15899( 5) - 08-Oct-2019 18:31:53 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15899 Version: 5 Check-in Time: 08-Oct-2019 18:31:53 STScI Edit Number: 1 Title Late-time UV spectroscopy of Tidal Disruption Events with HST ------------------------------------------------------------------------------------ Type Cycle GO 27 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Tzu-Yu Hung University of California - Santa Cruz Y CoI: Dr. Sjoert van Velzen New York University N CoI: Prof. Suvi Gezari University of Maryland N CoI: Dr. Stephen Bradley Cenko NASA Goddard Space Flight Center N CoI: Mr. Nathaniel Roth University of Maryland N CoI: Dr. Nadejda Blagorodnova Radboud Universiteit Nijmegen N CoI: Prof. Ryan Foley University of California - Santa Cruz N ------------------------------------------------------------------------------------ Abstract Once every 10^5 years, an accretion-powered flare may temporarily illuminate a supermassive black hole (SMBH) following a tidal disruption event (TDE), in which a star is shredded by tidal stresses close to the SMBH. TDEs provide a new window to study these otherwise dormant SMBHs and probe different accretion modes (super-Eddington to sub-Eddington) on a monthly timescale. Recent studies provide evidence for significant state changes in the late-time TDE emission properties (X-ray re-brightening, light curve flattening) that may be attributed to delayed disk formation or transitions in the accretion processes. These findings highlight our lack of understanding of how and when circularization occurs in TDEs. In light of this, we propose late-time follow-up UV spectroscopic observations of two TDEs that have been observed by HST in their early phases, in order to study the evolution of debris streams on the timescale of a year. Since the UV band captures the strongest atomic transitions (N, C, Si) expected from TDEs, UV spectroscopy will allow us to measure these lines and infer the kinematics, density, and abundance in this late-time TDE environment. Past HST STIS observations of these two targets have identified cold, fast outflows (0.05c) initiated by super-Eddington accretion. One of our targets, AT2018zr, has undergone a remarkable UV spectral hardening that may be related to an increased contribution from the disk. With the proposed HST observations, we wish to ascertain whether such disk exists by searching for high-ionization emission lines similar to those seen in QSOs. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to observe two TDE targets, AT2018zr and ZTF18abxftqm, in Cycle 27 in the wavelength range of 1200 - 3000 Angstrom. Extrapolating from existing data, we averages the results from a linear-fit and a t^-5/3 power-law fit to estimate the UVW2 mag of these two targets to be 21.4 and 20.4 mag in October 2019. For the NUV spectrum, we will use STIS NUV-MAMA with the G230L grating instead of COS because we require a broad wavelength coverage to capture NUV features in 1700 - 3000A (where the rest wavelengths of Fe and MgII transitions 15899( 5) - 08-Oct-2019 18:31:53 - [ 2] fall). We compared the FUV spectrum output from both COS and STIS with the G140L grating with HST ETC and found the required exposure time to reach the desired S/N to be comparable. Therefore, we choose to obtain the FUV spectrum with HST STIS with the G140L. To estimate the required exposure time, we assume the SED is a blackbody with a temperature of 20, 000 K, as typical of TDEs. We assume an orbital visibility of 54 min, an overhead of 6 min for guide star aquisition at the beginning of a visit, 4 min for guide star reaquisition at the beginning of each subsequent orbits, 6 min for target acquisition, 8 min spectroscopy overhead for each grating, and 4 min for each exposure time interval that exceeds 2300 seconds at the same grating position. This amounts to an on-source time of 34 min in the rst orbit, and 44 min in the subsequent orbits. A 5 orbits observation with STIS FUV-MAMA would result in a S/N >5 per resolution element from the continuum emission alone with the G140L grating for AT2018zr. A 3 HST orbits observation would result in a S/N >5 per resolution element from the continuum emission alone with the G230L grating with STIS NUV-MAMA. For ZTF18abxftqm, we require 2 HST orbits for the FUV spectrum, and 1 HSTorbit for the NUV spectrum with the same configuration to achieve the same level of S/N. The assumed S/N will allow us to detect strong narow (Wr > 1A) absorption lines in the spectrum. In summary, we request for (5 + 3) orbits for AT2018zr and (2 + 1) orbits for ZTF18abxftqm, totaling 11 orbits of HST time in Cycle 27. ------------------------------------------------------------------------------------ 15899( 5) - 08-Oct-2019 18:31:53 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AT2018ZR GALAXY, ACCRETION DISK, RA=07H56M54.5500S, J2000 V = 21 NUCLEUS DEC=+34D15'43.60" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 2 ZTF18ABXFTQM GALAXY, ACCRETION DISK, RA=01H07M33.6100S, J2000 V = 20.5 NUCLEUS DEC=+23D28'34.40" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 15899( 5) - 08-Oct-2019 18:31:53 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.55 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 15899( 5) - 08-Oct-2019 18:31:53 - [ 5] Visit: 01 Visit Requirements: BEFORE 01-MAY-2020:00:00:00 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 AT2018ZR STIS/CCD ACQ F28X50LP MIRROR 1 45 S (45 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2018ZR STIS/FUV ACCUM 52X0.2 G140L 1425 2 1000 S -MAMA (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2018ZR STIS/FUV ACCUM 52X0.2 G140L 1425 3 860 S PATTERN 2 3 -MAMA (5160 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2018ZR 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A AT2018ZR 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none 1 none 1 N/A -MAMA AT2018ZR 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none 2 none 1 N/A -MAMA AT2018ZR 3 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 1 none 2 N/A -MAMA AT2018ZR 3 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 2 none 2 N/A -MAMA AT2018ZR 3 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 3 none 2 N/A -MAMA AT2018ZR 3 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 1 none 3 N/A -MAMA AT2018ZR 3 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 2 none 3 N/A -MAMA AT2018ZR 3 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 3 none 3 N/A -MAMA 15899( 5) - 08-Oct-2019 18:31:53 - [ 6] Visit: 02 Visit Requirements: GROUP 02, 01 WITHIN 5D 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 AT2018ZR STIS/CCD ACQ F28X50LP MIRROR 1 45 S (45 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2018ZR STIS/FUV ACCUM 52X0.2 G140L 1425 2 1300 S PATTERN 2 2 -MAMA see below (4600 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2018ZR 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A AT2018ZR 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 1 none 1 1000.0 S -MAMA AT2018ZR 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 2 none 1 1000 S -MAMA AT2018ZR 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 1 none 2 N/A -MAMA AT2018ZR 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 2 none 2 N/A -MAMA 15899( 5) - 08-Oct-2019 18:31:53 - [ 7] Visit: 03 Visit Requirements: BEFORE 20-MAY-2020:00:00:00; GROUP 03, 01, 02 WITHIN 15D 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 AT2018ZR STIS/CCD ACQ F28X50LP MIRROR 1 45 S (45 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2018ZR STIS/NUV ACCUM 52X0.2 G230L 2376 2 1000 S -MAMA (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2018ZR STIS/NUV ACCUM 52X0.2 G230L 2376 3 860 S PATTERN 2 3 -MAMA (5160 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2018ZR 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A AT2018ZR 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none 1 none 1 N/A -MAMA AT2018ZR 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none 2 none 1 N/A -MAMA AT2018ZR 3 STIS/NUV ACCUM 52X0.2 G230L 2376 1 none 1 none 2 N/A -MAMA AT2018ZR 3 STIS/NUV ACCUM 52X0.2 G230L 2376 1 none 2 none 2 N/A -MAMA AT2018ZR 3 STIS/NUV ACCUM 52X0.2 G230L 2376 1 none 3 none 2 N/A -MAMA AT2018ZR 3 STIS/NUV ACCUM 52X0.2 G230L 2376 2 none 1 none 3 N/A -MAMA AT2018ZR 3 STIS/NUV ACCUM 52X0.2 G230L 2376 2 none 2 none 3 N/A -MAMA AT2018ZR 3 STIS/NUV ACCUM 52X0.2 G230L 2376 2 none 3 none 3 N/A -MAMA 15899( 5) - 08-Oct-2019 18:31:53 - [ 8] Visit: 04 Visit Requirements: BEFORE 27-FEB-2020:00:00:00 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 ZTF18ABXFTQ STIS/CCD ACQ F28X50LP MIRROR 1 60 S (60 M S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ZTF18ABXFTQ STIS/FUV ACCUM 52X0.2 G140L 1425 3 850 S PATTERN 2 2 M -MAMA see below (4440 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ZTF18ABXFTQ STIS/NUV ACCUM 52X0.2 G230L 2376 3 890 S M -MAMA (2670 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 | ------------------------------------------------------------------------------------------------------------------------------------ ZTF18ABXFTQM 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 1 none 1 630.0 S -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 2 none 1 630.0 S -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none 3 none 1 630.0 S -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 1 none 2 N/A -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 2 none 2 N/A -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none 3 none 2 N/A -MAMA ZTF18ABXFTQM 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none 1 none 3 N/A -MAMA ZTF18ABXFTQM 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none 2 none 3 N/A -MAMA ZTF18ABXFTQM 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none 3 none 3 N/A -MAMA 15899( 5) - 08-Oct-2019 18:31:53 - [ 9] Summary Form for Proposal 15899 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BEFORE 01-MAY-2020:00:00:00, BEFORE 20-MAY-2020:00:00:00, BEFORE 27-FEB-2020:00:00:00, GROUP 02, 01 WITHIN 5D, GROUP 03, 01, 02 WITHIN 15D, PATTERN 2 2, PATTERN 2 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2018ZR, ZTF18ABXFTQM ------------------------------------------------------------------------------------------------------------------------------------