Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Time delays & Hubble constant measurement

J. D. R. Pierel; E. E. Hayes; M. Millon; C. Larison; E. Mamuzic; A. Acebron; A. Agrawal; P. Bergamini; S. Cha; S. Dhawan; J. M. Diego; B. L. Frye; D. Gilman; G. Granata; C. Grillo; M. J. Jee; P. S. Kamieneski; A M. Koekemoer; A. K. Meena; A. B. Newman; M. Oguri; E. Padilla-Gonzalez; F. Poidevin; P. Rosati; S. Schuldt; L. G. Strolger; S. H. Suyu; S. Thorp; A. Zitrin

arXiv · 2509.12301

Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Time delays & Hubble constant measurement

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Abstract

Multiply-imaged supernovae (SNe) provide a novel means of constraining the Hubble constant ($H_0$). 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 $H_0$ thus far. Here we detail the third such measurement for SN Encore, a $z=1.95$ SNIa 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 $H_0$ 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 $H_0=66.9_{-8.1}^{+11.2} \rm{km} \rm{s}^{-1}\rm{Mpc}^{-1}$. 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 $H_0$.

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J. D. R. Pierel, E. E. Hayes, M. Millon, C. Larison, E. Mamuzic, A. Acebron, A. Agrawal, P. Bergamini, S. Cha, S. Dhawan, J. M. Diego, B. L. Frye, D. Gilman, G. Granata, C. Grillo, M. J. Jee, P. S. Kamieneski, A M. Koekemoer, A. K. Meena, A. B. Newman, M. Oguri, E. Padilla-Gonzalez, F. Poidevin, P. Rosati, S. Schuldt, L. G. Strolger, S. H. Suyu, S. Thorp, A. Zitrin. 2025-09-15. Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Time delays & Hubble constant measurement. https://arxiv.org/abs/2509.12301

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