The first two transient supersoft X-ray sources in M 31 globular clusters and the connection to classical novae

TL;DR

This study reports the discovery and analysis of the first two supersoft X-ray sources in M 31 globular clusters, linking one to a recent nova and providing insights into nova rates in these clusters.

Contribution

It presents the first identification of supersoft X-ray sources in M 31 globular clusters and connects one to a known nova, enhancing understanding of nova occurrence in such environments.

Findings

SS1 linked to nova M31N 2007-06b in Bol 111

SS2 shows no recent nova outburst evidence

Implications for nova rates in M 31 GCs

Abstract

Classical novae (CNe) have been found to represent the major class of supersoft X-ray sources (SSS) in our neighbour galaxy M 31. We determine properties and evolution of the two first SSSs ever discovered in the M 31 globular cluster (GC) system. We have used XMM-Newton, Chandra and Swift observations of the centre region of M 31 to discover both SSS and to determine their X-ray light curves and spectra. We performed detailed analysis of XMM-Newton EPIC PN spectra of the source in Bol 111 (SS1) using blackbody and NLTE white dwarf (WD) atmosphere models. For the SSS in Bol 194 (SS2) we used optical monitoring data to search for an optical counterpart. Both GC X-ray sources were classified as SSS. We identify SS1 with the CN M31N 2007-06b recently discovered in the M 31 GC Bol 111. For SS2 we did not find evidence for a recent nova outburst and can only provide useful constraints on the time of the outburst of a hypothetical nova. The only known CN in a M 31 GC can be identified with the first SSS found in a M31 GC. We discuss the impact of our observations on the nova rate for the M 31 GC system.