0 Assigned ID: 5927 ----- Attempting Submission 1 (Thu Sep 05 09:34:37 GMT 2024) ----- HST Phase I Proposal 5927 successfully submitted. Receipt: # 5927-1 0 ----- Attempting Submission 2 (Sat Sep 14 02:57:51 GMT 2024) ----- Late time constraints on a luminous LFBOT-TDE intermediate We request two orbits of Hubble Space Telescope/WFC3 imaging to measure the position and spectral energy distribution of an unprecedented optical transient. This event, AT 2024puz is one of the rare times where we are presented with something truly new: it is remarkably luminous in the optical (~1e45 erg/s) and X-rays (~1e44 erg/s) while being very fast (~20 days above half-peak) and persistently blue. It is located within a dwarf galaxy: from current constraints, it appears offset from the host center of light, although the sensitivty and resolution are not enough to be certain. AT 2024puz resembles either the slowest, most luminous fast blue optical transient (LFBOT), or a fast but luminous tidal disruption event (TDE). LFBOTs are a new, mysterious class of transients for which the trigger is unknown. One popular model is IMBH TDEs. AT 2024puz may represent the bridge between LFBOTs and more typical TDEs: it is consistent with being an LFBOT definitively produced by an IMBH TDE. However, it is also consistent with stellar LFBOT models. There are two questions that we must answer to distinguish between stellar LFBOT and TDE models for AT 2024puz: (1) where is AT 2024puz located within its host galaxy and (2) how does the late-time optical/UV emission evolve? If AT 2024puz is nuclear or located away from star formation, it is may be a TDE. If it is off-nuclear and in a star-forming region, it may be associated with a massive star. The rate of the late-time optical/UV luminosity decline is predicted to be different for stellar and TDE models. These questions cannot be answered with ground based telescopes: HST imaging in four bands spanning the transient SED is required. These are observations of a candidate LFBOT - TDE intermediate spanning two orbits using WFC3/IR and WFC3/UVIS. The combined IR and UVIS observations will 1) measure the transient position (using UVIS/F336W and IR/F160W) and 2) measure the late-time transient SED (using UVIS/F606W and IR/F105W), to be performed at the beginning of the scheduling window on Sept 19 or at the latest by Oct 7 (making this a non-disruptive ToO). Assuming the total observation time available (excluding overheads) is 2200s per orbit, our observations are as follows for a total of two orbits in a single visit: IR observations will include an 1100s observation (3-point dither) with F160W and an 1100s observation (3-point dither) with F105W. UVIS observations will include an 1100s observation with F336W (3-point dither) and an 1100s observation with F606W (3-point dither). PI and graduate student Jean Somalwar discovered AT 2024puz, and is leading the overall effort and publication of this event. PI Somalwar has extensive experience in the discovery and multi-wavelength characterization of TDE. PI Somalwar also has experience with HST imaging data (including published work Somalwar+2021, and proposal 16461 with 7 orbits of HST/ACS imaging awarded). Co-I Vikram Ravi, and Co-I Ryan Chornock are co-leads of the ZTF TDE/AGN working group. Together with Co-I Raffaela Margutti and Co-Is Yuhan Yao and Erica Hammerstein, they have extensive, world-leading experience in the discovery and characterization of various kinds of astronomical transient, including TDEs and LFBOTs. Co-I Edward Nathan has extensive experience with accretion disk models, including developing techniques for analyzing the late-time UV emission from TDEs. Supermassive Black Holes and Active Galaxies Galaxies 0 false SUPPORTED UNIDENTIFIED ACCRETION DISK ULTRAVIOLET EMITTER WIND X-RAY EMITTER NO J2000 J2000 WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F606W WFC3/IR Imaging No No F105W WFC3/IR Imaging No No F160W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false POS-TARG DITHER 0.605 LINE false false 3 41.788 WFC3-IR-DITHER-LINE-3PT 2 false /private/var/folders/rn/gwp5rt7d39nbd64f5d6tr_340000gn/T/OPVisit11163396225905176279.xml The scheduling data was generated by CASM 32.1.3. CASM was run in Phase II mode. 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