A massive millisecond magnetar may survive a merger of a neutron star (NS)
binary, which would continuously power the merger ejecta. We develop a generic
dynamic model for the merger ejecta with energy injection from the central
magnetar. The ejecta emission (the "merger-nova") powered by the magnetar peaks
in the UV band and the peak of lightcurve progressively shifts to an earlier
epoch with increasing frequency. A magnetar-powered mergernova could have an
optical peak brightness comparable to a supernova, which is a few tens or
hundreds times brighter than the radioactive-powered merger-novae (the
so-called macro-nova or kilo-nova). On the other hand, such a merger-nova would
peak earlier and have a significantly shorter duration than that of a
supernova. An early collapse of the magnetar could suppress the brightness of
the optical emission and shorten its duration. Such
millisecond-magnetar-powered merger-novae may be detected from NS-NS merger
events without an observed short gamma-ray burst, and could be a bright
electromagnetic counterpart for gravitational wave bursts due to NS-NS mergers.
If detected, it suggests that the merger leaves behind a massive NS, which has
important implications for the equation-of-state of nuclear matter.Comment: 6 pages, 4 figures, revised following the referee's report and
accepted for publication by ApJ
