Two type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions

Aims. The aim of this paper is to discuss the nature of two type Ic supernovae SN 2007bg and SN 2007bi and their host galaxies. Both supernovae were discovered in wide-field, non-targeted surveys and are found to be associated with sub-luminous blue dwarf galaxies identified in SDSS images. Methods. We present BVRI photometry and optical spectroscopy of SN 2007bg and SN 2007bi and their host galaxies. Their lightcurves and spectra are compared to those of other type Ic SNe and analysis of these data provides estimates of the energetics, total ejected masses and synthesised mass of Ni-56. Detection of the host galaxy emission lines allows for metallicity measurements. Results. Neither SNe 2007bg nor 2007bi were found in association with an observed GRB, but from estimates of the metallicities of their host-galaxies they are found to inhabit similar low-metallicity environments as GRB associated supernovae. The radio-bright SN 2007bg is hosted by an extremely sub-luminous galaxy of magnitude M-B = -12.4 +/- 0.6 mag and an estimated oxygen abundance of 12 + log(O/H) = 8.18 +/- 0.17 (on the Pettini & Pagel 2004 scale). The early lightcurve evolution of SN 2007bg matches the fast-pace decline of SN 19941 giving it one of the fastest post-maximum decline rates of all broad-lined type Ic supernovae known to date and, when combined with its high expansion velocities, a high kinetic energy to ejected mass ratio (E-K/M-ej similar to 2.7). We also show that SN 2007bi is possibly the most luminous type Ic known, reaching a peak magnitude of M-R similar to -21.3 mag and displays a remarkably slow decline, following the radioactive decay rate of Co-56 to Fe-56 throughout the course of its observed lifetime. SN 2007bi also displays an extreme longevity in its spectral evolution and is still not fully nebular at approximately one year post-maximum brightness. From a simple model of the bolometric light curve of SN 2007bi we estimate a total ejected Ni-56 mass of M-Ni = 3.5-4.5 M-circle dot, the largest Ni-56 mass measured in the ejecta of a supernova to date. There are two models that could explain the high luminosity and large ejected Ni-56 mass. One is a pair-instability supernova (PISN) which has been predicted to occur for massive stars at low metallicities. We measure the host galaxy metallicity of SN 2007bi to be 12 + log(O/H) = 8.15 +/- 0.15 (on the McGaugh 1991 scale) which is somewhat high to be consistent with the PISN model. An alternative is the core-collapse of a C+O star of 20-40 M-circle dot which is the core of a star of originally 50-100 M-circle dot.