This is a Time-Critical DDT request to obtain HST/STIS UV spectroscopy
of AT 2021aeuk during its currently ongoing third flare, which was
predicted by Sun et al. (2025) and confirmed by WFST on 27 May 2026
(TNS AstroNote, Wang et al. 2026-154). The source is brighter than the
second-flare peak and still rising, providing a unique and time-limited
opportunity to characterize the UV spectrum of this rpTDE candidate.
Givin each flare lasts only a few months, waiting for Cycle 34 would miss the observation window entirely.
The HST visibility window in the next two months is June 8 - June 12 and July 20 - July 21. Thus a scheduling requirement between today (June 1) to July 21 is specified.
Coordinated multi-wavelength campain is already in progress, including Einstein Probe, Chandra, LCOGT, and GTC has been scheduled or proposed.141022710Assigned ID: 7855
----- Attempting Submission 1 (Fri May 29 19:34:07 GMT 2026) -----
HST Phase I Proposal 7855 successfully submitted.
Receipt: # 7855-1
----- Attempting Submission 2 (Fri May 29 20:49:48 GMT 2026) -----
HST Phase I Proposal 7855 successfully submitted.
Receipt: # 7855-2
----- Attempting Submission 3 (Mon Jun 01 13:30:00 GMT 2026) -----
HST Phase I Proposal 7855 successfully submitted.
Receipt: # 7855-30----- Attempting Submission 1 (Wed Jun 10 17:50:54 GMT 2026) -----Confirming the First rpTDE in a Broad-Line AGN with the Rising Third Flare of AT 2021aeukAT 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.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 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.Jingbo Sun is the discoverer of this rpTDE candidate AT2021aeuk and led the publication of Sun et al. (2025), which precisely predicted the occurrence of this third flare. Hengxiao Guo is a multiwavelength expert specializing in TDEs and AGN. The team also includes X-ray experts Wenda Zhang, Joheen Chakraborty and Lorena Hernandez Garcia, who led the EP, XMM-Newton, Chandra, and XRISM observing proposals. Mingfeng Gu is an expert in radio observations and has led the radio monitoring campaign for this target, securing both VLA and EVN observations. Wanling Liu is a PhD student familiar with optical data analysis.Supermassive Black Holes and Active Galaxies0Time CriticalfalsefalseSUPPORTEDGALAXYACCRETION DISKSEYFERTJ2000J2000The scheduling data was generated by CASM 34.1.CASM was run in Phase I mode.STIS/FUV-MAMASpectroscopicNoNoG140L1425The scheduling data was generated by CASM 34.1.CASM was run in Phase I mode.STIS/NUV-MAMASpectroscopicNoNoG230L2376The scheduling data was generated by CASM 34.1.CASM was run in Phase I mode.STIS/CCDSpectroscopicNoNoG430L4300The scheduling data was generated by CASM 34.1.CASM was run in Phase II mode./private/var/folders/nr/56bxqsqs39q8y5r7cdzw5z6c0000gn/T/OPVisit6216889818955496899.xml(Tic-Visit
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