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Supersoft X-ray detections of M31 Novae with XMM-Newton

ATel #8228; M. Henze (IEEC/CSIC), M. Sasaki (IAAT), F. Haberl (MPE), D. Hatzidimitriou (U. Athens), for a larger collaboration
on 30 Oct 2015; 13:25 UT
Credential Certification: Martin Henze (henze@ice.cat)

Subjects: X-ray, Nova, Transient

Referred to by ATel #: 8825

We report the detection of three novae in recent XMM-Newton observations of the M31 disk (see also ATel #8227). Two 100-ks observations were carried out on 2015-06-28 (ObsID 0763120101; June) and 2015-08-11 (ObsID 0763120301; August) and have been analysed using the standard data reduction software. The observations had only a small overlap and consequently only a few sources are visible in both. Two of the three M31 novae were detected for the first time in X-rays. The other source was first announced in ATel #6564.

Nova M31N 2006-08a was discovered on 2006-08-16.518 UT by K. Itagaki. In the August observation, almost exactly nine years after the eruption (3282 days), it is faintly detected as an X-ray source with a combined EPIC emldetect likelihood of 10.5 and count rate of (1.3±0.5) × 10-3 ct s-1. No previous X-ray detection of this source is known. Almost all of the about 30 source counts are below 0.5 keV, which allows us to classify the object as a supersoft X-ray source (SSS). Within the M31 nova population, a very low effective temperature would be consistent with a long duration of the SSS phase (Henze et al. 2014)

Nova M31N 2012-06a was discovered on 2012-06-18.052 UT (ATel #4186) and spectroscopically classified as a FeII nova (ATel #4216). It was first detected as an X-ray source on 2014-08-09.89 UT, when it displayed a count rate of (9.6±0.7) × 10-3 ct s-1 and an SSS spectrum with a blackbody temperature of 41±5 eV (ATel #6564). One year later, on day 1105 after discovery (June), we find an increased count rate of (1.4±0.1) × 10-2 ct s-1. The X-ray spectrum is consistent with the 2014 results and can be parametrised by an absorbed blackbody with best-fit parameters kT = 45±7 eV and NH = (0.9±0.3) × 1021 cm-2. A relatively low blackbody temperature is consistent with an SSS visibility of at least three years after eruption (Henze et al. 2014).

Nova M31N 2013-11d was discovered on 2013-11-25.3 UT (ATel #5605). Additional optical photometry was reported in ATel #5744. We clearly detect an X-ray counterpart in June, 580 days after the eruption. This is the first detection of the nova in X-rays. The source is still present in the August observation, with consistent count rates of (5.6±1.1) × 10-3 ct s-1 (June) and (4.3±0.8) × 10-3 ct s-1 (August), respectively. A combined spectral analysis with an absorbed blackbody resulted in best-fit parameters of kT = 55±6 eV and NH = (0.9±0.3) × 1021 cm-2. This is consistent with an earlier SSS turn on. We would expect this nova to have a shorter SSS phase than the other two novae.