0 Assigned ID: 1060 ----- Attempting Submission 1 (Fri Apr 07 22:58:51 GMT 2017) ----- HST Phase I Proposal 1060 (ASSASN-15LH_first_sub.pdf.apt) successfully submitted. Receipt: # 1060-1 ----- Attempting Submission 2 (Fri Apr 07 23:34:59 GMT 2017) ----- ASASSN-15lh is the most luminous optical transient ever detected. Yet its nature is unclear. It was first thought to be a supernova. However, its host shows little sign of star formation, and the transient has remained extremely hot for nearly two years, making a supernova origin seem less likely. Aternatively, ASSASN-15lh may be a flare caused by the tidal disruption of a star that has flown too close to a supermassive black hole. However, this model is also far from an easy fit. The luminosty of the host's bulge implies a black hole mass so large that a star would almost certainly be swallowed before disrupted, unless the the black hole is spinning relativisitically. Furthermore, the double-humped shape of the light curve seen in the UV is highly unusual for a tidal disrution flare (TDF). Here we propose observations using nearly the entire wavelength range of HST to elucidate the nature of this object. We will be able to determine the location of the transient with respect to the host bulge light, and thus the likely position of the black hole, to ~20 pc, or nearly an order of magnitude better than the present best estimate. We will measure the temperature of the transient, which may prove too high for a supernova at such a late time, and study the host star-formation, which is essential for producing a supernova. Additionally, we will attempt to confirm (or refute) the apparent evidence for a recent merger we have found in the extant HST images. A merger should greatly enance the rate of stellar disruptions by a central black hole. Together these obsevations will dramatically improve our undertanding of this most extraordinary transient. Massive Black Holes And Their Host Galaxies 6 false true false false false false SUPPORTED This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F606W WFC3/IR Imaging No No F140W