Title:
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SN 1996N - A Type Ib supernova at late phases.

(Stockholm Observatory Supernova preprint No. STOCK_SN_007)


Authors:
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Jesper Sollerman (Stockholm Observatory)  and
Bruno Leibundgut & Jason Spyromilio (ESO, Garching)


Abstract:
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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 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 gamma-rays are leaking out of the ejecta. SN~1996N appears to be 
underluminous compared to SN 1993J, possibly indicating that less Ni-56 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] 6300, 6364, [Ca II] 7291, 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] 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-excited, thus revealing its existence.
Finally, we note that the emission lines of SN~1996N are blueshifted by roughly
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.


To appear in:
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Astronomy & Astrophysics, Main Journal