18132( 4) - 06-Apr-2026 12:11:40 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18132 Version: 4 Check-in Time: 06-Apr-2026 12:11:40 STScI Edit Number: 0 Title The first UV spectra of optically overluminous and featureless tidal disruption events ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Yuhan Yao University of California - Berkeley Y CoI: Prof. Ryan Chornock University of California - Berkeley N CoI: Dr. Raffaella Margutti University of California - Berkeley N CoI: Dr. Wenbin Lu University of California - Berkeley N ------------------------------------------------------------------------------------ Abstract We propose 8 orbits of HST STIS ultraviolet spectroscopic ToO observations to study a rare subclass of tidal disruption events (TDEs) characterized by featureless optical spectra. Only ~10 such events have been identified to date, and none have UV spectra. Compared to typical TDEs, these events exhibit higher UV and optical luminosities and are hosted by more massive black holes M_BH ~ 10^8 Msun), where general relativistic effects are more pronounced. They may arise from the disruption of massive stars and/or involve rapidly spinning black holes. Owing to their high luminosities, these events are expected to dominate the high-redshift TDE population to be uncovered by LSST, JWST, and Roman. This program will deliver the first UV spectroscopic observations of this elusive subclass, providing critical insights into their origin and enabling the first exploration of their rest-frame UV properties. These data will illuminate the physics of black hole accretion in extreme relativistic regimes and guide the identification of high-z TDE candidates in forthcoming surveys. ------------------------------------------------------------------------------------ Observations Description ------------------------ This proposal consists of non-disruptive ToO observations (up to 25 days response time) of 2 optically featureless tidal disruption events (TDEs), each for 4 orbits of STIS spectroscopic observations. Each trigger will be for a TDE that has (1) a nuclear location, (2) consistent blue colors, (3) an optical spectrum that is featureless, and (4) an apparent magnitude of g<19.5 at the time of triggering. Each TDE trigger will have two visits: - first visit: 2 orbits with STIS/FUV-MAMA using G140L, with the 52x0.2 aperture. - second visit: 2 orbits with STIS/NUV-MAMA using G230L, with the 52x0.2 aperture. Each visit requires an ACQ in the first orbit. As these are triggered observations, we do not yet know the exact brightness of the TDE or the host. We expect that the TDE will be a UV-bright point source on top of a (much redder) galaxy nucleus. The ACQ process uses the STIS/CCD with the F28X50LP 18132( 4) - 06-Apr-2026 12:11:40 - [ 2] filter, but the host galaxy may dominate in the optical. I have entered placeholder parameters for a diffuse acquisition. The exposure times and checkbox size may be adjusted depending on the actual properties of the target, so we have not provided ETC output until the actual fluxes are known. A ToO Activation on TDE-FEATURELESS1 should activate Visits 01 and 02 A ToO Activation on TDE-FEATURELESS2 should activate Visits 03 and 04 ------------------------------------------------------------------------------------ 18132( 4) - 06-Apr-2026 12:11:40 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2025AFJE UNIDENTIFIED, RA=02H48M9.8700S, J2000 V = 18 ULTRAVIOLET EMITTER DEC=+37D45'17.63" 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 ------------------------------------------------------------------------------------------------------------------------------------ 4 AT2025AFVR UNIDENTIFIED, RA=01H07M36.3000S, J2000 V = 18 ULTRAVIOLET EMITTER DEC=+24D22'39.89" 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 ------------------------------------------------------------------------------------------------------------------------------------ 5 AT2025ZMB UNIDENTIFIED, RA=06H09M46.7000S, J2000 V = 17 ULTRAVIOLET EMITTER DEC=+35D34'20.01" 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 ------------------------------------------------------------------------------------------------------------------------------------ 6 AT2026HIR UNIDENTIFIED, RA=08H44M12.3377S +/- 0.03", J2000 V = 17.5 +/- 0.5 ULTRAVIOLET EMITTER, DEC=+82D01'10.93" +/- 0.03" X-RAY EMITTER Reference Frame: ICRS Extended: NO Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0 ------------------------------------------------------------------------------------------------------------------------------------ 7 AT2023UQM UNIDENTIFIED, RA=23H10M39.0113S, J2000 V = 18 ULTRAVIOLET EMITTER DEC=+34D54'10.90" Reference Frame: ICRS Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 0.0 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18132( 4) - 06-Apr-2026 12:11:40 - [ 4] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 TDE-FEATUREL GALAXY, NUCLEUS (1) NUCLEAR (2) BLUE COLOR (3) V = 18 +/- 1.5 ESS-1 OPTICALLY FEATURELESS (4) G<19.5 MAG ------------------------------------------------------------------------------------------------------------------------------------ 2 TDE-FEATUREL GALAXY, NUCLEUS (1) NUCLEAR (2) BLUE COLOR (3) V = 18 +/- 1.5 ESS-2 OPTICALLY FEATURELESS (4) G<19.5 MAG ------------------------------------------------------------------------------------------------------------------------------------ 18132( 4) - 06-Apr-2026 12:11:40 - [ 5] Visit: 01 Visit Requirements: TOO RESPONSE TIME 25.0D 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 AT2026HIR STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, 1 50 S (50 DIFFUSE-CENTER=FLUX-CENTROID, S) CHECKBOX=13 ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2026HIR STIS/FUV ACCUM 52X0.2 G140L 1425 1 2094 S -MAMA (2094 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2026HIR STIS/FUV ACCUM 52X0.2 G140L 1425 1 2773 S -MAMA (2773 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2026HIR 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A AT2026HIR 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 1 N/A -MAMA AT2026HIR 3 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 2 N/A -MAMA 18132( 4) - 06-Apr-2026 12:11:40 - [ 6] Visit: 02 Visit Requirements: TOO RESPONSE TIME 25.0D 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 AT2026HIR STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, CHECKBOX=13, 1 50 S (50 DIFFUSE-CENTER=FLUX-CENTROID S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2026HIR STIS/NUV ACCUM 52X0.2 G230L 2376 1 2094 S -MAMA (2094 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2026HIR STIS/NUV ACCUM 52X0.2 G230L 2376 1 2773 S -MAMA (2773 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2026HIR 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A AT2026HIR 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA AT2026HIR 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 2 N/A -MAMA 18132( 4) - 06-Apr-2026 12:11:40 - [ 7] Visit: 03 Visit Requirements: TOO RESPONSE TIME 25.0D 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 TDE-FEATURE STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, 1 50 S (50 LESS-2 DIFFUSE-CENTER=FLUX-CENTROID, S) CHECKBOX=13 ------------------------------------------------------------------------------------------------------------------------------------ 2 TDE-FEATURE STIS/FUV ACCUM 52X0.2 G140L 1425 1 1800 S LESS-2 -MAMA (1800 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 TDE-FEATURE STIS/FUV ACCUM 52X0.2 G140L 1425 1 2400 S LESS-2 -MAMA (2400 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 | ------------------------------------------------------------------------------------------------------------------------------------ TDE-FEATURELESS-2 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A TDE-FEATURELESS-2 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 1 N/A -MAMA TDE-FEATURELESS-2 3 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 2 N/A -MAMA 18132( 4) - 06-Apr-2026 12:11:40 - [ 8] Visit: 04 Visit Requirements: TOO RESPONSE TIME 25.0D 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 TDE-FEATURE STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, CHECKBOX=13, 1 50 S (50 LESS-2 DIFFUSE-CENTER=FLUX-CENTROID S) ------------------------------------------------------------------------------------------------------------------------------------ 2 TDE-FEATURE STIS/NUV ACCUM 52X0.2 G230L 2376 1 1800 S LESS-2 -MAMA (1800 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 TDE-FEATURE STIS/NUV ACCUM 52X0.2 G230L 2376 1 2400 S LESS-2 -MAMA (2400 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 | ------------------------------------------------------------------------------------------------------------------------------------ TDE-FEATURELESS-2 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A TDE-FEATURELESS-2 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA TDE-FEATURELESS-2 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 2 N/A -MAMA 18132( 4) - 06-Apr-2026 12:11:40 - [ 9] Summary Form for Proposal 18132 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=DIFFUSE, CHECKBOX=13, DIFFUSE-CENTER=FLUX-CENTROID ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements TOO RESPONSE TIME 25.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2026HIR, TDE-FEATURELESS-2 ------------------------------------------------------------------------------------------------------------------------------------