18213( 5) - 26-Nov-2025 14:51:45 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18213 Version: 5 Check-in Time: 26-Nov-2025 14:51:45 STScI Edit Number: 1 Title Imaging of EP241021a during the flattening in the optical light curve: an exotic gamma-ray burst or a peculiar jetted tidal disruption event? ------------------------------------------------------------------------------------ Type Cycle GO/DD 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Luming Sun Anhui Normal University Y CoI: Xinwen Shu Anhui Normal University N CoI: Dr. Ning Jiang University of Science and Technology of N China CoI: Dr. Chichuan Jin National Astronomical Observatories of N China (NAOC) CoI: Prof. Jane Dai University of Hong Kong N CoI: Dr. Weimin Yuan Chinese Academy of Sciences N CoI: Mr. Jiazheng Zhu University of Science and Technology of N China CoI: Lei Yang Anhui Normal University N CoI: Prof. Bing Zhang University of Nevada - Las Vegas N CoI: Dr. Rongfeng Shen Sun Yat-Sen University N ------------------------------------------------------------------------------------ Abstract Since the launch of EP, peculiar fast X-ray transients (FXT) have been discovered, blurring the boundaries between the properties of Gamma-ray bursts (GRB) and jetted tidal disruption events (TDE). One example is the FXT EP241021a at z=0.748, which shows an X-ray plateau with luminosities of several 10^44 erg/s lasting for ~10 days. The time duration of the plateau is far longer than all known GRBs, while it is similar to jetted TDEs. The plateau luminosity is two orders of magnitude lower than jetted TDEs, suggesting a possible origin of TDE by an intermediate-mass black hole. In addition, during the X-ray plateau, there is a clear optical rebrightening by two magnitudes, peaking at around -21 mag, which is also rarely seen. Recent ground-based observations show that the optical light curve no longer declines in accordance with the early power-law trend but flattens. The flattening may originate in the thermal radiation from the transient source, or the continuum of the host galaxy, neither of which was detected previously. We propose a new and timely imaging observation of EP241021a at a phase of ~200 days with HST WFC3/UVIS and WFC3/IR. Our intermediate goal is to distinguish between the possible origins of the optical flattening, based on whether the target is extended and whether the central position coincides with the transient position measured by VLBA. In either case, we can further tell an exotic GRB or a peculiar jetted TDE as the favored origin of EP241021a with the constraints on the positional offset and broad-band SED. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will observe EP241021a with WFC3/UVIS with F606W and F814W filters, and with WFC3/IR with F110W and F160W filters. ------------------------------------------------------------------------------------ 18213( 5) - 26-Nov-2025 14:51:45 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 EP241021A GALAXY, HIGH REDSHIFT RA=01H55M23.4322S, J2000 V = 25 GALAXY DEC=+05D56'17.80" Reference Frame: ICRS Comments: The position of the target, used in WFC3/IR observations ------------------------------------------------------------------------------------------------------------------------------------ 2 EP241021A-UV GALAXY, HIGH REDSHIFT RA=01H55M22.9000S, J2000 V = 25 IS-CENTER GALAXY DEC=+05D57'3.00" Reference Frame: ICRS Comments: The center of the field of view that can observe four Gaia stars together with the target, used in WFC3/UVIS observations ------------------------------------------------------------------------------------------------------------------------------------ 18213( 5) - 26-Nov-2025 14:51:45 - [ 3] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 1.908 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 41.788 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 ------------------------------------------------------------------------------------------------------------------------------------ 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 ------------------------------------------------------------------------------------------------------------------------------------ 18213( 5) - 26-Nov-2025 14:51:45 - [ 4] Visit: 01 (EP241021a) Visit Requirements: ORIENT 15D TO 16 D; BEFORE 30-NOV-2025:00:00:00 On Hold Comments: Additional Comments: Observations of EP241021a in two UVIS fitlers and two IR filters 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 EP241021A-U WFC3/UVI ACCUM UVIS-CENT F606W 1 520 S PATTERN 2 1 VIS-CENTER S ER (2080 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 EP241021A-U WFC3/UVI ACCUM UVIS-CENT F814W FLASH=7 1 420 S (840 PATTERN 3 2 VIS-CENTER S ER S) ------------------------------------------------------------------------------------------------------------------------------------ 3 EP241021A WFC3/IR MULTIAC IR F110W NSAMP=6, SAMP-SEQ=SPARS25 1 DEF PATTERN 1 3 CUM (255.87 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 EP241021A WFC3/IR MULTIAC IR F160W NSAMP=6, SAMP-SEQ=SPARS25 1 DEF PATTERN 1 4 CUM (255.87 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 | ------------------------------------------------------------------------------------------------------------------------------------ EP241021A-UVIS-CENT 1 WFC3/UVI ACCUM UVIS-CEN F606W 1 none none none 1 N/A ER S TER EP241021A-UVIS-CENT 1 WFC3/UVI ACCUM UVIS-CEN F606W 2 none none none 1 N/A ER S TER EP241021A-UVIS-CENT 1 WFC3/UVI ACCUM UVIS-CEN F606W 3 none none none 1 N/A ER S TER EP241021A-UVIS-CENT 1 WFC3/UVI ACCUM UVIS-CEN F606W 4 none none none 1 N/A ER S TER EP241021A-UVIS-CENT 2 WFC3/UVI ACCUM UVIS-CEN F814W 1 none none none 2 N/A ER S TER EP241021A-UVIS-CENT 2 WFC3/UVI ACCUM UVIS-CEN F814W 2 none none none 2 N/A ER S TER EP241021A 3 WFC3/IR MULTIAC IR F110W 1 none none none 2 N/A CUM EP241021A 3 WFC3/IR MULTIAC IR F110W 2 none none none 2 N/A CUM EP241021A 4 WFC3/IR MULTIAC IR F160W 1 none none none 2 N/A CUM EP241021A 4 WFC3/IR MULTIAC IR F160W 2 none none none 2 N/A CUM 18213( 5) - 26-Nov-2025 14:51:45 - [ 5] Visit: 51 (EP241021a re-observation) Visit Requirements: On Hold Comments: Additional Comments: Observations of EP241021a in two UVIS fitlers and two IR filters - repeat of last four exposures 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 EP241021A WFC3/IR MULTIAC IR F110W NSAMP=6, SAMP-SEQ=SPARS25 1 DEF PATTERN 1 1 CUM (255.87 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 EP241021A WFC3/IR MULTIAC IR F160W NSAMP=6, SAMP-SEQ=SPARS25 1 DEF PATTERN 1 2 CUM (255.87 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 | ------------------------------------------------------------------------------------------------------------------------------------ EP241021A 1 WFC3/IR MULTIAC IR F110W 1 none none none 1 N/A CUM EP241021A 1 WFC3/IR MULTIAC IR F110W 2 none none none 1 N/A CUM EP241021A 2 WFC3/IR MULTIAC IR F160W 1 none none none 1 N/A CUM EP241021A 2 WFC3/IR MULTIAC IR F160W 2 none none none 1 N/A CUM 18213( 5) - 26-Nov-2025 14:51:45 - [ 6] Summary Form for Proposal 18213 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures IR, UVIS-CENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/IR, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=7, NSAMP=6, SAMP-SEQ=SPARS25 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BEFORE 30-NOV-2025:00:00:00, ORIENT 15D TO 16 D, PATTERN 1 1, PATTERN 1 2, PATTERN 1 3, PATTERN 1 4, PATTERN 2 1, PATTERN 3 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F110W, F160W, F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names EP241021A, EP241021A-UVIS-CENTER ------------------------------------------------------------------------------------------------------------------------------------