Recurrent nova M31N 2008-12a: Swift/XRT detection of the 2021 eruption
ATel #15050; M. J. Darnley (LJMU), K. L. Page (Leicester) on behalf of the 12a Collaboration
on 19 Nov 2021; 14:08 UT
Credential Certification: Matt Darnley (M.J.Darnley@ljmu.ac.uk)
Subjects: X-ray, Nova, Transient
The discovery of the 2021 eruption of the annually recurring recurrent nova M31N 2008-12a was reported in ATel #15034, with spectroscopic confirmation reported in ATel #15036. Upon discovery, we triggered our pre-approved GI programme on the Neil Gehrels Swift Observatory, reporting the Swift/UVOT uvw2 detection in ATel #15040. Follow-up observations have also been reported in ATels #15037, #15038, #15039, and #15045, with observations on-going at many facilities.
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 5.6-ks Swift observation between 2021-11-19.064 UT (MJD 59537.064) and 19.277 (MJD 59537.277).
We measured the preliminary XRT count rate to be (3.7 ± 1.2) × 10-3 ct/s. Although this detection is only fractionally above 3σ, there is a clear source in the XRT image at the location of M31N 2008-12a, with all detected X-ray photons below 1.5 keV.
No X-ray source was detected at a significant level in the preceding 1.9-ks Swift observation on 2021-11-18.5 UT with a 3σ upper limit of 4.9 × 10-3 ct/s (nor in any of the previous daily observations since the detection of this eruption).
If we assume an eruption date of 2021-11-14.2 ± 0.2 UT (see ATels #15034 and #15037), then the SSS counterpart appeared around day 5.0 ± 0.2 post-eruption. This preliminary estimate is broadly 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). However, given the faintness of the detection, this may be the earliest detection yet of SSS emission from this system.
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 the duty scientists, and the science planners.
