Rapidly Rising Transients in the Supernova - Superluminous Supernova Gap
  We present observations of four rapidly rising (t_{rise}~10d) transients with
peak luminosities between those of supernovae (SNe) and superluminous SNe
(M_{peak}~-20) - one discovered and followed by the Palomar Transient Factory
(PTF) and three by the Supernova Legacy Survey (SNLS). The light curves
resemble those of SN 2011kl, recently shown to be associated with an
ultra-long-duration gamma ray burst (GRB), though no GRB was seen to accompany
our SNe. The rapid rise to a luminous peak places these events in a unique part
of SN phase space, challenging standard SN emission mechanisms. Spectra of the
PTF event formally classify it as a Type II SN due to broad Halpha emission,
but an unusual absorption feature, which can be interpreted as either high
velocity Halpha (though deeper than in previously known cases) or Si II (as
seen in Type Ia SNe), is also observed. We find that existing models of white
dwarf detonations, CSM interaction, shock breakout in a wind (or steeper CSM)
and magnetar spindown can not readily explain the observations. We consider the
possibility that a "Type 1.5 SN" scenario could be the origin of our events.
More detailed models for these kinds of transients and more constraining
observations of future such events should help better determine their nature.
