Recurrent nova M31N 2008-12a: the 2015 outburst detected in X-rays with Swift XRT

ATel #7984; M. Henze (IEEC/CSIC), M. J. Darnley (LJMU), A. W. Shafter (SDSU), M. Kato (Keio University), I. Hachisu (U. Tokyo), M. F. Bode (LJMU), J.-U. Ness (ESA/ESAC), J. P. Osborne (U. Leicester), J. A. Kennea (PSU), N. Gehrels (NASA/GSFC), for a larger collaboration
on 3 Sep 2015; 08:52 UT
Credential Certification: Martin Henze (henze@ice.cat)

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In ATel #7964 we reported the discovery of the 2015 eruption of the recurrent nova M31N 2008-12a on 2015-08-28.55 UT. Comprehensive multi-wavelength studies of previous eruptions were published by Darnley et al. 2014, 2015, and Henze et al. 2014, 2015a. For additional optical photometry of the ongoing eruption see ATels #7965, #7967, #7968, #7969, #7974, #7976, #7979, and #7980. This is the 8th outburst of M31N 2008-12a in 8 consecutive years (Henze et al. 2015b).

Here we report the onset of the supersoft X-ray source (SSS) phase of M31N 2008-12a based on a faint detection during a 1-ks Swift observation on 2015-09-03.059 UT. The XRT count rate was (1.1±0.4) × 10^-2 ct/s (corrected for vignetting, dead time and PSF). Nothing was detected on 2015-09-02.262 UT with a 3σ upper limit of 0.8 × 10^-2 ct/s. The position of the X-ray source is in good agreement with the 2013/14 detections. A SSS turn-on time of about 5.8 days after eruption is in excellent agreement with the 5.9±0.5 days measured in 2014 (Henze et al. 2015). We will continue to monitor the SSS light curve until the predicted turn off around day 18.5 and will publish a comprehensive analysis soon.

The nova is still detected as an ultraviolet (UV) source but its magnitude has declined significantly since the first Swift UVOT detection on 2015-08-28.406 (Atel #7965). In less than six days the uvw1 filter (181-321 nm) magnitude dropped from 17.7±0.1 mag to 20.3±0.5 mag. This evolution is consistent with the 2014 UV light curve (cf. Darnley et al. 2015). Magnitudes are in the UVOT photometric system (Poole et al. 2008) and have not been corrected for extinction. A crowded field might introduce additional photometric uncertainties.

Near simultaneous optical photometry was obtained with the Liverpool Telescope (cf. ATel#7980), yielding the following magnitudes:

2015 Sep 02.973 u' = 20.6 ± 0.3
2015 Sep 02.984 B = 21.6 ± 0.1
2015 Sep 02.995 V = 21.4 ± 0.1
2015 Sep 03.014 r' = 20.9 ± 0.1
2015 Sep 03.018 I' = 20.9 ± 0.2

We wish to thank the Swift Science Operations Team for making these observations possible, in particular the duty scientists and the science planners.