  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 super-soft 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.

High Energy Astrophysical Phenomena