XMM-Newton X-ray and UV observations of the fast nova V2491 Cyg during the supersoft source phase

TL;DR

This study presents X-ray and UV observations of nova V2491 Cyg during its supersoft source phase, revealing variable light curves, high-velocity absorption lines, and spectral similarities with other novae, highlighting complex outburst phenomena.

Contribution

First simultaneous X-ray and UV observations of V2491 Cyg during the SSS phase, with detailed spectral analysis and comparison to similar novae, revealing new insights into nova outburst behavior.

Findings

Detected highly variable X-ray light curves with a 37.2 min period

Observed high-velocity absorption lines indicating expansion speeds of 3000-3400 km/s

Identified spectral similarities with R S Ophiuchi and differences with V4743 Sgr

Abstract

Two XMM observations of the fast classical nova V2491Cyg were carried out on days 39.93 and 49.62 after discovery, during the supersoft source (SSS) phase, yielding simultaneous X-ray and UV light curves and high-resolution X-ray spectra. The first X-ray light curve is highly variable with periodic oscillations (37.2 min) after an extended dip of factor of three lasting ~3 hours. The cause of the dip is currently unexplained and could have the same origin as similar events in V4743Sgr and RSOph, as it occurred on the same time scale. The 37-min period is not present during the dip and also not in the second observation. The UV light curves are variable but contain no dips and no period. High-resolution X-ray spectra are presented for 4 intervals of different intensity. All spectra are atmospheric continua with absorption lines and absorption edges. Interstellar lines of OI and NI are seen at their rest wavelengths, and a large number of high-ionization absorption lines are found at blue shifts indicating an expansion velocity of 3000-3400 km/s, which does not change significantly during the epochs of observation. Comparisons with the slower nova V4743Sgr and the symbiotic recurrent nova RSOph are presented. The SSS spectrum of V4743Sgr is much softer with broader and more complex photospheric absorption lines. Meanwhile, the absorption lines in RSOph are as narrow as in V2491Cyg, but they are less blue shifted. A remarkable similarity in the continua of V2491Cyg and RSOph is found. The only differences are smaller line shifts and additional emission lines in RSOph that are related to a dense stellar wind from the evolved companion. Three unidentified absorption lines are present in the X-ray spectra of all three novae, with rest wavelengths 26.05AA, 29.45AA, and 30.0AA. No satisfactory spectral model is currently available for the soft X-ray spectra of novae in outburst.