17849( 2) - 26-Aug-2024 12:21:21 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17849 Version: 2 Check-in Time: 26-Aug-2024 12:21:21 STScI Edit Number: 0 Title Probing the UV spectral evolution of a unique TDE with early bright X-ray emission ------------------------------------------------------------------------------------ Type Cycle GO/DD 31 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Ning Jiang University of Science & Technology of Y China CoI: Yibo Wang University of Science & Technology of N China CoI: Dr. Shifeng Huang University of Science & Technology of N China CoI: Mr. Jiazheng Zhu University of Science & Technology of N China CoI: Dr. Tinggui Wang University of Science & Technology of N China CoI: Mr. Lars Lund Thomsen University of Hong Kong N CoI: Prof. Jane Dai University of Hong Kong N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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 17849( 2) - 26-Aug-2024 12:21:21 - [ 2] 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. ------------------------------------------------------------------------------------ 17849( 2) - 26-Aug-2024 12:21:21 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AT2020AFHD GALAXY, ACCRETION DISK, RA=03H13M35.6670S +/- 0.02", J2000 V = 16.8 +/- 0.2 NUCLEUS, SEYFERT DEC=-02D09'6.21" +/- 0.02" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 17849( 2) - 26-Aug-2024 12:21:21 - [ 4] Visit: 01 Visit Requirements: 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 AT2020AFHD STIS/CCD ACQ F28X50LP MIRROR 1 60 S (60 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2020AFHD STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=676 1 1858 S -MAMA G (1858 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2020AFHD STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=676 1 2597 S -MAMA G (2597 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2020AFHD 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A AT2020AFHD 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 N/A -MAMA G AT2020AFHD 3 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 2 N/A -MAMA G 17849( 2) - 26-Aug-2024 12:21:21 - [ 5] Visit: 02 Visit Requirements: 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 AT2020AFHD STIS/CCD ACQ F28X50LP MIRROR 1 60 S (60 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2020AFHD STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=676 1 1858 S -MAMA G (1858 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2020AFHD STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=676 1 2597 S -MAMA G (2597 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2020AFHD 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A AT2020AFHD 2 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 N/A -MAMA G AT2020AFHD 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 N/A -MAMA G 17849( 2) - 26-Aug-2024 12:21:21 - [ 6] Summary Form for Proposal 17849 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=676 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2020AFHD ------------------------------------------------------------------------------------------------------------------------------------