Early emergence of super-soft-source emission in RS,Oph

Abstract

Early in the 2006 outburst of the recurrent nova RS,Oph, detected the onset of the super-soft-source (SSS) phase. An observation obtained on day 26.1 revealed a highly unusual soft X-ray spectrum containing transient narrow emission features that could not be identified with known transitions and are atypical of SSS spectra. Here, we analyse a new observation obtained at a comparable stage of the 2021 outburst. By presenting a complete census of temporal and spectral properties, we aim to demonstrate a high degree of complexity, deriving requirements for future model approaches. To minimise model-dependent bias, we used models only for demonstration purposes and based our interpretation primarily on observational evidence. We used the 2006d26.1 EPIC/pn spectrum to derive a minimal model for the onset of SSS emission and compare it with the simultaneous RGS spectrum. We investigated the detailed properties of the 2021d37.1 RGS spectrum through comparisons with spectra from other epochs of the 2006 and 2021 RS,Oph outbursts, spectra extracted from four variability intervals within the observation, and spectra of other novae with similar characteristics. These comparisons were used to identify plausible emission mechanisms and guide spectral interpretation. We also searched for 35 s periodic signals in the 2021d37.1 RGS and simultaneous NICER data, motivated by their detection during the 2006d26.1 observation. A blackbody model reproduces the low-resolution 2006d26.1 EPIC/pn spectrum but fails to describe the simultaneous RGS data. The 2021d37.1 RGS spectrum closely resembles that from 2006d26.1 and consists of a slowly fading shock component and a highly variable soft component with several unidentified features. While the shock emission evolves on timescales of days, the soft component varies on hourly timescales and dominates the observed variability, with individual features showing distinct behaviour. The soft emission resembles that of the novae V1716,Sco and V1494,Aql -- the latter of which has recently been interpreted with a charge--exchange model -- but not that of the persistent SSS Cal,87. No significant 35 s periodicity is detected. Our results demonstrate that low spectral-resolution observations can fail to reveal the complex line-dominated emission seen by higher resolution instruments. The rapidly variable soft component in RS,Oph, together with radiative recombination continua features, x emission, and the previously detected 35 s period, suggests that a hidden photoionising source is present. At the same time, the observed spectral complexity and variability indicate that quantitatively reproducing these spectra is likely to require modelling that accounts for geometry, density structure, dynamics, absorption, clumpiness, and other effects. We therefore focus on empirical comparisons and conclude that the enigmatic spectra observed shortly before the emergence of a clear SSS spectrum form an intermediate class between SSS emission (SSe) and SSS absorption (SSa) spectra, together with V1494 Aql and V1716 Sco. Future modelling efforts will be required to establish the relative importance of the physical processes responsible for this broader class of intermediate SSS spectra. Possible approaches include photoionisation models, which naturally arise from the presence of the luminous supersoft source, as well as charge--exchange models such as those recently proposed for V1716 Sco.