Cosmology with Supernova Encore in the Strong Lensing Cluster MACS J0138–2155: Time Delays and Hubble Constant Measurement

Multiply imaged supernovae (SNe) provide a novel means of constraining the Hubble constant (H₀). Such measurements require a combination of precise models of the lensing mass distribution and an accurate estimate of the relative time delays between arrival of the multiple images. Only two multiply imaged SNe, Refsdal and H0pe, have enabled measurements of H₀ thus far. Here we detail the third such measurement for SN Encore, a z = 1.95 Type Ia SN discovered in JWST/NIRCam imaging. We measure the time delay, perform simulations of additional microlensing and millilensing systematics, and combine with the mass models of Suyu et al. in a double-blind analysis to obtain our H₀ constraint. Our final time-delay measurement is Δt_{1b, 1a} = −39.8_−3.3^+3.9 days, which is combined with seven lens models weighted by the likelihood of the observed multiple image positions for a result of H₀ = 66.9_−8.1^+11.2 kms⁻¹ Mpc⁻¹. The uncertainty on this measurement could be improved significantly if template imaging is obtained. Remarkably, a sibling to SN Encore (SN “Requiem”) was discovered in the same host galaxy, making the MACS J0138.0–2155 cluster the first system known to produce more than one observed multiply imaged SN. SN Requiem has a fourth image that is expected to appear within a few years, providing an unprecedented decade-long baseline for time-delay cosmography and an opportunity for a high-precision joint estimate of H₀.