Beginning of the Super-Soft Phase of the Classical Nova V2491 Cygni

TL;DR

This study reports the detection of the onset of the super-soft X-ray phase in nova V2491 Cygni, analyzing Suzaku data with models suggesting either optically thin ejecta or thick white dwarf emission.

Contribution

First detailed Suzaku X-ray analysis of V2491 Cygni's super-soft phase, highlighting model ambiguities and systematic uncertainties in interpreting the emission origin.

Findings

Soft X-ray component appeared on day 29 post-outburst.

Spectral fitting yields two plausible models: blackbody and thin-thermal plasma.

Systematic errors dominate due to low spectral resolution and S/N.

Abstract

We present the results of soft X-ray studies of the classical nova V2491 Cygni using the Suzaku observatory. On day 29 after outburst, a soft X-ray component with a peak at ~0.5 keV has appeared, which is tantalising evidence for the beginning of the super-soft X-ray emission phase. We show that an absorbed blackbody model can describe the observed spectra, yielding a temperature of 57 eV, neutral hydrogen column density of 2x10^{21} cm^{-2}, and a bolometric luminosity of ~10^{36} erg s^{-1}. However, at the same time, we also found a good fit with an absorbed thin-thermal plasma model, yielding a temperature of 0.1 keV, neutral hydrogen column density of 4x10^{21} cm^{-2}, and a volume emission measure of ~10^{58} cm^{-3}. Owing to low spectral resolution and low signal-to-noise ratio below 0.6 keV, the statistical parameter uncertainties are large, but the ambiguity of the two very different models demonstrates that the systematic errors are the main point of concern. The thin-thermal plasma model implies that the soft emission originates from optically thin ejecta, while the blackbody model suggests that we are seeing optically thick emission from the white dwarf.