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

Strong gravitational lensing magnifies the light from a background source, allowing us to study these sources in detail. Here, we study the spectra of a 𝑧=1.95lensed Type Ia supernova SN Encore for its brightest Image A, taken 39 days apart. We infer the spectral age with template matching using the supernova identification (SNID) software and find the spectra 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 accounting for time-dilation. Since SN Ia measure dark energy properties by providing relative distances between low- and high-𝑧SN ,it is important to test for evolution of spectroscopic properties. Comparing the spectra to composite low-𝑧SN Ia spectra, we find strong evidence for similarity between the local sample of SN Encore. The line velocities of common SN Ia spectral lines, Si II 6355 and Ca II NIR triplet are consistent with the distribution for the low-𝑧sample as well as other lensed SNe Ia, e.g.iPTF16geu (𝑧=0.409)and SN H0pe (𝑧=1.78). The consistency in SN Ia spectra across cosmic time demonstrates the utility of using SNe Ia in the very high-𝑧universe for dark energy inference. We also find that the spectra of SN Encore match the predictions for explosion models very well. With future large samples of lensed SNe Ia, spectra at such late phases will beimportant to distinguish between different explosion scenarios