Jeffrey M. Silverman, Mohan Ganeshalingam, Alexei V. Filippenko
In this work we analyse late-time (t > 100 d) optical spectra of low-redshift (z < 0.1) Type Ia supernovae (SNe Ia) which come mostly from the Berkeley Supernova Ia Program dataset. We also present spectra of SN 2011by for the first time. The sample studied consists of 34 SNe Ia with 60 nebular spectra, which represents one of the largest sets of late-time SN Ia spectra ever analysed. The full width at half-maximum intensity (FWHM) and velocities of the [Fe III] {\lambda}4701, [Fe II] {\lambda}7155, and [Ni II] {\lambda}7378 emission features are measured in most of the spectra that are spectroscopically normal, have signal-to-noise ratios >20/px, and are older than 160 d past maximum brightness. The velocities of all three features are seen to be relatively constant with time, increasing only a few to ~20 km/s/d. The nebular velocity (v_neb, calculated by taking the average of the [Fe II] {\lambda}7155 and [Ni II] {\lambda}737 velocities) is correlated with the velocity gradient and near-maximum-brightness photospheric velocity; most high velocity gradient objects have redshifted nebular lines while most low velocity gradient objects have blueshifted nebular lines. A marginal correlation is found between v_neb and {\Delta}m_15(B), but for a given light-curve shape there is a large range of observed nebular velocities. The BSNIP data also indicate a strong correlation between observed (B-V)_max (which is likely mostly the intrinsic SN colour) and v_neb (a purely intrinsic quantity). Employing a relatively rudimentary search, evidence for light echoes in the late-time spectra is found for a handful of objects, though the presence of light echoes is not yet confirmed.
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http://arxiv.org/abs/1211.0279
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