BayeSN-TD: Time Delay and H 0 Estimation for Lensed SN H0pe

We present BayeSN-TD, an enhanced implementation of the probabilistic type Ia supernova (SN Ia) BayeSN SED model, designed for fitting multiply-imaged, gravitationally lensed type Ia supernovae (glSNe Ia). BayeSN-TD fits for magnifications and time-delays across multiple images while marginalising over an achromatic, Gaussian process-based treatment of microlensing, to allow for time-dependent deviations from a typical SN Ia SED caused by gravitational lensing by stars in the lensing system. BayeSN-TD is able to robustly infer time delays and produce well-calibrated uncertainties, even when applied to simulations based on a different SED model and incorporating chromatic microlensing, strongly validating its suitability for time-delay cosmography. We then apply BayeSN-TD to publicly available photometry of the glSN Ia SN H0pe, inferring time delays between images BA and BC of Δ TBA=121.9+9.5-7.5 days and Δ TBC=63.2+3.2-3.3 days along with absolute magnifications β for each image, β A = 2.38+0.72-0.54, β B=5.27+1.25-1.02 and β C=3.93+1.00-0.75. Combining our constraints on time-delays and magnifications with existing lens models of this system, we infer H₀=69.3+12.6-7.8 km s−1 Mpc−1, consistent with previous analysis of this system; incorporating additional constraints based on spectroscopy yields H₀=66.8+13.4-5.4 km s−1 Mpc−1. While this is not yet precise enough to draw a meaningful conclusion with regard to the ‘Hubble tension’, upcoming analysis of SN H0pe with more accurate photometry enabled by template images, and other glSNe, will provide stronger constraints on H0; BayeSN-TD will be a valuable tool for these analyses. The BayeSN-TD code is available at https://github.com/bayesn/bayesn-td.