Visit 0000001 requires 10033.2 seconds but separated by less than 0.0 seconds from Visit 0000002 Visit 0000002 requires 10033.2 seconds but separated by less than 0.0 seconds from Visit 0000001 Illegal Timing Link Cycle found for visits: (002 001 002) 0 Assigned ID: 7502 ----- Attempting Submission 1 (Thu Apr 16 17:37:30 GMT 2026) ----- HST Phase I Proposal 7502 successfully submitted. Receipt: # 7502-1 We use the AFTER OBS flag to indicate our requirement that the two visits (WFC3/UVIS) and ACS/SBC are taken within 30 days, although the order is unimportant. We show this scheduling constraint with AFTER OBS as there is no GROUP WITHIN option for the Phase I portion of APT. These constraints are further discussed in the Special Requirements section of the proposal. Initial Phase II submission for HST Program 18443. We await responses on several questions about our program implementation, specifically for the WFC3 exposures. These questions concern which aperture is best (since this source is faint and CTE is likely a concern), what level of post-flash is recommended (currently we have used 20 electrons to match the Phase I ETC calculations), and finally what dither pattern is recommended (especially to avoid droplets for high-precision photometry). 822504192 ----- Attempting Submission 1 (Tue Aug 18 17:40:45 GMT 2026) ----- 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. 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 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. PI Nicholas Stone is a professor at UW-Madison and an expert on the theory of tidal disruption events (TDEs). He was the lead theorist on the first paper (Leloudas et al. 2016) to propose that ASASSN-15lh was a TDE, and also on the discovery paper for tidal disruption plateau emission (van Velzen et al. 2019). Co-PI Jason Hinkle is a NHFP Einstein Fellow at UIUC, and a leading observer of TDEs. His work led to the discovery of a class of "extreme nuclear transients" which may represent even more extreme versions of ASASSN-15lh (Hinkle et al. 2025). He additionally has extensive experience reducing and analyzing space-based UV photometry. Co-I Yael Alush is a PhD student at the Hebrew University of Jerusalem and UW-Madison, working under the supervision of PI Stone. She has developed novel models (Alush & Stone 2025) for magnetized spreading TDE disks which have been used to fit dozens of observed TDE plateaus (Alush et al. 2025), and which will be used to fit the data collected in this proposal. Supermassive Black Holes and Active Galaxies Stellar Physics and Stellar Types 6 false true false false false false false SUPPORTED GALAXY ACCRETION DISK ELLIPTICAL J2000 J2000 The V-band magnitude includes the entire host galaxy flux and is estimated from pre-flare ASAS-SN V-band photometry. WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F336W ACS/SBC Imaging No No F125LP POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false POS-TARG DITHER 0.179 LINE 0.116 false false 63.65 4 20.02 ACS-SBC-DITHER-BOX 2 false 1786584341866 /Users/jhinkle/Documents/OP-cache/ASASSN-15lh_latetime_UV_PhaseII-v01.xml Timing link suitability includes constraints ([Group (01 02) Within 30.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. WFC3/UVIS F225W and F336W imaging of ASASSN-15lh. (Tic-Visit :proposal-id 18443 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "12-Aug-2026 21:25:41.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1732480 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18443 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18443 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2684 :al-overhead-at-end 127 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3064 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18443 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2683 :al-overhead-at-end 127 :internal-dur-at-end 352 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3064 :pcs-gap 0 ) true 2 0 1786584602615 /Users/jhinkle/Documents/OP-cache/ASASSN-15lh_latetime_UV_PhaseII-v02.xml Timing link suitability includes constraints ([Group (01 02) Within 30.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. ACS/SBC F125LP Imaging of ASASSN-15lh (Tic-Visit :proposal-id 18443 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "12-Aug-2026 21:30:02.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "28" :si-bright-earth-limb-angle 20.0 :data-volume 74064 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18443 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18443 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2683 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3064 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18443 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2684 :al-overhead-at-end 0 :internal-dur-at-end 117 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3064 :pcs-gap 0 ) true 2 0