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Recurrent nova M31N 2008-12a: Swift/XRT detection of the 2017 eruption

ATel #11130; M. Henze (SDSU), M. J. Darnley (LJMU), A. W. Shafter (SDSU), S. Kafka (AAVSO), M. Kato (Keio University), S. C. Williams (Lancaster), for a larger collaboration
on 5 Jan 2018; 19:18 UT
Credential Certification: Martin Henze (henze@ice.cat)

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

Referred to by ATel #: 11133, 11149, 11214

In ATel #11116 we announced the discovery of the predicted 2017 eruption of the recurrent nova M31N 2008-12a on 2017-12-31.77 UT. We reported the follow-up UV detection with Swift/UVOT in ATel #11121. This is the 10th observed eruption in 10 consecutive years of this unique nova system (cf. ATels #5607, #6527, #7964, #9848). Comprehensive multi-wavelength studies of previous eruptions were published by Darnley et al. (2014, 2015, 2016) and Henze et al. (2014, 2015a, 2015b, 2018 subm.). For additional optical photometry and spectroscopy of the ongoing eruption see ATels #11116, #11117, #11118, #11124, #11125, #11126.

Here we report the start of the supersoft X-ray source (SSS) phase of M31N 2008-12a. A faint X-ray counterpart was detected in a 5.2-ks Swift observation starting on 2018-01-05.48 UT. We measured the preliminary XRT count rate to be (3.1±1.0) × 10-3 ct/s (corrected for vignetting, dead time and PSF). No X-ray source was detected in the preceding 1.2-ks Swift observation on 2018-01-04.48 UT with an 3σ upper limit of 6.0 × 10-3 ct/s.

If we assume an eruption date of 2017-12-31.77 UT (MJD 58118.77), identical to the discovery date (ATel #11116), then the SSS counterpart appeared around day 4.7 after eruption. This preliminary estimate is somewhat earlier than 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 & Henze et al. 2016), and the 5.8 days observed for the peculiar 2016 eruption (ATel #9872). This might suggest an earlier eruption date, which we are currently working on constraining more accurately.

In addition, the nova is still detected as an UV source but its magnitude has declined significantly to uvw2 = 19.3±0.1 mag (cf. ATel #11121). 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 rapid scheduling of the observations, in particular B. Cenko, the duty scientists, and the science planners.