18261( 2) - 10-Jun-2026 18:01:25 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18261 Version: 2 Check-in Time: 10-Jun-2026 18:01:25 STScI Edit Number: 0 Title Confirming the First rpTDE in a Broad-Line AGN with the Rising Third Flare of AT 2021aeuk ------------------------------------------------------------------------------------ Type Cycle GO/DD 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Jingbo Sun Universiteit Leiden Y CoI: Joheen Chakraborty Massachusetts Institute of Technology N CoI: Prof. Hengxiao Guo Shanghai Astronomical Observatory, CAS N CoI: Dr. Wenda Zhang National Astronomical Observatories of N China (NAOC) CoI: Prof. Minfeng Gu Shanghai Astronomical Observatory, CAS N CoI: Ms. Wanling Liu Shanghai Astronomical Observatory, CAS N CoI: Dra. Lorena Hernandez-Garcia Universidad Diego Portales N ------------------------------------------------------------------------------------ Abstract AT 2021aeuk is the first candidate repeating partial tidal disruption event (rpTDE) in a Narrow-Line Seyfert 1 AGN, with a recurrence time of ~3.5 years between flares. Its third flare erupted in May 2026 as predicted and is still rising. Coordinated multi-wavelength observations of the previous second flare revealed an unusual anti-correlation between the X-ray and UV/optical evolution, unlike either a typical TDE or an AGN outburst, but supporting the interaction between the returning TDE stream and the pre-existing AGN accretion disk. We propose 4 orbits of HST/STIS UV/optical spectroscopy during this ongoing third flare to confirm the rpTDE nature of the recurrent eruptions through three diagnostics: (1) the UV-to-optical SED shape, (2) the UV emission-line ratios and their nitrogen enhancement, and (3) the kinematics of any high-velocity UV outflows. Successful confirmation would establish the first rpTDE in a pre-existing broad-line AGN, representing a population that has been largely overlooked in both TDE and AGN studies. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose four orbits of STIS UV/optical spectroscopic observations targeting the third flare of AT 2021aeuk. The observations will use the G140L (two orbits), G230L (one orbit), and G430L (one orbit) gratings, providing continuous wavelength coverage from 932 A to 4620 A in the rest frame. A slit width of 0.2" is adopted to minimize host galaxy contamination. The third flare is currently rising in the latest ATLAS data, having reached 17.1 mag in the o band and approaching the peak brightness of the first flare. Given that the nearest HST visibility window falls on July 19, we predict the o-band magnitude will decrease to 17.2 at the time of observation. To estimate the UV brightness, we decompose the UV-to-optical SED into two components: a pre-existing AGN continuum and a blackbody TDE component with a temperature of 10, 000 K, consistent with the second flare. This total SED yields an estimated FUV magnitude of 20.0, which we adopt as the basis for our ETC calculations. The resulting continuum S/N ratios are approximately 6, 15, and 50 for G140L, G230L, and G430L, respectively. The UV emission lines, which are generally 18261( 2) - 10-Jun-2026 18:01:25 - [ 2] prominent in both AGN and TDes, are expected to be detected at a higher S/N ratio. The HST combined spectrum, in comparison with the pre-outburst SED, will enable us to distinguish between a blackbody and power-law transient SED. It will also detect and resolve key diagnostic emission lines of both TDE and AGN activity, such as C IV, N III], C III], and Mg II. ------------------------------------------------------------------------------------ 18261( 2) - 10-Jun-2026 18:01:25 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AT2021AEUK GALAXY, ACCRETION DISK, RA=16H12M59.8424S, J2000 Z = V = 17.2 +/- 0.2 SEYFERT DEC=+42D19'40.25" 0.023 Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18261( 2) - 10-Jun-2026 18:01:25 - [ 4] Visit: 01 (FUV) Visit Requirements: BEFORE 01-AUG-2026:00:00:00 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 AT2021AEUK STIS/CCD ACQ 50CCD MIRROR 1 40 S (40 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2021AEUK STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1337 1 1917 S -MAMA G (1917 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2021AEUK STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1337 1 2593 S -MAMA G (2593 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2021AEUK 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A AT2021AEUK 2 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 N/A -MAMA G AT2021AEUK 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 N/A -MAMA G 18261( 2) - 10-Jun-2026 18:01:25 - [ 5] Visit: 02 (NUV) Visit Requirements: BEFORE 01-AUG-2026:00:00:00 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 AT2021AEUK STIS/CCD ACQ 50CCD MIRROR 1 40 S (40 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2021AEUK STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=1337 1 1917 S -MAMA G (1917 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2021AEUK 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A AT2021AEUK 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 N/A -MAMA G 18261( 2) - 10-Jun-2026 18:01:25 - [ 6] Visit: 03 (G430L) Visit Requirements: BEFORE 01-AUG-2026:00:00:00 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 AT2021AEUK STIS/CCD ACQ 50CCD MIRROR 1 40 S (40 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2021AEUK STIS/CCD ACCUM 52X0.2 G430L 4300 CR-SPLIT=3 1 1908 S (1908 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2021AEUK 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A AT2021AEUK 2 STIS/CCD ACCUM 52X0.2 G430L 4300 none none none 1 1 N/A AT2021AEUK 2 STIS/CCD ACCUM 52X0.2 G430L 4300 none none none 2 1 N/A AT2021AEUK 2 STIS/CCD ACCUM 52X0.2 G430L 4300 none none none 3 1 N/A 18261( 2) - 10-Jun-2026 18:01:25 - [ 7] Summary Form for Proposal 18261 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD, 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1337, CR-SPLIT=3 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BEFORE 01-AUG-2026:00:00:00 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, G430L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2021AEUK ------------------------------------------------------------------------------------------------------------------------------------