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 SN lacking spectroscopic signatures of H and He. SN 2010mb has a slowly-declining light curve ($sim600,$days) that cannot be powered by $^{56}$Ni/$^{56}$Co radioactivity, the common energy source for Type Ic SNe. We detect signatures of interaction with hydrogen-free CSM including a blue quasi-continuum and, uniquely, narrow oxygen emission lines that require high densities ($sim10^9$cm$^{-3}$). From the observed spectra and light curve we estimate that the amount of material involved in the interaction was $sim3$M$_{odot}$. Our observations are in agreement with models of pulsational pair-instability SNe described in the literature.