Recurrent nova M31N 2008-12a: Swift/XRT detection of the 2018 eruption

ATel #12207; 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 13 Nov 2018; 14:12 UT
Credential Certification: Martin Henze (henze@ice.cat)

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In ATel #12177 we announced the discovery of the 2018 eruption of the recurrent nova M31N 2008-12a on 2018-11-06.80 UT. We reported the follow-up UV detection with Swift/UVOT in ATel #12182. M31N 2008-12a has now been detected in eruption 11 times in 11 consecutive years (cf. ATels #5607, #6527, #7964, #9848, #11116). Comprehensive multi-wavelength studies of previous eruptions were published by Darnley et al. (2014, 2015, 2016) and Henze et al. (2014, 2015a, 2015b, 2018). For additional optical photometry and spectroscopy of the ongoing eruption see ATels #12179, #12181, #12189, #12190, #12195, #12200, #12203, #12204, #12205.

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 3.9-ks Swift observation starting on 2018-11-13.02 UT. We measured the preliminary XRT count rate to be (1.1±0.2) × 10^-2 ct/s (corrected for vignetting, dead time and PSF). No X-ray source was detected at a significant level in the preceding 4.7-ks Swift observation on 2018-11-12.03 UT with an 3σ upper limit of 4.0 × 10^-3 ct/s.

If we assume an eruption date of 2018-11-06.80 UT (MJD 58428.80), identical to the discovery date (ATel #12177), then the SSS counterpart appeared around day 6.2 after eruption. Taking into account the last non-detection, this preliminary estimate is well 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 & Henze et al. 2016), and the 5.8 days observed for the peculiar 2016 eruption (ATel #9872, Henze et al. 2018). Note the slightly earlier preliminary 4.7 days for the 2017 eruption (ATel #11130).

In addition, the nova is still detected as an ultraviolet (UV) source with a magnitude of uvw2 = 19.5±0.1 mag, well below the first UVOT measurement of uvw2 = 17.3±0.1 mag (ATel #12182). 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 excellent scheduling of the observations, in particular B. Cenko, the duty scientists, and the science planners.