The role of multiple images and model priors in measuring $H_0$ from supernova Refsdal in galaxy cluster MACS J1149.5+2223

Understand

Multiple image gravitational lensing systems with measured time delays provide a promising one-step method for determining $H_0$.

• MACS J1149, which lenses SN Refsdal into a quad S1-S4, and two other widely separated images, SX and SY, is a perfect candidate.
• If time delays are pinned down, the remaining uncertainty arises from the mass distribution in the lens.
• In MACS J1149, the mass in the relevant lens plane region can be constrained by (i) many multiple images, (ii) the mass of the galaxy splitting S1-S4 (which, we show, is correlated with $H_0$), (iii) magnification of SX (also correlated with $H_0$), and (iv) prior assumptions on the mass distribution.