18175( 3) - 16-Sep-2025 13:12:58 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18175 Version: 3 Check-in Time: 16-Sep-2025 13:12:58 STScI Edit Number: 0 Title Chasing Binary Black Hole Merger candidate AT2025smm ------------------------------------------------------------------------------------ Type Cycle GO/DD 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Mr. Xander James Hall Carnegie Mellon University Y CoI: Dr. Antonella Palmese Carnegie Mellon University N CoI: Prof. Barry Mckernan City University of New York Borough of N Manhattan Comm Coll CoI: Prof. Matthew J. Graham California Institute of Technology N CoI: Brendan O'Connor Carnegie Mellon University N CoI: Dr. Lei Hu Carnegie Mellon University N CoI: Dr. Rosalba Perna State University of New York at Stony N Brook CoI: Igor Andreoni University of North Carolina at Chapel N Hill CoI: Malte Busmann Ludwig Maximilian Universitat of Munich N CoI: Dr. Daniel Gruen Stanford University N ------------------------------------------------------------------------------------ Abstract We propose three single-orbit HST WFC3/UVIS observations to test whether AT2025smm is an EM counterpart to the BBH event S250727dc. Swift/UVOT found a blue transient 4 hours after the trigger that faded in less than 6 hours, with later non-detections, motivating rapid UV imaging. Only HST can establish whether the transient is nuclear by resolving and registering it relative to the host nucleus. Each visit uses F336W and F606W to ~27 mag (5 sigma) to measure UV color and evolution, separate host and transient light, and discriminate UV-breakout vs shock-cooling scenarios. The first visit should occur by Sep 21, followed by Oct-Nov and mid-Dec windows. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will obtain multi-filter observations with WFC3/UVIS of AT2025smm in the F336W and F606W. A first observation should occur in August-September, ideally by the end of August. Upon confirmation of a transient, the second observation will occur between October and November, the final observation can occur in December. ------------------------------------------------------------------------------------ 18175( 3) - 16-Sep-2025 13:12:58 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AT2025SMM UNIDENTIFIED, JET, RA=06H29M54.2900S, J2000 V = 26.5 +/- 0.2 SHOCK FRONT DEC=-37D13'55.56" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 18175( 3) - 16-Sep-2025 13:12:58 - [ 3] 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 WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18175( 3) - 16-Sep-2025 13:12:58 - [ 4] Visit: 01 Visit Requirements: BETWEEN 11-AUG-2025:00:00:00 AND 23-SEP-2025:00:00:00 On Hold Comments: Additional Comments: Visit that ideally occurs before the end of August, at the latest by September 23rd. 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 AT2025SMM WFC3/UVI ACCUM UVIS2-C1K F336W FLASH=19, CR-SPLIT=NO 1 374 S PATTERN 1 1-2 S 1C-SUB (1122 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2025SMM WFC3/UVI ACCUM UVIS2-C1K F606W FLASH=6, CR-SPLIT=NO 1 300 S (900 S 1C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F336W 1 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F336W 2 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F336W 3 none none none 1 N/A S K1C-SUB AT2025SMM 2 WFC3/UVI ACCUM UVIS2-C1 F606W 1 none none none 1 N/A S K1C-SUB AT2025SMM 2 WFC3/UVI ACCUM UVIS2-C1 F606W 2 none none none 1 N/A S K1C-SUB AT2025SMM 2 WFC3/UVI ACCUM UVIS2-C1 F606W 3 none none none 1 N/A S K1C-SUB 18175( 3) - 16-Sep-2025 13:12:58 - [ 5] Visit: 02 Visit Requirements: AFTER 01; BETWEEN 05-OCT-2025:00:00:00 AND 11-NOV-2025:00:00:00 On Hold Comments: Additional Comments: October 5th to November 11th Visit 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 AT2025SMM WFC3/UVI ACCUM UVIS2-C1K F606W 1 586 S PATTERN 2 1 S 1C-SUB (2344 S) ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 1 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 2 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 3 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 4 none none none 1 N/A S K1C-SUB 18175( 3) - 16-Sep-2025 13:12:58 - [ 6] Visit: 03 Visit Requirements: AFTER 02; BETWEEN 07-DEC-2025:00:00:00 AND 20-DEC-2025:00:00:00 On Hold Comments: Additional Comments: December 7th to December 20th 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 AT2025SMM WFC3/UVI ACCUM UVIS2-C1K F606W 1 586 S PATTERN 2 1 S 1C-SUB (2344 S) ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 1 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 2 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 3 none none none 1 N/A S K1C-SUB AT2025SMM 1 WFC3/UVI ACCUM UVIS2-C1 F606W 4 none none none 1 N/A S K1C-SUB 18175( 3) - 16-Sep-2025 13:12:58 - [ 7] Summary Form for Proposal 18175 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO, FLASH=19, FLASH=6 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01, AFTER 02, BETWEEN 05-OCT-2025:00:00:00 AND 11-NOV-2025:00:00:00, BETWEEN 07-DEC-2025:00:00:00 AND 20-DEC-2025:00:00:00, BETWEEN 11-AUG-2025:00:00:00 AND 23-SEP-2025:00:00:00, PATTERN 1 1-2, PATTERN 2 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F336W, F606W ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2025SMM ------------------------------------------------------------------------------------------------------------------------------------