18013( 3) - 26-Nov-2025 18:46:53 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18013 Version: 3 Check-in Time: 26-Nov-2025 18:46:53 STScI Edit Number: 0 Title Searching for Strong Lensing Signatures in a Possible Lensed GRB. ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. David Stephen O'Neill University of Birmingham Y CoI: Dr. Benjamin Paul Gompertz University of Birmingham N CoI: Prof. Graham Smith University of Birmingham N CoI: Dr. Dimple Dimple University of Birmingham N ------------------------------------------------------------------------------------ Abstract Gamma-ray bursts (GRBs) are among the most energetic phenomena in the universe, capable of being detected at high redshifts with sub-second timing precision. This makes them compelling candidates for probing cosmological parameters through gravitational lensing. However, despite nearly 20 years of observations, no strong lensing candidate has been conclusively identified, largely due to uncertainties in the prompt emission and localisation of GRBs. Recently, we have identified a potential lensed GRB pair. We now aim to leverage the excellent depth and resolution capabilities of the HST to search for a potential host galaxy, determine image multiplicity, and identify any intervening lensing object(s) along the line of sight. A confirmed detection of gravitational lensing in a GRB would be a historic first and return an immediate, high-impact scientific result. ------------------------------------------------------------------------------------ Observations Description ------------------------ Images will be taken with ACS/WFC F435W, F814W and WFC/IR F160W. With this data, we will be able to 1) confirm the presence or absence of any host galaxy and enable a sensitive search for lensing systems in the foreground, i.e. z <0.824. 2) determine if lensing signatures are present, in particular, multiply imaged host galaxy candidates and 3) derive a photometric redshift for possible lenses and host galaxies. HST /ACS high resolution capabilities and multi-band colour information are essential in determining the existence of multiply imaged galaxies. Using this data it will be possible to construct a lensing model. We assume the the maximum possible redshift of the lens as z=1.6. The majority of past LGRB hosts can be recovered with a limiting magnitude of r <26 AB mag with all hosts being recovered with r <27 mag. ([5] figure 9). Therefore we require imaging to recover at minimum a r =27 AB mag Sc-galaxy-like host at z=1.6 as 1" extended source with S/N>5 in any of the filters. For ACS observations, we will use 4 dithered images to allow for the removal of artifacts and cosmic rays. Each set of 4 images will require 4 + 3 x 2.5 minutes with an additional 6.5 minutes for guide star acquisition and 1.5 minutes to allow for maneuvering for dithering. This results in a total overhead of 19.5 minutes per orbit. This leaves ~1950s in the orbit to carry out the exposures. For a 26 and 27 AB mag target this would result in a S/N in F435W ~15.1 and 6.1, S/N F814W ~6.5 and 2.6 18013( 3) - 26-Nov-2025 18:46:53 - [ 2] respectively. For WFC3/IR observations, we will also use a set of 4 dithered images. Total overheads from guide star acquisition, dither maneuvering and 4x1 min for each exposure results in a total overhead of 12 minutes. The remaining 1800s integration time split across the 4 exposures returns a S/NF 160W ~16 for a r =27 AB mag galaxy. These observations will allow for the unambiguous confirmation on the presence or absence of the host galaxy, whether it has been multiply imaged as well as the existence of nearby possible lensing objects. To avoid diffraction spikes from a nearby star, constraints on orientation are required. To mitigate the effects of CTE, the target will be placed in the corner of the B amplifier in ACS/WFC observations. ------------------------------------------------------------------------------------ 18013( 3) - 26-Nov-2025 18:46:53 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 240205B GALAXY, GRAVITATIONAL RA=23H25M50.1200S +/- 0.5", J2000 V = 27 +/- 0.1 LENS DEC=-55D07'5.50" +/- 0.5" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18013( 3) - 26-Nov-2025 18:46:53 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.2637 Line_Spacing 0.1856 Coordinate_Frame POS-TARG Pattern_Orient 41.859 Angle_Between_Sides 69.02 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.2637 Line_Spacing 0.1856 Coordinate_Frame POS-TARG Pattern_Orient 20.7 Angle_Between_Sides 69.02 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 3 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-BLOB Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 5.183 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 41.859 Angle_Between_Sides Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 18013( 3) - 26-Nov-2025 18:46:53 - [ 5] Visit: 01 (ACS/WFC) Visit Requirements: 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 240205B ACS/WFC ACCUM WFC1 F435W FLASH=8 1 485 S POS TARG see below 57.250586000784864, (1948 S) 5.975122184783851; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 240205B ACS/WFC ACCUM WFC1 F814W 1 490 S POS TARG see below 57.250586000784864, (2008 S) 5.975122184783851; PATTERN 2 2 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 240205B 1 ACS/WFC ACCUM WFC1 F435W 1 none none none 1 487.0 S 240205B 1 ACS/WFC ACCUM WFC1 F435W 2 none none none 1 487.0 S 240205B 1 ACS/WFC ACCUM WFC1 F435W 3 none none none 1 487.0 S 240205B 1 ACS/WFC ACCUM WFC1 F435W 4 none none none 1 487.0 S 240205B 2 ACS/WFC ACCUM WFC1 F814W 1 none none none 2 511.0 S 240205B 2 ACS/WFC ACCUM WFC1 F814W 2 none none none 2 499.0 S 240205B 2 ACS/WFC ACCUM WFC1 F814W 3 none none none 2 499.0 S 240205B 2 ACS/WFC ACCUM WFC1 F814W 4 none none none 2 499.0 S 18013( 3) - 26-Nov-2025 18:46:53 - [ 6] Visit: 02 (ACS/WFC3-IR) Visit Requirements: 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 240205B WFC3/IR MULTIAC IR F160W NSAMP=13, SAMP-SEQ=SPARS50 1 DEF PATTERN 3 1 CUM (2411.751 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 | ------------------------------------------------------------------------------------------------------------------------------------ 240205B 1 WFC3/IR MULTIAC IR F160W 1 none none none 1 N/A CUM 240205B 1 WFC3/IR MULTIAC IR F160W 2 none none none 1 N/A CUM 240205B 1 WFC3/IR MULTIAC IR F160W 3 none none none 1 N/A CUM 240205B 1 WFC3/IR MULTIAC IR F160W 4 none none none 1 N/A CUM 18013( 3) - 26-Nov-2025 18:46:53 - [ 7] Summary Form for Proposal 18013 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures IR, WFC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/IR ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=8, NSAMP=13, SAMP-SEQ=SPARS50 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 2 2, PATTERN 3 1, POS TARG 57.250586000784864, 5.975122184783851 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F160W, F435W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names 240205B ------------------------------------------------------------------------------------------------------------------------------------