SN 2010mb: direct evidence for a supernova interacting with a large amount of hydrogen-free circumstellar material

We present our observations of SN 2010mb, a Type Ic supernova (SN) lacking spectroscopic signatures of H and He. SN 2010mb has a slowly declining light curve (LC) (~600 days) that cannot be powered by 56Ni/56Co radioactivity, the common energy source for Type Ic SNe. We detect signatures of interaction with hydrogen-free circumstellar material including a blue quasi-continuum and, uniquely, narrow oxygen emission lines that require high densities (~109 cm–3). From the observed spectra and LC, we estimate that the amount of material involved in the interaction was ~3 M ☉. Our observations are in agreement with models of pulsational pair-instability SNe described in the literature.