Recent searches by unbiased, wide-field surveys have uncovered a group
of extremely luminous optical transients. The initial discoveries of SN
2005ap by the Texas Supernova Search and SCP-06F6 in a deep Hubble
pencil beam survey were followed by the Palomar Transient Factory
confirmation of host redshifts for three similar transients. The
transients share the common properties of high optical luminosities
(peak magnitudes ~ -21 to -23), blue colors, and a lack of H or He
spectral features. The physical mechanism that produces the luminosity
is uncertain, with suggestions ranging from jet-driven explosion to
pulsational pair-instability. Here we report the most detailed
photometric and spectral coverage of an ultra-bright transient (SN
2010gx) detected in the Pan-STARRS1 sky survey. In common with other
transients in this family, the early-time spectra show a blue continuum,
and prominent broad absorption lines of O II. However, about 25d after
discovery, the spectra developed type Ic supernova features, showing the
characteristic broad Fe II and Si II absorption lines. Detailed,
post-maximum follow-up may show that all SN 2005ap and SCP-06F6 type
transients are linked to supernovae Ic. This poses problems in
understanding the physics of the explosions: there is no indication from
late-time photometry that the luminosity is powered by 56Ni, the broad
lightcurves suggest very large ejected masses, and the slow spectral
evolution is quite different from typical Ic timescales. The nature of
the progenitor stars and the origin of the luminosity are intriguing and
open questions.
