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  • Liverpool Telescope Discovers Sixth Eruption of a Remarkable Recurrent Nova in M31

    Liverpool Telescope quasi-true colour image of the region of M31 around M31N 2008-12a. The ‘yellow’ backdrop is the unresolved stellar population of the disk of M31, where the dark patches are regions of high extinction. The majority of the stars seen in the image are in the foreground and belong to our own Galaxy. The image has been enhanced by the use of a Hα image to highlight regions of excited Hydrogen gas. The nova can be seen in eruption in the centre of the image (marked by the white arrow).


    The true recurrent nature of the Nova system designated M31N 2008-12a was finally characterized following its fifth detected optical eruption in 2013. An international study co-led by Matt Darnley (Astrophysics Research Institute, Liverpool John Moores University) and Martin Henze (European Space Astronomy Centre, Spain), along with independent work by the Intermediate Palomar Transient Factory (iPTF), uncovered the progenitor system of M31N 2008-12a, and inferred the presence of an extremely high mass white dwarf and a high mass accretion rate; the tell-tale signs that M31N 2008-12a may one day evolve to a Type Ia Supernova explosion.

    Such a high mass white dwarf leads to a very rapid evolution, and decline, of the optical lightcurve of each eruption. Despite five optical eruptions, and three separate X-ray detections, very little was known about the behaviour of the system during its eruptions.

    In anticipation of a sixth eruption towards the end of 2014, Darnley led a campaign on the Liverpool Telescope (LT) to monitor M31N 2008-12a to detect any changes in its behaviour. This LT campaign was also designed to react rapidly following a newly detected eruption, to obtain as much data on the system as possible.

    Nightly monitoring of M31N 2008-12a by the LT began towards the end of July 2014, and just before 10pm (GMT) on 2nd October a sixth eruption was detected. As planned, intensive photometric monitoring of the eruption using the IO:O optical imaging CCD camera on the LT was immediately implemented.

    In addition, and for the first time, we deployed the newly commissioned SPRAT (SPectrograph for the Rapid Acquisition of Transients) instrument on the LT, a low-resolution, and high throughput, spectrograph designed specifically for the classification of transients.

    SPRAT has been mounted on the LT for less than a month, and we used this new instrument to obtain the first spectra of an extragalactic nova ever taken by the LT. These data allowed us to spectroscopically confirm the nature of the eruption; and also to determine the expansion velocity of its ejecta.

    As well as Matt Darnley and Martin Henze, the international collaboration also includes; Mike Bode (LJMU), Steve Williams (LJMU), Allen Shafter (San Diego State University, USA), Jan-Uwe Ness (ESAC), and former LJMU PhD student Rebekah Hounsell (Space Telescope Science Institute, USA). Iain Steele, Rob Smith, and Andrzej Piascik, all from the LT Group at LJMU, were instrumental in obtaining and analysing the SPRAT spectroscopic observations.

    Liverpool Telescope SPRAT spectrum of M31N 2008-12a taken on the night of 3rd October 2014. The spectrum shows the tell-tale Hydrogen, Helium, and Nitrogen emission lines of a ‘He/N’ nova in eruption.