18433( 3) - 12-Aug-2026 12:23:38 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18433 Version: 3 Check-in Time: 12-Aug-2026 12:23:38 STScI Edit Number: 0 Title Decoding the Remarkable Transient AT2026hir: Offset and Black Hole Mass with HST and XMM ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Yuhan Yao University of California - Berkeley Y CoI: Prof. Ryan Chornock University of California - Berkeley N CoI: Dr. Ning Jiang University of Science and Technology of N China CoI: Dr. Andy Mummery Institute For Advanced Study N CoI: Nayana AJ University of California - Berkeley N CoI: Dr. Raffaella Margutti University of California - Berkeley N CoI: Prof. Jane Dai University of Hong Kong N CoI: Dr. Charlotte Ward Princeton University N ------------------------------------------------------------------------------------ Abstract EP260324a/AT2026hir is a luminous X-ray and optical nuclear transient at z=0.087 that defies classification within known transient populations. Its nuclear location and quiescent massive host galaxy are consistent with a luminous tidal disruption event (LTDE), yet its anomalously rapid light-curve evolution is more reminiscent of Luminous Fast Blue Optical Transients (LFBOTs). Panchromatic early-time follow-up leaves the nature of AT2026hir ambiguous: it may be a TDE powered by the supermassive black hole in the galaxy nucleus, or an LFBOT powered by a much lower-mass black hole at a slightly off-nuclear location. To resolve this ambiguity, we propose joint late-time HST (2 orbits) + XMM-Newton (30 ks) observations targeting two key measurements: the positional offset of the transient relative to the host nucleus, and the mass of the central black hole (Mbh). HST UV+optical imaging will improve the current ground-based astrometric uncertainty of 0.3 arcsec by an order of magnitude, enabling detection or exclusion of offsets down to 50 pc. Joint HST+XMM-Newton broadband SED modeling will precisely constrain Mbh via relativistic disk theory, leveraging both the X-ray spectral shape and the normalization of the UV/optical plateau. ------------------------------------------------------------------------------------ Observations Description ------------------------ This proposal consists of two orbits of HST imaging obseravtions of the optical transient AT2026hir. The HST observation is coordinated with a 30ks XMM-Newton observation. First orbit: ACS/SBC Imaging, F150LP filter Second orbit: WFC3/UVIS Imaging, F625W (optical) and F275W (NUV) filters. For the scheduling, we focus on the first half of Cycle 34 to maximize the probability of getting a high-SNR X-ray spectrum before the disk cools significantly. Three suitable HST visibility windows are listed below (based on the Phase II Visit Planner result). 18433( 3) - 12-Aug-2026 12:23:38 - [ 2] 2026 Nov 7-22 2027 Feb 9-21 2027 March 29-30; April 2-9 ------------------------------------------------------------------------------------ 18433( 3) - 12-Aug-2026 12:23:38 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AT2026HIR UNIDENTIFIED, RA=08H44M12.3716S +/- 0.1", J2000 V = 17 +/- 0.5 ULTRAVIOLET EMITTER DEC=+82D01'10.91" +/- 0.1" Reference Frame: ICRS Extended: YES Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18433( 3) - 12-Aug-2026 12:23:38 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE-3PT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.135 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-SBC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.472 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 44.4 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18433( 3) - 12-Aug-2026 12:23:38 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: - 1 orbit: ACS/SBC Imaging in F150LP - 1 orbit: WFC3/UVIS imaging in F225W+F625W Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 AT2026HIR ACS/SBC ACCUM SBC F150LP 1 1330 S PATTERN 2 1 (2660 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2026HIR WFC3/UVI ACCUM UVIS2-C1K F275W FLASH=21.0 1 207 S (621 PATTERN 1 2 S 1C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2026HIR WFC3/UVI ACCUM UVIS2-C1K F625W FLASH=5 1 566 S PATTERN 1 3 S 1C-SUB (1698 S) Comments: This F625W image will measure the nucleus of the host galaxy. ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ AT2026HIR 1 ACS/SBC ACCUM SBC F150LP 1 none none none 1 N/A AT2026HIR 1 ACS/SBC ACCUM SBC F150LP 2 none none none 1 N/A AT2026HIR 2 WFC3/UVI ACCUM UVIS2-C1 F275W 1 none none none 2 N/A S K1C-SUB AT2026HIR 2 WFC3/UVI ACCUM UVIS2-C1 F275W 2 none none none 2 N/A S K1C-SUB AT2026HIR 2 WFC3/UVI ACCUM UVIS2-C1 F275W 3 none none none 2 N/A S K1C-SUB AT2026HIR 3 WFC3/UVI ACCUM UVIS2-C1 F625W 1 none none none 2 N/A S K1C-SUB AT2026HIR 3 WFC3/UVI ACCUM UVIS2-C1 F625W 2 none none none 2 N/A S K1C-SUB AT2026HIR 3 WFC3/UVI ACCUM UVIS2-C1 F625W 3 none none none 2 N/A S K1C-SUB 18433( 3) - 12-Aug-2026 12:23:38 - [ 6] Summary Form for Proposal 18433 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures SBC, UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/SBC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=21.0, FLASH=5 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 2, PATTERN 1 3, PATTERN 2 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F150LP, F275W, F625W ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2026HIR ------------------------------------------------------------------------------------------------------------------------------------