X-ray Study of Rekindled Accretion in the Classical Nova V2491 Cygni

TL;DR

This study used X-ray spectroscopy to observe the evolution of the classical nova V2491 Cygni, revealing a transition from ejecta-dominated emission to rekindled accretion around day 50 post-outburst.

Contribution

It provides the first detailed X-ray spectral timeline showing the transition from ejecta to accretion phases in V2491 Cygni, highlighting early reestablishment of accretion processes.

Findings

Detection of Fe XXV emission line pre-nova and on day 9.

Disappearance of Fe XXV line on day 29.

Reappearance of Fe lines indicating resumed accretion around day 50.

Abstract

We conducted an X-ray spectroscopic study of the classical nova V2491 Cygni using our target-of-opportunity observation data with the Suzaku and XMM-Newton satellites as well as archived data with the Swift satellite. Medium-resolution (R~10-50) spectra were obtained using the X-ray CCD spectrometers at several post-nova epochs on days 9, 29, 40, 50, and 60-150 in addition to a pre-nova interval between days -322 and -100 all relative to the time when the classical nova was spotted. We found remarkable changes in the time series of the spectra: (a) In the pre-nova phase and on day 9, the 6.7 keV emission line from Fe XXV was significantly detected. (b) On day 29, no such emission line was found. (c) On day 40, the 6.7 keV emission line emerged again. (d) On days 50 and 60-150, three emission lines at 6.4, 6.7, and 7.0 keV respectively from quasi-neutral Fe, Fe XXV, and Fe XXVI were found. Statistically significant changes of the Fe K line intensities were confirmed between day 29 and 50. Based on these phenomena, we conclude that (1) the post-nova evolution can be divided into two different phases, (2) ejecta is responsible for the X-ray emission in the earlier phase, while rekindled accretion is for the later phase, and (3) the accretion process is considered to be reestablished as early as day 50 when the quasi-neutral Fe emission line emerged, which is a common signature of accretion from magnetic cataclysmic variables.