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Recurrent nova M31N 2008-12a: The surprisingly early 2016 supersoft X-ray turn-off seen by Swift/XRT

ATel #9907; M. Henze (CSIC-IEEC), M. J. Darnley (LJMU), A. W. Shafter (SDSU), S. Kafka (AAVSO), M. Kato (Keio University), for a larger collaboration
on 28 Dec 2016; 13:59 UT
Credential Certification: Martin Henze (henze@ice.cat)

Subjects: X-ray, Nova, Transient

Referred to by ATel #: 9910, 11214

In ATel #9872 we announced the emergence of the supersoft X-ray source (SSS) emission from the rapidly recurring nova M31N 2008-12a (Henze et al. 2014, 2015a, 2015b; Darnley et al. 2014, 2015, 2016) during its 2016 eruption. The optical discovery of the eruption was reported in ATel #9848. For additional multi-wavelength follow-up coverage see ATels #9852, #9853, #9857, #9861, #9864, #9865, #9874, #9881, #9883, #9885, #9891, #9906.

Here we report the end of the SSS phase around December 28. After the last (marginal) detection in a 2.8-ks XRT observation on 2016-12-27.73 UT, with a count rate of (0.2±0.1) × 10-2 ct/s, nothing was seen in the subsequent 3.2-ks pointing on 2016-12-28.19. Therefore, we estimate that the SSS phase ended prior to day 16 after eruption. This is significantly earlier than for the previous observed eruptions in 2013 (day 19±1; Henze et al. 2014), 2014 (day 18.4±0.5; ATel #6604 and Henze et al. 2015a), and 2015 (day 18.6±0.7; ATel #8062 and Darnley et al. 2016). This surprising deviation from previous behaviour might be connected to the somewhat delayed eruption date of the 2016 outburst. All results are preliminary. A detailed analysis is in preparation.

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.