Breaking the Habit: The Peculiar 2016 Eruption of the Unique Recurrent Nova M31N 2008-12a
2018
Since its discovery in 2008, the Andromeda galaxy nova M31N
2008-12a has been observed in eruption every single year. This
unprecedented frequency indicates an extreme object, with a
massive white dwarf and a high accretion rate,which is the most
promising candidate for the single-degenerate progenitor of a
Type Ia supernova known to date. The previous three eruptions
of M31N 2008-12a have displayed remarkably homogeneous
multiwavelength properties:. (i) from a faint peak, the optical
light curve declined rapidly by two magnitudes in less than two
days, (ii) early spectra showed initial high velocities that
slowed down significantly within days and displayed clear He/N
lines throughout, and (iii) the supersoft X-ray source (SSS)
phase of the nova began extremely early, six days after
eruption, and only lasted for about two weeks. In contrast, the
peculiar 2016 eruption was clearly different. Here we report
(i) the considerable delay in the 2016 eruption date, (ii) the
significantly shorter SSS phase, and (iii) the brighter optical
peak magnitude (with a hitherto unobserved cusp shape). Early
theoretical models suggest that these three different effects
can be consistently understood as caused by a lower quiescence
mass accretion rate. The corresponding higher ignition mass
caused a brighter peak in the free-free emission model. The
less massive accretion disk experienced greater disruption,
consequently delaying the re-establishment of effective
accretion. Without the early refueling, the SSS phase was
shortened. Observing the next few eruptions will determine
whether the properties of the 2016 outburst make it a genuine
outlier in the evolution of M31N 2008-12a.
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