Constraining the Progenitor Companion of the Nearby Type Ia SN 2011fe with a Nebular Spectrum at +981 Days

Abstract

We present an optical nebular spectrum of the nearby Type Ia supernova 2011fe, obtained 981 days after explosion. SN 2011fe exhibits little evolution since the +593 day optical spectrum, but there are several curious aspects in this new extremely late-time regime. We suggest that the persistence of the $\sim5800$~\AA\ feature is due to Na I D, and that a new emission feature at $\sim7300$~\AA\ may be [Ca II]. Also, we discuss whether the new emission feature at $\sim6400$~\AA\ might be [Fe I] or the high-velocity hydrogen predicted by Mazzali et al. The nebular feature at 5200~\AA\ exhibits a linear velocity evolution of $\sim350$ $\rm km\ s^{-1}$ per 100 days from at least +220 to +980 days, but the line's shape also changes in this time, suggesting that line blending contributes to the evolution. At $\sim 1000$ days after explosion, flux from the SN has declined to a point where contribution from a luminous secondary could be detected. In this work we make the first observational tests for a post-impact remnant star and constrain its temperature and luminosity to $T \gtrsim 10^4$ $\rm K$ and $L \lesssim 10^4$ $\rm L_{\odot}$. Additionally, we do not see any evidence for narrow H$\alpha$ emission in our spectrum. We conclude that observations continue to strongly exclude many single-degenerate scenarios for SN 2011fe.Comment: 11 pages, 10 figures, published by MNRA