SN 1996N - A TYPE IB SUPERNOVA AT LATE PHASES

We present photometric and spectroscopic observations of the Type Ib s upernova (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 a nd 1.93+/-0.24 mag (100d)(-1) respectively, substantially faster than the decay rate of Co-56. 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 ga mma-rays are leaking out of the ejecta. SN 1996N appears to be underlu minous compared to SN 1993J, possibly indicating that less Ni-56 was e jected in the explosion, but uncertainties in intrinsic absorption pre vents 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] l ambda lambda 6300, 6364, [Ca II] lambda lambda 7291, 7324, and the Ca II near-IR triplet. In particular, these spectra closely resemble thos e of SN 1993J. We speculate about the possibility that the broad emiss ion feature seen redward of [O I] lambda 6364 in SN 1996N could be due to H alpha, 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-exci ted, thus revealing its existence. Finally, we note that the emission lines of SN 1996N are blueshifted by similar to 1000 km s(-1). With th e scenarios proposed for the blueshifts in SN 1993J in mind, we discus s the possible cause for the shifts in SN 1996N. The lineshifts may in fact indicate large scale asymmetries in the supernova explosion, rat her than the formation of dust at early epochs.