Supersoft X-rays reveal a classical nova in the M 31 globular cluster Bol 126^⋆,⋆⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany
e-mail: mhenze@mpe.mpg.de
² INAF-Napoli, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
³ International Centre for Relativistic Astrophysics, Piazzale della Repubblica 2, 65122 Pescara, Italy
⁴ University Observatory Munich, Scheinerstrasse 1, 81679 München, Germany
⁵ Departament de Física i Enginyeria Nuclear, EUETIB, Universitat Politècnica de Catalunya, c/Comte d’Urgell 187, 08036 Barcelona, Spain
⁶ Institut d’Estudis Espacials de Catalunya, c/Gran Capità 2–4, Ed. Nexus-201, 08034 Barcelona, Spain
⁷ Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis, GR15784 Zografos, Athens, Greece
⁸ Foundation for Research and Technology Hellas, IESL, Greece
⁹ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, USA
¹⁰ Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹¹ Astronomical Institute, Academy of Sciences, CZ-251 65 Ondřejov, Czech Republic
¹² Itagaki Astronomical Observatory, Teppo-cho, Yamagata 990-2492, Japan
¹³ Miyaki-Argenteus Observatory, Miyaki-cho, Saga-ken, Japan
¹⁴ Xingming Observatory, Mt. Nanshan, Urumqi, Xinjiang, PR China
¹⁵ Graduate Institute of Astronomy, National Central University, Jhongli 32001, Taiwan
¹⁶ Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA

Received: 9 August 2012
Accepted: 2 November 2012

Abstract

Context. Classical novae (CNe) represent the main class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. Only three confirmed novae and three SSSs have been discovered in globular clusters (GCs) of any galaxy so far, of which one nova and two SSSs (including the nova) were found in M 31 GCs.

Aims. To study the SSS state of CNe we carried out a high-cadence X-ray monitoring of the M 31 central area with XMM-Newton and Chandra. This project is supplemented by regular optical monitoring programmes at various observatories.

Methods. We analysed X-ray and optical monitoring data of a new transient X-ray source in the M 31 GC Bol 126, discovered serendipitously in Swift observations. Our optical data set was based on regular M 31 monitoring programmes from five different small telescopes and was reduced using a homogeneous method. Additionally, we made use of Pan-STARRS 1 data obtained during the PAndromeda survey. We extracted light curves of the source in the optical and X-rays, as well as X-ray spectra.

Results. Our observations reveal that the X-ray source in Bol 126 is the third SSS in an M 31 GC and can be confirmed as the second CN in the M 31 GC system. This nova is named M31N 2010-10f. Its properties in the X-ray (high black-body temperature, short SSS phase) and optical (relatively high maximum magnitude, fast decline) regimes agree with a massive white dwarf (M_WD  ≳ 1.3 M_⊙) in the binary system. Incorporating the data on previously found (suspected) novae in M 31 GCs we used our high-cadence X-ray monitoring observations to estimate a tentative nova rate in the M 31 GC system of 0.05 yr^-1 GC^-1. An optical estimate, based on the recent 10.5-year WeCAPP survey, gives a lower nova rate, which is compatible with the X-ray rate on the 95% confidence level.

Conclusions. Although still based on small-number statistics, there is growing evidence that the nova rate in GCs is higher than expected from primordial binary formation and under conditions as in the field. Dynamical binary formation and/or additional accretion from the intracluster medium are possible scenarios for an increased nova rate, but observational confirmation for this enhancement has been absent, so far. Regular X-ray monitoring observations of M 31 provide a promising strategy to find these novae.