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

¹ Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany e-mail: mhenze@mpe.mpg.de
² Departament de Física i Enginyeria Nuclear, EUETIB (UPC/IEEC), Comte d'Urgell 187, 08036 Barcelona, Spain
³ University of Texas, Austin TX, 78712, USA
⁴ Institut de Ciències de l'Espai (CSIC-IEEC), Campus UAB, Fac. Ciències, 08193 Bellaterra, Spain
⁵ European Southern Observatory (ESO), 85748 Garching, Germany
⁶ INAF-Napoli, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁷ International Centre for Relativistic Astrophysics, Piazzale della Repubblica 2, 65122 Pescara, Italy
⁸ Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁹ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, USA
¹⁰ Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
¹¹ Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic

Received: 9 October 2008
Accepted: 21 April 2009

Abstract

Context. Classical novae (CNe) have been found to represent the major class of supersoft X-ray sources (SSSs) in our neighbour galaxy M 31.

Aims. We determine the properties and evolution of the two first SSSs ever discovered in the M 31 globular cluster (GC) system.

Methods. We have used XMM-Newton, Chandra and Swift observations of the centre region of M 31 to discover both SSSs 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.

Results. 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.

Conclusions. The only known CN in a M 31 GC can be identified with the first SSS found in a M 31 GC. We discuss the impact of our observations on the nova rate for the M 31 GC system.