantly fainter than in 2006. However, 0.3-1 keV light-curves from 2021 reveal a 35 s quasi-periodic oscillation consistent in frequency with the 2006 data. The Swift X-ray spectra from 2021 are featureless, with the soft emission typically being well parametrized by a simple blackbody, while the 2006 spectra showed much stronger evidence for superimposed ionized absorption edges. Considering the data after day 60 following each eruption, during the supersoft phase the 2021 spectra are hotter, with smaller effective radii and lower wind absorption, leading to an apparently reduced bolometric luminosity. We explore possible explanations for the gross differences in observed SSS behaviour between the 2006 and 2021 outbursts.

On the Observability of Recurrent Nova Super-Remnants

“…However, this is still much fainter than typical nova X-ray luminosities such as, for example, M31N 2004-01b, 2005-02a, and 2006-06b with 𝐿 X = (11.1 ± 1.6) × 10 36 erg s −1 , 2.6 × 10 37 erg s −1 and (3.6 ± 0.3) × 10 36 erg s −1 , respectively 5 (see Henze et al 2010Henze et al , 2011, for a large sample of M 31 CNe X-ray luminosities). Instead, this X-ray luminosity is more akin to that seen in quiescent novae such as ∼6 × 10 31 erg s −1 for RS Ophiuchi (Page et al 2022).…”