0 Assigned ID: 4169 ----- Attempting Submission 1 (Tue May 23 20:15:20 GMT 2023) ----- This target is not used in this proposal. 0 ----- Attempting Submission 1 (Wed Sep 13 08:00:52 GMT 2023) ----- HST Phase II Proposal 17449 successfully submitted. Receipt: # 17449-1 ----- Attempting Submission 2 (Tue Feb 06 19:49:02 GMT 2024) ----- ----- Attempting Submission 3 (Wed Feb 07 18:54:02 GMT 2024) ----- HST Phase II Proposal 17449 successfully submitted. Receipt: # 17449-3 ----- Attempting Submission 4 (Wed Feb 07 19:40:06 GMT 2024) ----- HST Phase II Proposal 17449 successfully submitted. Receipt: # 17449-4 ----- Attempting Submission 5 (Wed Feb 07 19:42:57 GMT 2024) ----- Resubmission after I changed STIS acquisition time to 40 sec Early spectroscopy of tidal disruption events: outflow signatures or chemical composition ? Tidal Disruption Events (TDEs) happen when stars get tidally disrupted by the gravitational field of a supermassive black hole. TDEs are bright panchromatic events and evolve on human-friendly time scales, which makes them prime tools to study accretion physics and black holes in all types of galaxies. Despite considerable progress achieved in the last few years, and the increasing number of TDEs discovered in the optical/UV wavelengths, we still do not know what powers the optical/UV emission of these events, with reprocessed accretion and debris stream collisions being the leading candidates. Important clues to their nature can be derived from spectroscopy, including the presence of Bowen fluorescent emission and the study of kinematical offsets. A growing number of optical TDEs show transient blueshifted line profiles, which are particularly prominent at the He + Bowen blend and at early phases. These have been interpreted as signatures of outflows, which are an inherent prediction of super-Eddington accretion models. However, the blended nature of optical lines makes it hard to disentangle outflows (measured by blueshifts) from chemical composition (the relative contributions of He II and N III). Early UV spectroscopy is required to break this degeneracy by observing the profiles of isolated He II and high-ionisation lines before maximum light, something which has never been possible to achieve before. We therefore propose a ToO program to obtain the first pre-max spectrum of a TDE in the UV and solve this outstanding question. In addition, our proposed COS-STIS observations will contribute to slowly increasing a small legacy sample of TDEs with UV spectroscopy. This is a target of opportunity program. We propose to obtain spectra of a TDE with STIS + COS in order to cover both the NUV and FUV wavelength range, and study a range of spectral lines at different wavelengths. In total, we request for observations at three separate epochs: the first spectrum should be obtained as soon as possible after the early discovery and spectroscopic classification (by means of an optical spectrum) of a TDE showing blueshifted features. Hence, this is a disruptive ToO. Two more epochs will be obtained around the peak of the light curve (or a bit after the peak) and at a few weeks after the peak. These observations will allow us to monitor the evolution of the line profiles, strengths and offsets. These two observations will be linked to be separated by ~3 and ~6 weeks from the first trigger and they will therefore be nondisruptive for the telescope schedule. At the same time, this is an adequate time scale for probing TDE evolution. We will apply strict selection criteria in order to ensure that the data will be adequate to address our goals. We will only trigger our program on a TDE which i) is discovered and spectroscopically classified before peak and ii) shows a blueshifted broad emission line of He II (possibly blended with N III) and iii) is sufficiently bright. Based on our S/N calculations we have set our brightness cut-off to UVM2 < 15.5 mag (Vega) at peak, corresponding to a typical TDE at z < 0.05. During the rise (time of first trigger) and decay, this translates to UVM2 < 16.5 mag. To obtain a quick estimate of the UV magnitude, we will request for a Swift visit within 24 hours from spectroscopic classification and identification as a potential target. However, this is not required and it will not be part of our triggering criteria. If Swift observations are not available or possible within a fast time scale, we will estimate the UV color, using typical TDE optical - UV colors (g-UVW2 = 2.1+/-0.46 based on a large number of TDEs), where the optical color will be provided by the discovery survey. Prior to triggering, a safety control will be performed for nearby bright objects, in compliance with the COS + STIS regulations. Disruptive nature of ToO and justification: As explained, we request a disruptive ToO. This is the only way to obtain a spectrum of a TDE prior to maximum light. TDEs have typical rise times of 20-25 days and the earliest available classifications (i.e. spectroscopic confirmation of their nature) are 1-2 weeks before peak. In addition, spectroscopic evolution is fast at these phases and the line profiles we wish to observe are of transient nature. We can therefore not afford waiting 3 weeks for a non-disruptive trigger but we request a turnaround of 2-5 days. The two follow-up observations will not be disruptive but will be linked to be ~3 and ~6 weeks after the first trigger. Our team includes both senior and junior scientists with extensive experience within transient astrophysics and with a particular focus on the study of TDEs in a multi wavelength approach. We are all actively involved in the advanced Public ESO Spectroscopic Survey for Transient Objects (ePESSTO+) scientific collaboration and members of the ePESSTO+ TDE science group. In the last few years, the members of this group have produced many significant publications in the field of TDEs, thus contributing significantly to the progress achieved in this field (e.g. Leloudas et al. 2019, 2022; Wevers et al. 2019, 2020, 2021, 2022; Onori et al. 2019, 2022; Gromadzki et al. 2019; Nicholl et al. 2020, 2022; Jonker et al. 2020; Arcavi et al. 2022; Charalampopoulos et al. 2022, 2023). In addition, we have co-authored many of the papers belonging to the dedicated Space Science Series of ISSI: The Tidal Disruption of Stars by Massive BH (https://link.springer.com/book/9789402421453). We have access to a number of TDE-related programs which include the use of various facilities such as the ESO instruments, but also LT, WHT, NOT, TNG, Gemini, LCO, Campo Imperatore and ATCA. In particular, Iair Arcavi is one of the pioneers in the field of optical TDEs and his group is focused in the daily monitoring of the ZTF public feed and in classifying all promising nuclear transients. Matt Nicholl, Giorgos Leloudas, Thomas Wevers have experience using HST including for transient objects and including UV spectroscopy. They have the necessary expertise to design the observations and reduce the data in a timely manner. Francesca Onori has experience in the transient field, particularly TDEs, in different bands (optical, NIR and X-ray). Ilya Mandel is an expert theorist in the modelling of TDEs. Panos Charalampopoulos is an expert on TDE spectroscopy and spectral polarimetry. Miika Pursiainen has expertise in seacrhing for fast evolving transients and spectroscopic analysis. Supermassive Black Holes and Active Galaxies Stellar Physics and Stellar Types 6 false false false false false false SUPPORTED EXT-STAR ACCRETION DISK UNDESIGNATED X-RAY TRANSIENT NO The V-magnitude is a difference magnitude provided by the ATLAS survey (in the orange filter) The UV magnitude is measured by Swift UVOT in the UVM2 filter on 2024-02-03 ATLAS18BCNL ZTF18AABDAJX J2000 J2000 UV (2000 AA): 16.1 mag (Vega) EXT-STAR ACCRETION DISK UNDESIGNATED X-RAY TRANSIENT The target will be a TDE satisfying the following criteria: i) it will be discovered and spectroscopically classified before peak AND ii) the optical spectrum will show a blueshifted broad emission line of He II (possibly blended with N III) AND iii) it will be sufficiently nearby/bright. We have set our brightness cut-off to UVM2 < 15.5 mag (Vega) at peak, corresponding to a typical TDE at z < 0.05. During the rise (time of first trigger), this translates to UVM2 < 16.5 mag (where UVM2 is the Swift band). The target will be by definition found in the nucleus of a host galaxy that could be of similar brightness as the transient in the optical (V band). However, in the UV wavelengths we do not expect significant contamination from the host. Bright Tidal Disruption Event (TDE), discovered pre-peak UV (2000 AA): 14-16.5 mag (Vega) COS/FUV Spectroscopic No No G140L 800 STIS/NUV-MAMA Spectroscopic No No G230L 2376 COS/FUV Spectroscopic No No G140L 800 STIS/NUV-MAMA Spectroscopic No No G230L 2376 COS/FUV Spectroscopic No No G140L 800 STIS/NUV-MAMA Spectroscopic No No G230L 2376 Timing link suitability includes constraints ([02 After 01 by 0.1 to 0.1 days] [03 After 01 by 12.0 to 30.0 days]) The scheduling data was generated by CASM 32.0. 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CASM was run in Phase II mode. /System/Volumes/Data/solaris/user1/weck/work/OP-cache/17449-v03.xml 1708645249000 240.0 (Tic-Visit :proposal-id 17449 :visit-id "03" :proposal-version -1 :trans-version "20230626" :date "07-Feb-2024 09:13:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 435008 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17449 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17449 :visit-id "03" :orbit-number 1 :pcs-time 443 :si-vis-utilization 2474 :al-overhead-at-end 0 :internal-dur-at-end 369 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2920 :pcs-gap 0 ) false 1 0 Timing link suitability includes constraints ([04 After 03 by 0.1 to 0.1 days]) The scheduling data was generated by CASM 32.0. CASM was run in Phase II mode. /System/Volumes/Data/solaris/user1/weck/work/OP-cache/17449-v04.xml 1708650926000 241.0 (Tic-Visit :proposal-id 17449 :visit-id "04" :proposal-version -1 :trans-version "20230626" :date "12-Feb-2024 15:59:33.0" :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 87408 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17449 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17449 :visit-id "04" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2220 :al-overhead-at-end 0 :internal-dur-at-end 394 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2920 :pcs-gap 0 ) true 1 0 Timing link suitability includes constraints ([05 After 03 by 15.0 to 27.0 days] [06 After 05 by 0.1 to 0.1 days]) The scheduling data was generated by CASM 32.0. 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