Recent discoveries of supersoft X-ray sources in M 31

TL;DR

This paper reviews recent X-ray observations of M 31, highlighting the discovery of supersoft X-ray sources in globular clusters, short SSS phases indicating massive white dwarfs, and potential binary orbital periods.

Contribution

It reports the first detection of SSSs in M 31 globular clusters and links them to classical novae, providing new insights into their rates and characteristics.

Findings

Discovered SSSs in M 31 globular clusters.

Identified a nova with a very short SSS phase of 29-52 days.

Detected short-term variability suggesting a ~2-hour orbital period.

Abstract

Classical novae (CNe) have recently been reported to represent the major class of supersoft X-ray sources (SSSs) in the central area of our neighbouring galaxy M 31. This paper presents a review of results from recent X-ray observations of M 31 with XMM-Newton and Chandra. We carried out a dedicated optical and X-ray monitoring program of CNe and SSSs in the central area of M 31. We discovered the first SSSs in M 31 globular clusters (GCs) and their connection to the very first discovered CN in a M 31 GC. This result may have an impact on the CN rate in GCs. Furthermore, in our optical and X-ray monitoring data we discovered the CN M31N 2007-11a, which shows a very short SSS phase of 29 - 52 days. Short SSS states (durations < 100 days) of CNe indicate massive white dwarfs (WDs) that are candidate progenitors of supernovae type Ia. In the case of M31N 2007-11a, the optical and X-ray light curves suggest a binary containing a WD with M_WD > 1.0 M_sun. Finally, we present the discovery of the SSS counterpart of the CN M31N 2006-04a. The X-ray light curve of M31N 2006-04a shows short-time variability, which might indicate an orbital period of about 2 hours.