SN 1996N - A type Ib supernova at late phases

We present photometric and spectroscopic observations of the Type Ib supernova (SN Ib) 1996N in NGC 1398. The supernova has been observed at several occasions between 179 and 337 days after discovery. The light curves in V , R and I have decline rates of 1.67±0.23, 1.72±0.10 and 1.93±0.24 mag (100d)−1 respectively, substantially faster than the decay rate of 56Co. The late light curves of SN 1996N are rather similar to the light curves of SN 1993J, and the decline is consistent with simple models of a radioactively powered supernova, where the γ-rays are leaking out of the ejecta. SN 1996N appears to be underluminous compared to SN 1993J, possibly indicating that less 56Ni was ejected in the explosion, but uncertainties in intrinsic absorption prevents us from definite conclusions. The late time spectra of SN 1996N are similar to spectra of other SNe Ib/c, and are dominated by [O i] λλ6300, 6364, [Ca ii] λλ7291, 7324, and the Ca ii near-IR triplet. In particular, these spectra closely resemble those of SN 1993J. We speculate about the possibility that the broad emission feature seen redward of [O i] λ6364 in SN 1996N could be due to Hα, as it was in SN 1993J. In this scenario, small amounts of hydrogen might go unnoticed in the early spectra of SN Ib/c. Later on, when the SN continuum has faded, the hydrogen layer might be re-excited, thus revealing its existence. Finally, we note that the emission lines of SN 1996N are blueshifted by ∼1000 km s−1. With the scenarios proposed for the blueshifts in SN 1993J in mind, we discuss the possible cause for the shifts in SN 1996N. The lineshifts may in fact indicate large scale asymmetries in the supernova explosion, rather than the formation of dust at early epochs.