  The hydrogen-poor supernova PTF11rka (z = 0.0744), reported by the Palomar
Transient Factory, was observed with various telescopes starting a few days
after the estimated explosion time of 2011 Dec. 5 UT and up to 432 rest-frame
days thereafter. The rising part of the light curve was monitored only in the
R_PTF filter band, and maximum in this band was reached ~30 rest-frame days
after the estimated explosion time. The light curve and spectra of PTF11rka are
consistent with the core-collapse explosion of a ~10 Msun carbon-oxygen core
evolved from a progenitor of main-sequence mass 25--40 Msun, that liberated a
kinetic energy (KE) ~ 4 x 10^{51} erg, expelled ~8 Msun of ejecta (Mej), and
synthesised ~0.5 Msun of 56Nichel. The photospheric spectra of PTF11rka are
characterised by narrow absorption lines that point to suppression of the
highest ejecta velocities ~>15,000 km/s. This would be expected if the ejecta
impacted a dense, clumpy circumstellar medium. This in turn caused them to lose
a fraction of their energy (~5 x 10^50 erg), less than 2% of which was
converted into radiation that sustained the light curve before maximum
brightness. This is reminiscent of the superluminous SN 2007bi, the light-curve
shape and spectra of which are very similar to those of PTF11rka, although the
latter is a factor of 10 less luminous and evolves faster in time. PTF11rka is
in fact more similar to gamma-ray burst supernovae (GRB-SNe) in luminosity,
although it has a lower energy and a lower KE/Mej ratio.

High Energy Astrophysical Phenomena