  Modern wide-field high-cadence surveys have revealed the significant
diversity of optical transient phenomena in their luminosity and timescale
distributions, which led to the discovery of some mysterious fast optical
transients (FOTs). These FOTs can usually rise and decline remarkably in a
timescale of a few days to weeks, which are obviously much rapider than
ordinary supernovae. SN 2019bkc/ATLAS19dqr is one of the fastest detected FOTs
so far and, meanwhile, it was found to be un-associated with a host galaxy.
These discoveries provide a good chance to explore the possible origins of
FOTs. So, we model the light curves of SN 2019bkc in details. It is found that
SN 2019bkc can be well explained by the thermal emission of an explosion ejecta
that is powered by a long-lasting central engine. The engine could be a
spinning-down millisecond magnetar or a fallback accretion onto a compact
object. Combining the engine property, the mass of the ejecta, and the
hostlessness of SN 2019bkc, we suggest that this FOT is likely to originate
from a merger of a white dwarf and a neutron star.
