Recurrent nova M31N 2008-12a: Swift/XRT detection of the 2019 eruption

ATel #13290; M. J. Darnley (LJMU), M. Henze (SDSU), I. Hachisu (Tokyo), M. Kato (Keio), J. P. Osborne (Leicester), J.-U. Ness (ESA/ESAC), A. W. Shafter (SDSU), on behalf of the 12a Collaboration
on 14 Nov 2019; 11:16 UT
Credential Certification: Matt Darnley (M.J.Darnley@ljmu.ac.uk)

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In ATel #13269 we announced the discovery of the 2019 eruption of the recurrent nova M31N 2008-12a on 2019-11-06.71 UT (almost exactly one year since the 2018 eruption was discovered; ATel #12177). We reported the follow-up UV detection with Neil Gehrels Swift Observatory/UVOT in ATel #13274. M31N 2008-12a has now been detected in eruption 12 times in 12 consecutive years (cf. ATels #5607, #6527, #7964, #9848, #11116, #12177). Comprehensive multi-wavelength studies of previous eruptions were published by Darnley et al. (2014, 2015, 2016) and Henze et al. (2014, 2015a, 2015b, 2018), a recent review article has been published by Darnley & Henze (2019). For additional optical photometry and spectroscopy of the ongoing eruption see ATels #13273, #13274, #13279, #13281.

Here we report the emergence of the supersoft X-ray source (SSS) phase of M31N 2008-12a. A faint X-ray counterpart was detected in a 3.6-ks Swift observation starting on 2019-11-12.33 UT (MJD 58799.33). We measured the preliminary XRT count rate to be (7.3 ± 1.6) × 10^-3 ct/s. No X-ray source was detected at a significant level in the preceding 0.7-ks Swift observation on 2019-11-11.33 UT with an 3σ upper limit of 1.02 × 10^-2 ct/s (nor in any of the previous daily observations since the detection of this eruption).

If we assume an eruption date of 2019-11-06.6 UT (MJD 58793.6), determined by fitting the Liverpool Telescope and LCO optical photometric data of the 2019 eruption to the eruption light-curve template (see Darnley et al. 2016), then the SSS counterpart appeared around day 5.7 after eruption. This preliminary estimate is consistent with the 5.9 ± 0.5 days measured in 2014 (ATel #6558, Henze et al. 2015), the 5.7 ± 0.5 days seen in 2015 (ATel #7984, Darnley et al. 2016), and the 5.8 days observed for the peculiar 2016 eruption (ATel #9872, Henze et al. 2018). The X-ray spectrum at this time is consistent with SSS emission

We wish to thank the Swift Team for the excellent scheduling of the observations, in particular B. Cenko, the duty scientists, and the science planners.