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The recurrent nova RS Ophiuchi (RS Oph) underwent its most recent eruption on8 August 2021 and became the first nova to produce both detectable GeV and TeVemission. We used extensive X-ray monitoring with the Neutron Star InteriorComposition Explorer Mission (NICER) to model the X-ray spectrum and probe theshock conditions throughout the 2021 eruption. The rapidly evolving NICERspectra consisted of both line and continuum emission that could not beaccounted for using a single-temperature collisional equilibrium plasma modelwith an absorber that fully covered the source. We successfully modelled theNICER spectrum as a non-equilibrium ionization collisional plasma withpartial-covering absorption. The temperature of the the non-equilibrium plasmashow a peak on Day 5 with a kT of approximately 24 keV. The increase intemperature during the first five days could have been due to increasingcontribution to the X-ray emission from material behind fast polar shocks or adecrease is the amount of energy being drained from shocks into particleacceleration during that time period. The absorption showed a change from fullycovering the source to having a covering fraction of roughly 0.4, suggesting ageometrical evolution of the shock region within the complex globaldistribution of the circumstellar material. These findings show the evidence ofthe ejecta interacting with some dense equatorial shell initially and with lessdense material in the bipolar regions at later times during the eruption.

X-rays from RS Ophiuchi's 2021 eruption: shocks in and out of ionization equilibrium