The central region of M 31 observed with - ^*

II. Variability of the individual sources

¹ Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
² NIS Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
³ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, RH5 6NT, UK
⁴ Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
⁵ Istituto di Fisica Cosmica "G. Occhialini" , Via Bassini 15, 20133 Milano, Italy
⁶ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁷ Max Planck Institut für Extraterrestrische Physik, Giessenbachstraße, 85741 Garching bei München, Germany
⁸ XMM-Newton SOC, VILSPA-ESA, Apartado 50727, 28080 Madrid, Spain

Corresponding author: K. N. Borozdin, kbor@lanl.gov

Received: 5 June 2001
Accepted: 31 August 2001

Abstract

We present the results of a study of the variability of X-ray sources in the central 30´of the nearby Andromeda Galaxy (M 31) based on XMM-Newton Performance Verification observations. Two observations of this field, with a total exposure time of about 50 ks, were performed in June and December of 2000. We found 116 sources brighter than a limiting luminosity of 610³⁵ erg s^-1 (0.3-12 keV, kpc). For the ~60 brightest sources, we searched for periodic and non-periodic variability; at least 15% of these sources appear to be variable on a time scale of several months. We discovered a new bright transient source ~2.9´from the nucleus in the June observation; this source faded significantly and was no longer detected in December. The behaviour of the object is similar to a handful of Galactic LMXB transients, most of which are supposed to harbor black holes. We detected pulsations with a period of ~865 s from a source with a supersoft spectrum. The flux of this source decreased significantly between the two XMM observations. The detected period is unusually short and points to a rapidly spinning magnetized white dwarf. The high luminosity and transient nature of the source suggest its possible identification with classical or symbiotic nova, some of which were observed earlier as supersoft sources.