X-ray monitoring of classical novae in the central region of M 31 III. Autumn and winter 2009/10, 2010/11, and 2011/12^⋆,⋆⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
² European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: mhenze@sciops.esa.int
³ INAF-Napoli, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁴ International Centre for Relativistic Astrophysics, Piazzale della Repubblica 2, 65122 Pescara, Italy
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 Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, Fac. Ciències, 08193 Bellaterra, Spain
⁹ Department of Physics and Astronomy, Clemson University, Clemson SC 29634-0978, USA

Received: 1 August 2013
Accepted: 28 November 2013

Abstract

Context. Classical novae (CNe) represent the major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31.

Aims. We performed a dedicated monitoring of the M 31 central region, which aimed to detect SSS counterparts of CNe, with XMM-Newton and Chandra between Nov. and Mar. of the years 2009/10, 2010/11, and 2011/12.

Methods. We systematically searched our data for X-ray counterparts of CNe and determined their X-ray light curves and also their spectral properties in the case of XMM-Newton data. Additionally, we determined luminosity upper limits for all previously known X-ray emitting novae, which are not detected anymore, and for all CNe in our field of view with recent optical outbursts.

Results. In total, we detected 24 novae in X-rays. Seven of these sources were known from previous observations, including the M 31 nova with the longest SSS phase, M31N 1996-08b, which was found to fade below our X-ray detection limit 13.8 yr after outburst. Of the new discoveries, several novae exhibit significant variability in their short-term X-ray light curves with one object showing a suspected period of about 1.3 h. We studied the SSS state of the most recent outburst of a recurrent nova, which had previously shown the shortest time ever observed between two outbursts (~5 yr). The total number of M 31 novae with X-ray counterpart was increased to 79, and we subjected this extended catalogue to detailed statistical studies. Four previously indicated correlations between optical and X-ray parameters could be confirmed and improved. Furthermore, we found indications that the multi-dimensional parameter space of nova properties might be dominated by a single physical parameter, and we provide interpretations and suggest implications. We studied various outliers from the established correlations and discuss evidence of a different X-ray behaviour of novae in the M 31 bulge and disk.

Conclusions. Exploration of the multi-wavelength parameter space of optical and X-ray measurements is shown to be a powerful tool for examining properties of extragalactic nova populations. While there are hints that the different stellar populations of M 31 (bulge vs. disk) produce dissimilar nova outbursts, there is also growing evidence that the overall behaviour of an average nova might be understood in surprisingly simple terms.