Recurrent Nova M31N 2008-12a: The 2016 eruption may be imminent
ATel #9415; M. J. Darnley (LJMU), M. Henze (CSIC-IEEC)
on 26 Aug 2016; 10:33 UT
Credential Certification: Matt Darnley (M.J.Darnley@ljmu.ac.uk)
Subjects: Optical, Nova, Transient
The remarkable recurrent nova M31N 2008-12a has been observed undergoing eruptions eleven times, which includes annual eruptions between 2008 and 2015 (see Darnley et al. 2016, for a recent review), with the system having a predicted recurrence period of just under six months (Henze et al. 2015).
In order to detect the next eruption, high cadence monitoring observations have been on-going since the last detected eruption (in late August 2015) involving a large number of telescopes around the world. The predicted window for the 2016 eruption is August 21 2016 to October 13 2016 (1 sigma range).
The Liverpool Telescope (Steele et al. 2004) has been monitoring M31N 2008-12a with a high cadence (typically a few hours) in the u'-band. On the night of August 24 2016 a series of four 180s u'-band observations found no object at the position of the recurrent nova down to a 3 sigma limiting magnitude of 23.1. A 60s r'-band observation of the field taken by the LCOGT 2m (Hawaii; aka Faulkes Telescope North) around 12 hours later also found no object at the position of M31N 2008-12a.
However, on the night of August 25 2016, a similar series of observations (each around an hour apart) indicated a positive detection at the position of M31N 2008-12a. The detection has a significance of 3.5 sigma, and the object has photometry of u' = 23.51 ± 0.32. We also note that a faint object is visible by eye in the stacked observation, and that there is a marginal detection (around 2 sigma) in each of the four single frames. This marks the first potential detection of M31N 2008-12a since the monitoring of the 2015 eruption ceased at the end of September 2015.
While we do not claim that this is the start of the 2016 eruption, we do believe that this may be a precursor event, as similar behaviour may have been detected in Liverpool Telescope data prior to the 2015 eruption (Henze et al. in prep).
Therefore we recommend that any on-going or planned observations of this system are increased in urgency and cadence, particularly in bluer bands.
