V723 Cassiopeia still on in X-rays: A bright Super Soft Source 12 years after outburst

TL;DR

This study reports that the classical nova V723 Cas remains an active Super Soft X-ray source over 12 years post-outburst, with variable emission and indications of ongoing nuclear burning or accretion processes.

Contribution

It provides the first long-term X-ray monitoring of V723 Cas post-outburst, confirming persistent Super Soft X-ray emission and analyzing its spectral and temporal properties.

Findings

V723 Cas is still an active X-ray source after 12 years.

X-ray emission shows variability within a factor of two.

Spectral analysis suggests temperatures of (2.8-3.8)×10^5 K and high luminosity.

Abstract

We find that the classical nova V723 Cas (1995) is still an active X-ray source more than 12 years after outburst and analyze seven X-ray observations carried out with Swift between 2006 January 31 and 2007 December 3. The average count rate is 0.022+/-0.01 cts s^-1 but the source is variable within a factor of two of the mean and does not show any signs of turning off. We present supporting optical observations which show that between 2001 and 2006 an underlying hot source was present with steadily increasing temperature. In order to confirm that the X-ray emission is from V723 Cas, we extract a ROSAT observation taken in 1990 and find that there was no X-ray source at the position of the nova. The Swift XRT spectra resemble those of the Super Soft X-ray binary Sources (SSS) which is confirmed by RXTE survey data which show no X-ray emission above 2 keV between 1996 and 2007. Using blackbody fits we constrain the effective temperature to between T_eff=(2.8-3.8)x10^5 K and a bolometric luminosity >5x10^36 erg s^-1 and caution that luminosities from blackbodies are generally overestimated and temperatures underestimated. We discuss a number of possible explanations for the continuing X-ray activity, including the intriguing possibility of steady hydrogen burning due to renewed accretion.