0 Assigned ID: 5924 ----- Attempting Submission 1 (Tue Aug 06 08:59:00 GMT 2024) ----- HST Phase I Proposal 5924 successfully submitted. Receipt: # 5924-1 ----- Attempting Submission 2 (Tue Aug 06 12:53:45 GMT 2024) ----- HST Phase I Proposal 5924 successfully submitted. Receipt: # 5924-2 0 ----- Attempting Submission 1 (Sun Aug 25 05:53:25 GMT 2024) ----- Probing the UV spectral evolution of a unique TDE with early bright X-ray emission The 2024 outburst of AT2020afhd was identified as a unique tidal disruption event (TDE) with bright soft X-ray emission (~10^43 erg/s) although with a large amplitude of variability. A timely UV spectrum around its optical peak has been obtained, which showed a very steep continuum slope. Interestingly, the UV continuum below 2000 A shows a notable excess relative to the extrapolation of the blackbody inferred from the photometric spectral energy distribution (SED). The overall SED can be better fitted by a steep powerlaw function with an index of about -3. This discovery indicates that the blackbody assumption of the SED may have severely underestimated the intrinsic bolometric luminosity, leading to the so-called "missing energy" puzzle. Such SEDs, with a steep rise towards the blue, have previously been predicted by the reprocessing model. Here we propose an additional epoch of UV spectroscopic observations with a small amount of time (4 orbits) for this ongoing unique TDE. At the time of observation, its luminosity (accretion rate) will have decreased by an order of magnitude compared to the first visit (26 February). Therefore, it will provide us an unprecedented opportunity to study the intrinsic SED of a TDE at different accretion phases, which will definitely serve as a new observational test of models (e.g. reprocessing) that account for the optical emission of TDEs. Meanwhile, the evolution of the UV emission lines, including some unknown ones, can also be studied. Furthermore, the detection or not of BAL features in a lower X-ray phase will help to confirm or exclude whether the dramatic X-ray variability is caused by the change in absorption. We request to use G140L and G230L gratings to obtain a continuous wavelength coverage from 1100 to 3000 $\AA$ in the rest frame. Since our primary goal is to obtain a SED in this wavelength range, we prefer to perform the observation in a single epoch. If this is not possible, we hope that the observations of the two gratings can be made as close together as possible. A slit width of 0.2$\arcsec$ is preferred to cover the galaxy's nucleus and minimize the starlight contamination. The UVW1 and UVW2 magnitudes measured from the latest Swift epoch on August 02 are 17.94 and 18.13, respectively. We expect that the source will be $\sim0.5$ mag fainter in a couple of months when the proposed observation is conducted. Thus we try to estimate the needed exposure time assuming a magnitude of 18.6 in UVW2, the band of which is less contaminated by host emission. The calculations were performed with the Exposure Time Calculator (ETC) of HST/STIS. We assume a powerlaw continuum with an index of $-2.78$, that is the continuum shape of the UV spectroscopic observation around the optical peak. To achieve a signal-to-noise ratio (S/N) of 10 at $\sim1500$~\AA, it requires an exposure time of $\sim3600$ seconds with the G140L grating. Based on our previous experiences in analyzing the STIS data, a similar time in G230L is also sufficient. The estimate is also roughly consistent with the estimated from the observation on Feb. 26. Our target is visible in the next few months by selecting the "Increase Scheduling Flexibility" flag in APT. The estimated orbit visibility is 49 minutes per orbit. Therefore, we request a total of 4 orbits (two orbits for each), including overheads. Expertise: Our team has been focusing on various transients associated with supermassive black holes (SMBHs) in the past five years, particularly on tidal disruption events (TDEs). The PI, Dr. Ning Jiang, is also the PI of two previous HST DDT proposals, including the first DDT visit of the current target (ID:17609), which have resulted in two papers to be submitted. The PI is also the co-author of two TDE publications basing on HST/STIS UV spectra (Yang et al.2017,ApJ,846,150; Sheng et al.2021,ApJL,920,25). Other CoIs are also experts on multi-wavelength observations of TDEs and numerical simulation of TDEs. Supermassive Black Holes and Active Galaxies Supermassive Black Holes and Active Galaxies 0 false SUPPORTED GALAXY ACCRETION DISK NUCLEUS SEYFERT J2000 J2000 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 /tmp/OPVisit3271157513844070267.xml The scheduling data was generated by CASM 32.1.3. CASM was run in Phase II mode. 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