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

Abstract

We present a strong-lensing analysis of the galaxy cluster MACS J0138.0-2155 (z_ =0.336), the first known cluster lens 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 distribution of this cluster has been investigated with several independent lens models, using a sample of 23 spectroscopically confirmed multiple images from eight background sources spanning 0.767 < z < 3.420, identified from Hubble Space Telescope and James Webb Space Telescope imaging, and VLT/MUSE spectroscopy. For this study, we developed a new lens model with the Bayesian parametric lens-modelling framework , using the same strong-lensing dataset. Under a fiducial flat Lambda Cold Dark Matter cosmology, our reference mass model accurately reproduces the observed image positions, with an image-plane root-mean-square positional residual of Δ_ l s Gravity.jl ̊m rms =0.15''. Assuming H_0=70, , ^ km s -1 , ^ Mpc -1 , we predicted the future reappearances of highly delayed SN counter images, finding Δ t_ ̊m 1d,1a =3122^ +75 _ -70 days (from approximately April to August 2032) for SN Encore and Δ t_ ̊m 2d,2a =3902^ +67 _ -72 days (from approximately January to May 2027) for SN Requiem. By allowing the Hubble constant to vary and including the measured relative time delays of both SN Encore and SN Requiem, together with their statistical uncertainties, as observables, we inferred the value of H_0 jointly with the lens-model parameters. We performed the cosmographic analysis with three alternative parametrisations of the large-scale cluster halo and combined the resulting H_0 posteriors through Bayesian model averaging, using the corresponding Bayesian evidences as model weights. The resulting model-averaged constraint is H_0=69.2^ +9.6 _ -8.0 , , ^ km s -1 , ^ Mpc -1 , consistent with the value inferred from independent lens models of this cluster. The current uncertainty remains dominated by the relatively large fractional uncertainties of the measured SN time delays (gtrsim10%), while the scatter among the mass parametrisations considered here is subdominant. The forthcoming long-baseline reappearance of SN Requiem offers an immediate opportunity to improve the statistical precision on the Hubble constant, provided that lens-model systematics are sufficiently controlled. Together, these results establish MACS J0138.0-2155 as a premier anchor for high-precision cluster-scale time-delay cosmography with multiply imaged SNe.