High-resolution X-ray spectra of RS Ophiuchi (2006 and 2021): Revealing the cause of SSS variability

Abstract

Context. The ∼ 10 − 20 year recurrent symbiotic nova RS Oph exploded on 2021 August 9, the seventh confirmed recorded outburst since 1898. During the previous outburst in 2006, the current fleet of X-ray space observatories was already in operation, and thanks to the longevity of Swift, XMM-Newton, and Chandra, a direct comparison between these two outbursts is possible. The Swift monitoring campaign revealed similar behaviour during the early shock phase but very different behaviour during the super-soft source (SSS) phase. Two XMM-Newton ⋆ observations were made during the 2021 SSS phase on days 37.1 and 55.6 after the 2021 optical peak. We focus in this work on the bright SSS observation on day 55.6 and compare to SSS Chandra and XMM-Newton grating observations made on days 39.7, 54, and 66.9 after the 2006 optical peak. Aims. By exploring the reasons for the differences between the 2006 and 2021 outbursts, we aim to obtain a better general understanding of the emission and absorption mechanisms. While the emission mechanisms hold the key to the physics of novae and nuclear burning, absorption processes may dominate what we observe, and we aim to explore the cause of the gross initial variability in the observed SSS emission. Methods. We present a novel approach to down-scaling the observed (brighter) 2006 SSS spectra to match the 2021 day 55.6 spectrum by parameter optimisation of: (1) a constant factor (representing fainter source emission, smaller radius, eclipses, etc.), (2) a multi-ionisation photoelectric absorption model (representing different line-of-sight absorption), and (3) scaling with a ratio of two blackbody models with different effective temperatures (representing different brightness and colours). This model approach does not depend on a source model assuming the intrinsic source to be the same. It is therefore more sensitive to incremental changes than modelling approaches where source and absorption are modelled simultaneously. Results. The 2021d55.6 spectrum can be reproduced remarkably well by multiplying the (brighter) 2006d39.7 and 2006d54 spectra with the absorption model, while the 2006d66.9 spectrum requires additional colour changes to match the 2021.d55.6 spectrum. The 2006d39.7 spectrum much more closely resembles the 2021d55.6 spectrum in shape and structure than the same-epoch 2006d54 spectrum: The spectra on days 2006d39.7 and 2021d55.6 are richer in absorption lines with a deeper O i absorption edge, and blueshifts are higher (∼ 1200 km s −1 ) than on day 2006d54 (∼ 700 km s −1 ). In the SSS light curves on days 2006d39.7, 2006d54, and 2021d55.6, brightness and hardness variations are correlated, indicating variations of the O i column density. Only on day 2006d39.7, a 1000s lag is observed. The 35s period was detected on day 2021d55.6 with lower significance compared to 2006d54. Conclusions. We conclude that the central radiation source is the same, while absorption is the principal reason for observing lower soft-X-ray emission in 2021 than in 2006. This is consistent with a...

The RS Oph Outburst of 2021 Monitored in X-Rays with NICER

“…At least one plasma component at 217 eV is necessary to improve the fit; a second thermal component around 80 eV improves the fit even more. Ness et al (2023) find that an absorbing medium with varying ionization states can explain the SSS irregular variability. For an XSPEC global fit, we did not have a model with varying ionization states, but the TBVARABS model allows to test instead varying the abundances of the absorbing medium.…”

“…One challenge posed by the luminous supersoft X-ray spectra of novae in general, and specifically of RS Oph in 2006 and 2021, is that most high-resolution X-ray spectra obtained with the Chandra and XMM-Newton gratings show blueshifted absorption lines (e.g., Rauch et al 2010;Ness et al 2011;Orio et al 2021;Ness et al 2023), indicating that we are not simply observing an atmosphere at rest. The blueshift can be large, corresponding to even 2000 km s −1 for many features of some novae; this is in contrast with a key assumption in the models, that, by the time the central WD becomes observable, mass loss has ceased (Kato & Hachisu 2020, e.g.).…”

“…The blackbody has a temperature of 36.6 eV (only ; 425,000 K), but its bolometric luminosity would have to be much over Eddington level, namely about 2 × 10 40 erg s −1 (a blackbody is known to overestimate the luminosity, e.g., Heise et al 1994). We do not consider this a realistic fit, but it is an experiment to guide a future analysis in which the XMM-Newton Reflection Grating Spectrometer (RGS) spectra (see Ness et al 2023) will be the starting point. A rigorous model for the SSS spectrum should first rely on the high-resolution spectra, and we leave it for future work.…”

“…The NICER data are not compatible with changes in the emitting region size or in the temperature; our qualitative fits with atmospheric models to the spectra of days 40-67 always converge toward T eff ; 750,000 K, no matter what the count rate is. However, given that our model fits for this phase are still qualitative and not rigorous, we refer here to the discussion of the highresolution X-ray spectra of RS Oph by Ness et al (2023), who compared a high-resolution XMM-Newton RGS spectrum of RS Oph obtained in 2021, with those of 2006 at different times after the outburst. They discussed three different possible scenarios:…”

“…The absorption varies on short timescales along the line of sight. Given that varying the column density N(H) does not explain the variability, Ness et al (2023) explored the effect of changing ionization stages in the ejecta and in the preexisting wind.…”