X-Rays from RS Ophiuchi's 2021 Eruption: Shocks In and Out of Ionization Equilibrium

The recurrent nova RS Ophiuchi (RS Oph) underwent its most recent eruption on 2021 August 8 and became the first nova to produce both detectable GeV and TeV emission. We used extensive X-ray monitoring with the Neutron Star Interior Composition Explorer Mission (NICER) to model the X-ray spectrum and probe the shock conditions throughout the 2021 eruption. The rapidly evolving NICER spectra consisted of both line and continuum emission that could not be accounted for using a single-temperature collisional equilibrium plasma model with an absorber that fully covered the source. We successfully modeled the NICER spectrum as a nonequilibrium ionization collisional plasma with partial covering absorption. The temperature of the nonequilibrium plasma shows a peak on day 5 with a kT of approximately 24 keV. The increase in temperature during the first five days could have been due to increasing contribution to the X-ray emission from material behind fast polar shocks or a decrease is the amount of energy being drained from the shocks into particle acceleration during that period. The absorption showed a change from fully covering the source to having a covering fraction of roughly 0.4, suggesting a geometrical evolution of the shock region within the complex global distribution of the circumstellar material. These findings show evidence of the ejecta interacting with some dense equatorial shell initially, and with less dense material in the bipolar regions at later times during the eruption.