Searching for light echoes due to CSM in SN Ia spectra
  We present an analytical model for light echoes (LEs) coming from
circumstellar material (CSM) around Type Ia Supernovae (SNe Ia). Using this
model we find two spectral signatures at 4100 {\AA} and 6200 {\AA} that are
useful to identify LEs during the Lira law phase (between 35 and 80 days after
maximum light) coming from nearby CSM at distances of 0.01-0.25 pc. We analyze
a sample of 89 SNe Ia divided in two groups according to their B-V decline rate
during the Lira law phase, and search for LEs from CSM interaction in the group
of SNe with steeper slopes by comparing their spectra with our LE model. We
find that a model with LEs + pure extinction from interstellar material (ISM)
fits better the observed spectra than a pure ISM extinction model that is
constant in time, but we find that a decreasing extinction alone explains
better the observations without the need of LEs, possibly implying dust
sublimation due to the radiation from the SN.
