A new $H_0$ measurement with SNe Requiem and Encore using Gravity.jl

Abstract: We present a strong-lensing (SL) analysis of the galaxy cluster MACS J0138.0-2155 (z=0.336), the first known lens cluster discovered to host two distinct multiply imaged Type Ia supernovae (SNe): SN Requiem and SN Encore. Both transients are located in the massive, multiply imaged red galaxy MRG-M0138 at z=1.949. The projected total mass of this cluster has been investigated with several independent lens models (Suyu+26; Pierel+26), using a sample of 23 spectroscopically confirmed multiple images from 8 background sources (0.767<z\<3.420), identified from the HST and the JWST imaging data, and VLT/MUSE spectroscopy. In this work, we develop a new SL model based on a novel Bayesian parametric lens-modelling framework Gravity.jl, exploiting the same SL dataset. Our reference mass model accurately reproduces the observed image positions, with an image-plane rms image position residual of 0.24''. Assuming H0 = 70 km/s/Mpc, we predict the future reappearances of highly delayed SNe counter-images, finding Dt(1d,1a) = 3177_{-59}^{+78} d (May-September, 2032) for SN Encore and Dt(2d,2a) = 3938_{-77}^{+90} d (February-July, 2027) for SN Requiem. By allowing H0 to vary, and using the measured time delays of both SN Encore and SN Requiem together with their statistical uncertainties as observables, we infer the value of H0 jointly with the other lens-model free parameters. From this analysis, we obtain a new measurement of H0 = 67.0_{-7.8}^{+9.3} km/s/Mpc, consistent with the value inferred from the aforementioned independent lens models. This error is currently dominated by the large relative uncertainty on the measured time delays (\>10%). The forthcoming reappearance of SN Requiem offers an immediate opportunity to significantly improve constraints on H0, provided that lens-model systematics are controlled. These results establish M0138 as a premier anchor for high-precision cluster-scale TDC.