18443( 2) - 18-Aug-2026 18:07:16 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18443 Version: 2 Check-in Time: 18-Aug-2026 18:07:16 STScI Edit Number: 0 Title Unraveling the Mysteries of ASASSN-15lh: How do the Brightest Tidal Disruption Events Evolve? ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Nicholas Stone University of Wisconsin - Madison Y CoI: Dr. Jason Hinkle University of Illinois at Urbana - N Champaign CoI: Yael Alush The Hebrew University N ------------------------------------------------------------------------------------ Abstract When it was first discovered more than a decade ago, the enigmatic nuclear transient ASASSN-15lh was interpreted as either the most luminous supernova ever seen or the most luminous tidal disruption event (TDE) ever seen. The ensuing decade of observations has favored, though not definitively proven, the TDE interpretation, in which the transient was produced by an unlucky star torn to shreds by a supermassive black hole. Leveraging the long baseline afforded by a 2019 HST UV epoch, we propose multi-band HST UV photometry to unambiguously settle the question of ASASSN-15lh's origins by searching for late-time evolution in a faint, slowly spreading accretion disk. If the TDE interpretation is confirmed, we will fit time-dependent general-relativistic disk models to the HST light curve in order to test key modeling assumptions and probe decades-old questions in accretion theory related to the stress at the inner boundary of accretion disks. ASASSN-15lh's unique combination of high black hole mass, extreme peak luminosity, and long temporal baseline with multi-wavelength coverage gives it exquisite potential to test and calibrate existing models for late-time disk emission in TDEs. This is an urgently needed stress test for TDE parameter inference prior to the arrival of thousands of TDEs from the Vera Rubin Observatory and ULTRASAT samples, the immense volumes of which make us increasingly sensitive to rare and extreme transients like ASASSN-15lh. ------------------------------------------------------------------------------------ Observations Description ------------------------ This program is designed to obtain late-time imaging in 3 UV filters for the enigmatic TDE candidate ASASSN-15lh. These UV data will (1) search for temporal evolution from earlier WFC3/UVIS UV imaging in 2019 from HST Program 15499 to differentiate between SLSN and TDE origins for the transient and (2) if evolution is detected, supporting the TDE interpretation, measure the properties of a late-time accretion disk. We will obtain NUV imaging with WFC3/UVIS F225W and F336W and FUV imaging with ACS/SBC F125LP. The choice of WFC3/UVIS filters is designed to leverage previous exposures in HST Program 15499. The ACS/SBC F125LP imaging will provide an important higher energy constraint on the SED to aid in differentiating between a star formation and accretion disk origin. Constraining temporal evolution and disentangling transient and host-galaxy 18443( 2) - 18-Aug-2026 18:07:16 - [ 2] emission at faint UV fluxes requires high-precision photometry. Our baseline exposure time estimates assume no evolution from the measured total fluxes of m_F225W = 22.816 +/- 0.014 mag and m_F336W = 22.056 +/- 0.012 in 2019 (calculated using a standard 10 pixel radius aperture). To constrain UV temporal evolution at the <2% level, we need a precision matching that of the 2019 epochs. The expected F225W and F336W backgrounds are low, so we have added 20 electrons of post-flash exposure to mitigate CTE issues. We estimate the exposure time for the F125LP imaging based on a S/N of 50 for the host-galaxy only flux. The WFC3/UVIS and ACS/SBC visits can be scheduled separately, provided that they are taken within 1 month, probing the disk on timescales much shorter than the dynamical time at 9-10 years post disruption. ------------------------------------------------------------------------------------ 18443( 2) - 18-Aug-2026 18:07:16 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ASASSN-15LH GALAXY, ACCRETION DISK, RA=22H02M15.4510S, J2000 Z = V = 18.60 +/- 0.05 The V-band magnitude ELLIPTICAL DEC=-61D39'34.60" 0.2326 includes the entire host galaxy flux and is estimated from pre-flare ASAS-SN V-band photometry. Reference Frame: ICRS Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18443( 2) - 18-Aug-2026 18:07:16 - [ 4] 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 ACS-SBC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.179 Line_Spacing 0.116 Coordinate_Frame POS-TARG Pattern_Orient 20.02 Angle_Between_Sides 63.65 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18443( 2) - 18-Aug-2026 18:07:16 - [ 5] Visit: 01 (WFC3/UVIS Imaging) Visit Requirements: GROUP 01, 02 WITHIN 30D On Hold Comments: Additional Comments: WFC3/UVIS F225W and F336W imaging of ASASSN-15lh. 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 ASASSN-15LH WFC3/UVI ACCUM UVIS2 F336W FLASH=20 1 396 S PATTERN 1 1 S see below (1191 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ASASSN-15LH WFC3/UVI ACCUM UVIS2 F225W FLASH=20 1 1067 S PATTERN 1 2 S see below (3564 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 | ------------------------------------------------------------------------------------------------------------------------------------ ASASSN-15LH 1 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 397.0 S S ASASSN-15LH 1 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 1 397.0 S S ASASSN-15LH 1 WFC3/UVI ACCUM UVIS2 F336W 3 none none none 1 397.0 S S ASASSN-15LH 2 WFC3/UVI ACCUM UVIS2 F225W 1 none none none 1 1048.0 S S ASASSN-15LH 2 WFC3/UVI ACCUM UVIS2 F225W 2 none none none 2 1258.0 S S ASASSN-15LH 2 WFC3/UVI ACCUM UVIS2 F225W 3 none none none 2 1258.0 S S 18443( 2) - 18-Aug-2026 18:07:16 - [ 6] Visit: 02 (ACS/SBC Imaging) Visit Requirements: GROUP 02, 01 WITHIN 30D On Hold Comments: Additional Comments: ACS/SBC F125LP Imaging of ASASSN-15lh 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 ASASSN-15LH ACS/SBC ACCUM SBC F125LP 1 1508 S PATTERN 2 1 see below (5106 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 | ------------------------------------------------------------------------------------------------------------------------------------ ASASSN-15LH 1 ACS/SBC ACCUM SBC F125LP 1 none none none 1 1271.0 S ASASSN-15LH 1 ACS/SBC ACCUM SBC F125LP 2 none none none 1 1271.0 S ASASSN-15LH 1 ACS/SBC ACCUM SBC F125LP 3 none none none 2 1282.0 S ASASSN-15LH 1 ACS/SBC ACCUM SBC F125LP 4 none none none 2 1282.0 S 18443( 2) - 18-Aug-2026 18:07:16 - [ 7] Summary Form for Proposal 18443 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures SBC, UVIS2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/SBC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=20 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 01, 02 WITHIN 30D, GROUP 02, 01 WITHIN 30D, PATTERN 1 1, PATTERN 1 2, PATTERN 2 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F125LP, F225W, F336W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ASASSN-15LH ------------------------------------------------------------------------------------------------------------------------------------