0 Assigned ID: 4959 ----- Attempting Submission 1 (Wed Feb 07 17:13:44 GMT 2024) ----- HST Phase I Proposal 4959 successfully submitted. Receipt: # 4959-1 We hope the observations can be scheduled ASAP. Also it would be great if the visit 02 (FUV) can be conducted immediately following visit 01 (NUV), thus the FUV and NUV spectra are obtained almost simultaneously, God bless! 2095659524 ----- Attempting Submission 1 (Fri Feb 16 11:34:20 GMT 2024) ----- The 2024 optical outburst of AT2020afhd is a newly identified TDE at a redshift of 0.024 and is currently approaching its peak. In addition to an obvious bump in the optical rising light curves, AT2020afhd exhibits an unusually bright soft X-ray emission (~10^43 erg/s), which has decreased by a factor of ~10 within only one week, making it the second TDE with similar behaviors after AT2022dsb. This behavior can be ideally explained in the reprocessing model, in which the high-speed outflow launched at the high accretion rate stage serves as the obscuring material. Therefore, AT2020afhd offers an extremely rare opportunity to explore TDE physics in real time. This target is UV bright and is currently in the HST visibility window (ends on March 4) and the next window does not begin until December 2024. We propose to conduct timely STIS UV spectroscopic observations on the intriguing system with a small amount of time (2 orbits). Our immediate objectives are twofold: 1) to detect broad blueshifted UV absorption lines with which we can check whether the rapid X-ray fading is caused by the obscuration of the high-speed outflow (reprocessing layer); 2) to detect potential characteristic UV emission lines of TDEs to further confirm the TDE nature given the existence of AGN activity (eROSITA DR1 detection) prior to the outburst. We anticipate that the proposed observation can significantly contribute to understanding the optical emission origin of TDEs in combination with other follow-up observations. We request a single epoch (that could be split into two visits, if necessary) of UV spectroscopy of a TDE, the 2024 outburst of AT2020afhd, over the wavelength range from 1200-3000A. HST, with its sensitive UV spectrographs and exquisite angular resolution, is the only facility that can conduct such observations. A slit width of 0.2" is preferred to cover the galaxy's nucleus and minimize the starlight contamination. The magnitudes in UVW1 (central wavelength 2688.46A) and UVW2 (2140.26A) bands measured from the latest Swift/UVOT observations on Feb 13 are 16.16+/-0.04 and 16.23+/-0.03, respectively, which should be almost their peak magnitudes based on its optical light curves. We expect that the TDE is going to decline soon and could be 0.5-1 mag fainter after a couple of weeks when the observations are conducted. Thus we try to estimate the resulted signal to noise ratio (SNR) assuming an AB magnitude of 17. The calculations were performed with the Exposure Time Calculator (ETC) of HST/STIS. We assume a blackbody spectrum with temperature of 20,000K, which is a typical value for TDE, and an AB magnitude of 17 in the UVW1 band. To reliably detect a possible C IV absorption line, a continuum signal-to-noise ratio (SNR) of 10 at 1586A is needed. We assume that our targets are visible for 55 minutes each orbit. After including overheads for guide-star acquisition (6 min), target acquisition (~8 min for an extended source, no peak-up required for 0.2" slit width), and wavelength calibration (4 min per orbit), we find that we can obtain 37 min of effective exposure. We request to allocate one-orbit time for G230L and G140L, respectively. That will result in 2100 second exposure time for each grism and meet with our scientific goal. According to the exposure time calculator by inputting our assumed SED, we would expect a SNR per resolution element >=10 for the range from 1650-3100 A. To summarize, we request a single epoch of STIS spectroscopy of AT2020afhd, for a total of 2 orbits (1 FUV + 1 NUV). 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 the PI of the other HST DDT proposal (ID:16943), which has resulted in a paper 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 AGN outbrusts. 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 POS-TARG DITHER 0.4165 LINE false false 3 90.0 STIS-ALONG-SLIT 1 false 1708080807019 The scheduling data was generated by CASM 32.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 17609 :visit-id "01" :proposal-version -1 :trans-version "20230626" :date "16-Feb-2024 18:53:26.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 435200 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17609 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17609 :visit-id "01" :orbit-number 1 :pcs-time 443 :si-vis-utilization 2691 :al-overhead-at-end 0 :internal-dur-at-end 440 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3136 :pcs-gap 0 ) true 1 0 1708080861402 The scheduling data was generated by CASM 32.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 17609 :visit-id "02" :proposal-version -1 :trans-version "20230626" :date "16-Feb-2024 18:54:21.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 435200 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17609 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17609 :visit-id "02" :orbit-number 1 :pcs-time 443 :si-vis-utilization 2691 :al-overhead-at-end 0 :internal-dur-at-end 440 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3136 :pcs-gap 0 ) false 1 0