Refsdal Redux: Precise Measurements of the Reappearance of the First Supernova with Multiple Resolved Images

In 1964, Refsdal first considered the possibility that a powerful gravitational lens could create multiple images of a well-aligned background supernova (SN) explosion. For such a system, the time delays between the SN images depend on both the matter distribution in the lens, and the cosmic expansion rate. In HST exposures of the MACSJ1149 cluster taken in November 2014, we discovered the first multiply imaged SN, which we have named 'Refsdal.' Four images of the SN create an Einstein Cross around an early-type cluster member. SN Refsdal is expected to reappear during Cycle 23 in a different image of the host galaxy (offset by 8'') created by the cluster's potential. Our HST imaging and spectroscopy now show that SN Refsdal is a peculiar Type IIP SN similar to 1987A at redshift z=1.5. We propose a WFC3 imaging program to detect the reappearance (10 orbits) and a follow-up ToO program to construct near-IR light curves in two bands (up to 28 orbits). Comparing the new light curve to the current four images, we will measure precise relative magnifications and time delays. This will make SN Refsdal into a powerful probe of the cluster mass distribution and lay a foundation for the future use of SN time delays for constraints on cosmological parameters. Comparison among the five light curves will also allow us to measure microlensing fluctuations, which can constrain the elliptical lens' and intracluster stellar populations. Finally, this Cycle 23 imaging will provide late-time light curves for the four highly magnified images (~15-20x) in the Einstein Cross. These will enable an improved comparison to the low-redshift 87A-like population and an estimate of the 56Ni mass.