Spectroscopic analysis of the strongly lensed SN Encore: Constraints on cosmic evolution of Type Ia supernovae

Strong gravitational lensing magnifies the light from abackground source, allowing us to study these sources in detail. Here, we studythe spectra of a z = 1.95 lensed Type Ia supernova (SN Ia) SN Encore for itsbrightest image A, taken 39 d apart. We infer the spectral age with templatematching using the supernova identification (SNID) software and find thespectra to be at 29.0 ± 5.0 and 37.4 ± 2.8 rest-frame days post-maximum,respectively, consistent with separation in the observer frame after accountingfor time dilation. Since SNe Ia measure dark energy properties by providingrelative distances between low- and high-z SNe, it is important to test for theevolution of spectroscopic properties. Comparing the spectra to composite low-zSN Ia spectra, we find strong evidence of the similarity between the localsample and SN Encore. The line velocities of common SN Ia spectral lines, Si II6355 Å and Ca II near-infrared triplet, are consistent with the distributionfor the low-z sample as well as other lensed SNe Ia, e.g. iPTF16geu (z = 0.409) and SN H0pe (z = 1.78). The consistency between the low-z sample and lensed SNeat high-z suggests no obvious cosmic evolution demonstrating their use ashigh-z distance indicators, though this needs to be confirmed/refuted via alarger sample. We also find that the spectra of SN Encore match the predictionsfor explosion models very well. With future large samples of lensed SNe Ia,e.g. with the Vera C. Rubin Observatory, spectra at such late phases will beimportant to distinguish between different explosion scenarios.