Unbeamed tidal disruption events at hard X-rays
  Owing to their thermal emission, tidal disruption events (TDEs) were
regularly detected in the soft X-rays and sometimes in the optical. Only a few
TDEs have been detected at hard X-rays: two are high redshift beamed events,
one of which occurred at the core of a nearby galaxy, and the most recent one
is of a different nature, involving a compact object in the Milky Way. The aims
of this work are to obtain a first sample of hard X-ray-selected unbeamed TDEs,
to determine their frequency and to probe whether TDEs usually or exceptionally
emit at hard X-ray energies. We performed extensive searches for hard X-ray
flares at positions in over 53000 galaxies, up to a distance of 100 Mpc in the
Swift BAT archive. Light curves were extracted and parametrized. The quiescent
hard X-ray emission was used to exclude persistently active galactic nuclei.
Significant flares from non-active galaxies were derived and checked for
possible contamination. We found a sample of nine TDE candidates, which
translates into a rate of $2 \times 10^{-5}\,{\rm galaxy}^{-1}\,{\rm yr}^{-1}$
above the BAT detection limit. This rate is consistent with those observed by
XMM-Newton at soft X-rays and in the optical from SDSS observations, and is as
expected from simulations. We conclude that hard X-ray emission should be
ubiquitous in un-beamed TDEs and that electrons should be accelerated in their
accretion flow.
