17805( 3) - 21-Aug-2025 21:32:14 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17805 Version: 3 Check-in Time: 21-Aug-2025 21:32:14 STScI Edit Number: 0 Title Ultra-rapid observations of a gravitational wave source ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Eleonora Troja Universita degli Studi di Roma Tor Vergata Y CoI: Simone Dichiara The Pennsylvania State University N CoI: Dr. Chris Fryer Los Alamos National Laboratory N CoI: Ryan Wollaeger Los Alamos National Laboratory N CoI: Brendan O'Connor Carnegie Mellon University N CoI: Dra. Rosa Leticia Becerra National Autonomous University of Mexico N CoI: Josefa Becerra-Gonzalez Instituto de Astrofisica de Canarias N ------------------------------------------------------------------------------------ Abstract The discovery of GW170817 ushered in a new era of multi-messenger astrophysics, in which gravitational waves and light are used to explore the most extreme phenomena, such as gamma-ray bursts and kilonovae. We propose to use the unique sensitivity and spectral UV coverage of HST to probe the earliest phases of a young kilonova, promptly (< 4 hr) localized through gravitational waves and light. A similar event, although rare, would offer us the unique opportunity to open a new window into the physics of kilonovae and probe an unexplored stage of their life. These observations would constrain the physical properties of the emitting ejecta (e.g. temperature, expansion velocity) and critically distinguish between competing models (e.g. radioactivity, shock interactions, magnetar) to power the luminous blue kilonova. ------------------------------------------------------------------------------------ Observations Description ------------------------ For events at > 100 Mpc, we request 5 orbits of UV imaging (F336W, F275W, F225W) split in two visits, one (2 orbits) as soon as possible (IMA V1) and the second one (3 orbits) within 1 day (IMA V2) since V1. For an event <100 Mpc, we would instead opt for spectroscopic observations using the WFC3/G280 grism and the same number of orbits. Depending on the reaction time and information available at the time of the trigger, a hybrid configuration (spectroscopy followed by imaging) may be adopted. ------------------------------------------------------------------------------------ 17805( 3) - 21-Aug-2025 21:32:14 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2025ULZ STAR, EJECTA, NEUTRON RA=15H51M54.1711S, J2000 V = 28 STAR DEC=+30D54'8.33" Reference Frame: Other: ZTF Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 17805( 3) - 21-Aug-2025 21:32:14 - [ 3] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 KN GW COUNTERPART V = 19 ------------------------------------------------------------------------------------------------------------------------------------ 17805( 3) - 21-Aug-2025 21:32:14 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 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 ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 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: 3 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.145 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 5 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-LINE-3PT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.605 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 41.788 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 17805( 3) - 21-Aug-2025 21:32:14 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 6 Primary_Pattern Secondary_Pattern Pattern_Type LINE WFC3-IR-DITHER-BOX-MIN Pattern_Purpose DITHER DITHER Number_Of_Points 2 4 Point_Spacing 1 0.572 Line_Spacing 0.365 Coordinate_Frame POS-TARG POS-TARG Pattern_Orient 5 18.528 Angle_Between_Sides 74.653 Center_Pattern NO NO ------------------------------------------------------------------------------------------------------------------------------------ 17805( 3) - 21-Aug-2025 21:32:14 - [ 6] Visit: 01 (VISIT) Visit Requirements: TOO RESPONSE TIME 2.0D On Hold Comments: Additional Comments: Imaging, single filter F225W, 1 orbit Recommended post-flash of 20 e/pixels Aperture chosen to target near the C amplifier to reduce CTE losses If you need to reduce the FOV, use UVIS2-C1K1C-SUB 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 AT2025ULZ WFC3/UVI ACCUM UVIS2-C1K F336W CR-SPLIT=NO, FLASH=19 1 60 S (120 PATTERN 3 1 S 1C-CTE 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025ULZ 1 WFC3/UVI ACCUM UVIS2-C1 F336W 1 none none none 1 N/A S K1C-CTE AT2025ULZ 1 WFC3/UVI ACCUM UVIS2-C1 F336W 2 none none none 1 N/A S K1C-CTE 17805( 3) - 21-Aug-2025 21:32:14 - [ 7] Visit: 02 (IRKN) Visit Requirements: SCHED 100%; TOO RESPONSE TIME 2.0D On Hold Comments: Additional Comments: 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 AT2025ULZ WFC3/IR MULTIAC IR F110W NSAMP=2, SAMP-SEQ=SPARS5 1 DEF PATTERN 6 1 CUM (63.464 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 4 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 4 none none 1 N/A CUM 17805( 3) - 21-Aug-2025 21:32:14 - [ 8] Visit: 03 (IRKN) Visit Requirements: SCHED 100%; ON HOLD ; TOO RESPONSE TIME 2.0D On Hold Comments: Awaiting GW trigger Additional Comments: 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 AT2025ULZ WFC3/IR MULTIAC IR F110W NSAMP=2, SAMP-SEQ=SPARS5 1 DEF PATTERN 6 1 CUM (63.464 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 1 4 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F110W 2 4 none none 1 N/A CUM 17805( 3) - 21-Aug-2025 21:32:14 - [ 9] Visit: 04 (IRKN) Visit Requirements: SCHED 100%; TOO RESPONSE TIME 2.0D On Hold Comments: Additional Comments: 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 AT2025ULZ WFC3/IR MULTIAC IR F160W NSAMP=2, SAMP-SEQ=SPARS5 1 DEF PATTERN 6 1 CUM (63.464 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 4 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 4 none none 1 N/A CUM 17805( 3) - 21-Aug-2025 21:32:14 - [ 10] Visit: 05 (IRKN) Visit Requirements: SCHED 100%; ON HOLD ; TOO RESPONSE TIME 2.0D On Hold Comments: Awaiting GW trigger Additional Comments: 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 AT2025ULZ WFC3/IR MULTIAC IR F160W NSAMP=2, SAMP-SEQ=SPARS5 1 DEF PATTERN 6 1 CUM (63.464 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 1 4 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 1 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 2 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 3 none none 1 N/A CUM AT2025ULZ 1 WFC3/IR MULTIAC IR F160W 2 4 none none 1 N/A CUM 17805( 3) - 21-Aug-2025 21:32:14 - [ 11] Summary Form for Proposal 17805 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures IR, UVIS2-C1K1C-CTE ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/IR, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO, FLASH=19, NSAMP=2, SAMP-SEQ=SPARS5 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD , PATTERN 3 1, PATTERN 6 1, SCHED 100%, TOO RESPONSE TIME 2.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F110W, F160W, F336W ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2025ULZ ------------------------------------------------------------------------------------------------------------------------------------