0 Assigned ID: 1342 ----- Attempting Submission 1 (Wed Feb 01 00:09:21 GMT 2017) ----- HST Phase I Proposal 1342 (midcycle_m1149.apt) successfully submitted. Receipt: # 1342-1 ----- Attempting Submission 2 (Wed Feb 01 03:10:40 GMT 2017) ----- HST Phase I Proposal 1342 (midcycle_m1149.apt) successfully submitted. Receipt: # 1342-2 ----- Attempting Submission 3 (Wed Feb 01 04:02:23 GMT 2017) ----- HST Phase I Proposal 1342 (midcycle_m1149.apt) successfully submitted. Receipt: # 1342-3 ----- Attempting Submission 4 (Wed Feb 01 04:19:09 GMT 2017) ----- This is a resubmission of our Phase-II proposal. Following Tricia Royale's suggestion, we have opened up the ORIENT constraint to 90--104 for visit #5 to make it possible to use a "Shared Risk Pair" of guide stars for those observations. 1077357291 ----- Attempting Submission 1 (Sat Mar 25 18:48:28 GMT 2017) ----- ----- Attempting Submission 2 (Mon Mar 27 20:18:30 GMT 2017) ----- Probing the Nature of Dark Matter with Individual Stars Highly Magnified by a Galaxy Cluster A star highly magnified by a galaxy cluster should provide a powerful new tool for probing the composition of dark matter. We discovered the first-known such star (named ''Icarus'') close to the critical curve of the MACS J1149 galaxy cluster in April 2016. The star's spectral energy distribution (SED) shows no change over the last several years, including during its rise to peak brightness in May 2016. The SED is consistent with that of a mid-to-late B-type star at the redshift z=1.49 of the background lensed galaxy, and our models suggest that the star became temporarily magnified by a factor of at least ~2000 at peak. While continuing to monitor the field, we discovered a new, entirely unexpected source (dubbed ''Chucky'') with very high significance ~0.26 arcseconds away from Icarus on October 30, 2016. If Chucky is a counterimage of Icarus, a population of massive objects (>20 solar masses) may be needed to hide it during the ~12 years while the field has been observed. When a follow-up program was able to observe the field to measure its SED, Chucky had faded. However, in a subsequent visit, we detected an additional potential ~4-sigma source (''Perdix'') offset from both Icarus and Chucky. If we continue to detect transients at large offsets, dark matter corresponding to an ultra-light scalar field (Hui et al. 2016) may be required to account for the rate of such events. An ultra-light scalar field would be granular instead of smoothly distributed, leading to microlensing events with large angular separations. The observations of the field, at an optimal cadence for finding microlensing events, will be able to identify distinct stars from their Balmer jumps. Two transients that we understand to be indiidual magnified stars have appeared close (<0.1") to the critical curve of the MACS J1149 galaxy cluster in HST observations that we acquired during Cycle 23 and Cycle 24. As part of this program, we will continue to monitor the field for similar events, which should help us to understand the lens system and place constraints on dark matter in the form of compact objects. This program will consist of five separate one-orbit visits each split between integrations in the WFC3-UVIS F606W and WFC3-IR F125W wideband filters. These observer-frame optical and near-IR filters span the Balmer jump of stars at the redshift (z=1.49) of the lensed background galaxy. 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