Lessons from the first multiply imaged supernova II: A parametric strong-lensing model for the galaxy cluster MACS J1149.5+2223

TL;DR

This paper develops a parametric strong-lensing model for galaxy cluster MACS J1149.5+2223, focusing on the multiply imaged supernova Refsdal, to compare with an existing light-traces-mass model and assess systematic uncertainties.

Contribution

It introduces a new parametric grid-based lens model for MACS J1149.5+2223, enabling comparison with previous models to evaluate systematic differences and uncertainties.

Findings

Time delay predictions differ between models, with the parametric method estimating ~326 days and the LTM method ~224 days.

Models agree within 1-3 sigma for most lensing properties, indicating reasonable consistency.

Systematic differences highlight the need for refined measurements to better constrain the models.

Abstract

We present a parametric, grid-based lens model for the galaxy cluster MACS J1149.5+2223, concentrating on the properties of the first multiply imaged supernova Refsdal. This model complements our updated light-traces-mass (LTM) strong-lensing model for this cluster, described in a companion paper, and is generated using the same pipeline but with a different parametrization. Together these two models probe different possible solutions in a relatively self-consistent manner and can be used to examine systematic uncertainties and relevant differences between the two parameterizations. We obtain reasonably similar (agreeing to within $\simeq1-3\sigma$, in most cases) time delays and magnification ratios, with respect to S1, from the two different methods, although the LTM predictions seem to be systematically shorter/smaller for some of the images. Most notably, the time delay [and 95\% CI] between the Einstein cross (in particular, image S1), and SX, the image that appeared about a year after the original discovery of the cross, differs substantially between the parametric method (326 [300 -- 359] days) and the LTM method (224 [198 -- 306] days), which seems to underestimates the true reappearance time. The cause for this systematic difference is unclear at present. We speculate on its possible origin and note that a refined measurement of SN Refsdal's properties should help to more strongly discriminate between the two solutions, and thus between the two descriptions for the intrinsic shape of the underlying matter distribution. We also discuss the implications of our results for the Hubble constant.