18217( 3) - 08-Dec-2025 17:49:05 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18217 Version: 3 Check-in Time: 08-Dec-2025 17:49:05 STScI Edit Number: 0 Title X-ray Flashes as Dirty Fireballs or Off-axis GRBs ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Anna Yen Qin Ho Cornell University Y CoI: Dr. Daniel Perley Liverpool John Moores University N CoI: Genevieve Schroeder Northwestern University N CoI: Gokul Prem Srinivasaragavan University of Maryland N CoI: Dr. Stephen Bradley Cenko NASA Goddard Space Flight Center N ------------------------------------------------------------------------------------ Abstract The collapsar model provides a very good model for the energetics, prompt gamma-ray emission, and the subsequent afterglow emission of long-duration GRBs. However, it does not provide a ready explanation for the apparent variations in the opening angle of the jet and in the explosive energy release. It has been speculated that there exist classes of relativistic explosions which have a lower Lorentz factor and wider opening angle than classical GRBs that peak in the X-ray band, known as dirty fireballs, with afterglows similar to classical GRBs. A decade long search with ground- based optical surveys has found a dozen afterglows without a gamma-ray trigger, but their physical interpretation have been limited by a lack of coverage by high-energy missions, as well as a dearth of late-time sensitive afterglow observations. The recently launched Einstein Probe (EP) and Space Variable Objects Monitor (SVOM) are changing this landscape, offering a clear path to identify dirty fireballs. Here, we propose a comprehensive program with the VLA, XMM-Newton, and HST to help distinguish dirty fireballs from their primary false positives, GRBs viewed slightly off-axis. The discovery of a single dirty fireball would have a dramatic impact on our view of the GRB phenomenon. ------------------------------------------------------------------------------------ Observations Description ------------------------ This is a target-of-opportunity proposal. We will trigger on one object with HST. For that object, we will obtain three epochs of ToO observations with WFC3. The first will be one orbit at 1-2 months after discovery (i.e., non-disruptive) in the F606W filter. The second will be at 2-4 months, also in F606W, this time two orbits. And the third will be after the transient light has faded, 2 additional orbits in F606W for the purpose of subtracting out the host-galaxy contribution. So, the total HST observation comes out to 5 orbits of WFC3/F606W observations. ------------------------------------------------------------------------------------ 18217( 3) - 08-Dec-2025 17:49:05 - [ 2] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 XRF SOFT X-RAY FLASH (EP<25 KEV, V = 26 EISO>1E51 ERG) ------------------------------------------------------------------------------------------------------------------------------------ 18217( 3) - 08-Dec-2025 17:49:05 - [ 3] 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 ------------------------------------------------------------------------------------------------------------------------------------ 18217( 3) - 08-Dec-2025 17:49:05 - [ 4] Visit: 01 Visit Requirements: TOO RESPONSE TIME 30.0D 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 XRF WFC3/UVI ACCUM UVIS F606W 1 551 S PATTERN 1 1 S (2204 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 | ------------------------------------------------------------------------------------------------------------------------------------ XRF 1 WFC3/UVI ACCUM UVIS F606W 1 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 2 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 3 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 4 none none none 1 N/A S 18217( 3) - 08-Dec-2025 17:49:05 - [ 5] Visit: 02 Visit Requirements: TOO RESPONSE TIME 30.0D 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 XRF WFC3/UVI ACCUM UVIS F606W 1 551 S PATTERN 1 1 S (2204 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 XRF WFC3/UVI ACCUM UVIS F606W 1 546 S PATTERN 1 2 S (2184 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 | ------------------------------------------------------------------------------------------------------------------------------------ XRF 1 WFC3/UVI ACCUM UVIS F606W 1 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 2 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 3 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 4 none none none 1 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 1 none none none 2 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 2 none none none 2 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 3 none none none 2 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 4 none none none 2 N/A S 18217( 3) - 08-Dec-2025 17:49:05 - [ 6] Visit: 03 Visit Requirements: TOO RESPONSE TIME 30.0D 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 XRF WFC3/UVI ACCUM UVIS F606W 1 551 S PATTERN 1 1 S (2204 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 XRF WFC3/UVI ACCUM UVIS F606W 1 546 S PATTERN 1 2 S (2184 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 | ------------------------------------------------------------------------------------------------------------------------------------ XRF 1 WFC3/UVI ACCUM UVIS F606W 1 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 2 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 3 none none none 1 N/A S XRF 1 WFC3/UVI ACCUM UVIS F606W 4 none none none 1 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 1 none none none 2 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 2 none none none 2 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 3 none none none 2 N/A S XRF 2 WFC3/UVI ACCUM UVIS F606W 4 none none none 2 N/A S 18217( 3) - 08-Dec-2025 17:49:05 - [ 7] Summary Form for Proposal 18217 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2, TOO RESPONSE TIME 30.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W ------------------------------------------------------------------------------------------------------------------------------------ Target Names XRF ------------------------------------------------------------------------------------------------------------------------------------