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Recurrent nova M31N 2008-12a: Swift/XRT detection of the 2016 eruption

ATel #9872; M. Henze (CSIC-IEEC), M. J. Darnley (LJMU), A. W. Shafter (SDSU), S. Kafka (AAVSO), M. Kato (Keio University), for a larger collaboration
on 18 Dec 2016; 13:08 UT
Credential Certification: Martin Henze (henze@ice.cat)

Subjects: Ultra-Violet, X-ray, Nova, Transient

Referred to by ATel #: 9874, 9881, 9883, 9885, 9891, 9906, 9907, 11130

In ATel #9848 we announced the discovery of the predicted 2016 eruption of the recurrent nova M31N 2008-12a on 2016-12-12.4874 UT. The 9th eruption in 9 consecutive years was predicted by Darnley & Henze et al. (2016) in their comprehensive study of the 2015 eruption (see also the initial prediction by Henze et al. 2015b). Detailed multi-wavelength studies of earlier eruptions were published by Darnley et al. (2014, 2015), Henze et al. (2014, 2015a), and Tang et al. (2014). For additional optical photometry and spectroscopy of the ongoing eruption see ATels #9852, #9857, #9861, #9864, and #9865. A fast Swift/UVOT detection was announced in ATel #9853.

Here we report the onset of the supersoft X-ray source (SSS) phase of M31N 2008-12a which was clearly detected in a 3.9-ks Swift observation starting on 2016-12-18.101 UT. The XRT count rate was (1.0±0.2) × 10-2 ct/s (corrected for vignetting, dead time and PSF). No source was detected in the preceding 1.1-ks Swift observation on 2016-12-16.38 UT with an 3σ upper limit of 0.7 × 10-2 ct/s. Assuming an eruption date of 2016-12-12.32 UT (MJD 57734.32), based on the ATel #9848 constraints, the SSS detection occurred on day 5.8 after eruption. This is well consistent with the 5.9±0.5 days measured in 2014 (ATel #6558, Henze et al. 2015) and the 5.7±0.5 days seen in 2015 (ATel #7984, Darnley & Henze et al. 2016).

In addition, the nova is still detected as an ultraviolet (UV) source but its magnitude has declined significantly to uvw2 = 19.3±0.1 mag (cf. ATel #9853). Our preliminary magnitudes use the UVOT photometric system (Poole et al. 2008, Breeveld et al. 2011) and have not been corrected for extinction.

We wish to thank the Swift Team for the scheduling of the ToO observations, in particular N. Gehrels, B. Cenko, and especially the duty scientists and science planners who made the early detection possible.