Time-Varying Sodium Absorption in the Type Ia Supernova 2013gh
  Temporal variability of narrow absorption lines in high-resolution spectra of
Type Ia supernovae (SNe Ia) is studied to search for circumstellar matter. Time
series which resolve the profiles of absorption lines such as Na I D or Ca II
H&K are expected to reveal variations due to photoionisation and subsequent
recombination of the gases. The presence, composition, and geometry of
circumstellar matter may hint at the elusive progenitor system of SNe Ia and
could also affect the observed reddening law. To date, there are few known
cases of time-varying Na I D absorption in SNe Ia, all of which occurred during
relatively late phases of the supernova evolution. Photoionisation, however, is
predicted to occur during the early phases of SNe Ia, when the supernova peaks
in the ultraviolet. We therefore attempt to observe early-time absorption-line
variations by obtaining high-resolution spectra of SNe before maximum light. We
have obtained photometry and high-resolution spectroscopy of SNe Ia 2013gh and
iPTF 13dge, to search for absorption- line variations. Furthermore, we study
interstellar absorption features in relation to the observed photometric
colours of the SNe. Results. Both SNe display deep Na I D and Ca II H&K
absorption features. Furthermore, small but significant variations are detected
in a feature of the Na I D profile of SN 2013gh. The variations are consistent
with either geometric effects of rapidly moving or patchy gas clouds or
photoionisation of Na I gas at R \approx 1019 cm from the explosion. Our
analysis indicates that it is necessary to focus on early phases to detect
photoionisation effects of gases in the circumstellar medium of SNe Ia.
Different absorbers such as Na I and Ca II can be used to probe for matter at
different distances from the SNe.
