X-ray monitoring of classical novae in the central region of M 31

Abstract

Context. Classical novae (CNe) have recently been reported to represent the major class of supersoft X-ray sources (SSSs) in the central region of our neighbour galaxy M 31. Aims. We carried out a dedicated monitoring of the M 31 central region with XMM-Newton and Chandra in order to find SSS counterparts of CNe, determine the duration of their SSS phase and derive physical outburst parameters. Methods. We systematically searched our data for X-ray counterparts of CNe and determined their X-ray light curves and spectral properties. Additionally, we determined luminosity upper limits for all novae from previous studies which are not detected anymore and for all CNe in our field of view with optical outbursts between May 2005 and March 2007. Results. We detected eight X-ray counterparts of CNe in M 31, four of which were not previously known. Seven sources can be classified as SSSs, one is a candidate SSS. Two SSSs are still visible more than nine years after the nova outburst, whereas two other nova counterparts show a short SSS phase of less than 150 days. Of the latter sources, M31N 2006-04a exhibits a short-time variable X-ray light curve with an apparent period of (1.6 ± 0.3) h. This periodicity could indicate the binary period of the system. There is no X-ray detection for 23 out of 25 CNe which were within the field of view of our observations and had their outburst from about one year before the start of the monitoring until its end. From the 14 SSS nova counterparts known from previous studies, ten are not detected anymore. Additionally, we found four SSSs in our XMM-Newton data without a nova counterpart, one of which is a new source. Conclusions. Out of eleven SSSs detected in our monitoring, seven are counterparts of CNe. We therefore confirm the earlier finding that CNe are the major class of SSSs in the central region of M 31. We use the measured SSS turn-on and turn-off times to estimate the mass ejected in the nova outburst and the mass burned on the white dwarf. Classical novae with short SSS phases seem to be an important contributor to the overall population.

M31N 2008-12a—THE REMARKABLE RECURRENT NOVA IN M31: PANCHROMATIC OBSERVATIONS OF THE 2015 ERUPTION

“…A dedicated multi-year follow-up program with the same telescopes studied the multiwavelength population properties of M 31 novae in detail (Henze et al 2010(Henze et al , 2014b. A major result of this work was the discovery of strong correlations between various observable parameters, indicating that novae with a faster visible decline tend to show a shorter SSS phase with a higher temperature (Henze et al 2014b).…”

“…But it is the nearby Andromeda Galaxy (M 31), with an annual nova rate of -+ 65 15 16 yr −1 (Darnley et al 2006), that provides the leading laboratory for the study of galaxy-wide nova populations (see, for example, Ciardullo et al 1987Ciardullo et al , 1990aShafter & Irby 2001;Darnley et al 2004Darnley et al , 2006Henze et al 2008Henze et al , 2010Henze et al , 2011Henze et al , 2014bShafter et al 2011aShafter et al , 2011bShafter et al , 2015aWilliams et al 2014Williams et al , 2016. Since the discovery of the first M 31 nova by Ritchey (1917, also spectroscopically confirmed) and the pioneering work of Hubble (1929), more than 1000 nova candidates have been discovered (see Pietsch et al 2007;Pietsch 2010, and their on-line database 46 ), with over 100 now spectroscopically confirmed (see, for example, Shafter et al 2011b).…”

Nova M31N 2007-12b: supersoft X-rays reveal an intermediate polar?

“…Henze et al (2010) report a ∼5900 s period for the SSS counterpart of M31N 2006-04a. Owing to the duration of the observation, only three cycles could be followed.…”

“…As in Henze et al (2010), we assume the volume of the nova shell, expanding at constant velocity v, to be V ∼ 4π × v 3 × t 3 × f , where the fill factor f is a dimensionless parameter describing the thickness Table 5). …”

“…This leads to an ejected mass of M e j H = 2.0 × 10 −6 M . The burned mass can be estimated from the turn-off time of the SSS phase as M burn,H = (L bol · t off )/(X H ), where L bol is the bolometric luminosity, t off the SSS turn-off time, X H the hydrogen fraction of the burned material, and = 5.98 × 10 18 erg g −1 (see Sala & Hernanz 2005b;Henze et al 2010). From the preferred envelope mixing model of Sala & Hernanz (2005b) (see above), we derive X H ≈ 0.5.…”