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Temporal variability of narrow absorption lines in high-resolution spectra ofType Ia supernovae (SNe Ia) is studied to search for circumstellar matter. Timeseries which resolve the profiles of absorption lines such as Na I D or Ca IIH&K are expected to reveal variations due to photoionisation and subsequentrecombination of the gases. The presence, composition, and geometry ofcircumstellar matter may hint at the elusive progenitor system of SNe Ia andcould also affect the observed reddening law. To date, there are few knowncases of time-varying Na I D absorption in SNe Ia, all of which occurred duringrelatively late phases of the supernova evolution. Photoionisation, however, ispredicted to occur during the early phases of SNe Ia, when the supernova peaksin the ultraviolet. We therefore attempt to observe early-time absorption-linevariations by obtaining high-resolution spectra of SNe before maximum light. Wehave obtained photometry and high-resolution spectroscopy of SNe Ia 2013gh andiPTF 13dge, to search for absorption- line variations. Furthermore, we studyinterstellar absorption features in relation to the observed photometriccolours of the SNe. Results. Both SNe display deep Na I D and Ca II H&Kabsorption features. Furthermore, small but significant variations are detectedin a feature of the Na I D profile of SN 2013gh. The variations are consistentwith either geometric effects of rapidly moving or patchy gas clouds orphotoionisation of Na I gas at R \approx 1019 cm from the explosion. Ouranalysis indicates that it is necessary to focus on early phases to detectphotoionisation 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 atdifferent distances from the SNe.

Time-Varying Sodium Absorption in the Type Ia Supernova 2013gh