15679( 9) - 25-Jan-2019 22:18:17 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15679 Version: 9 Check-in Time: 25-Jan-2019 22:18:17 STScI Edit Number: 5 Title Searching for Outflow Signatures in the Tidal Disruption Event ZTF18abxftqm ------------------------------------------------------------------------------------ Type Cycle GO/DD 26 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Tzu-Yu Hung University of California - Santa Cruz Y CoI: Prof. Suvi Gezari University of Maryland N CoI: Dr. Sjoert van Velzen New York University N CoI: Dr. Nadejda Blagorodnova California Institute of Technology N CoI: Mr. Nathaniel Roth University of Maryland N ------------------------------------------------------------------------------------ Abstract Wide-field optical transient surveys have discovered about a dozen of tidal disruption events (TDEs), which belong to a rare class of transient phenomena that occur when an unlucky star is tidally disrupted by a supermassive black hole (SMBH). In recent years, two emission mechanisms (circularization v.s. reprocessing) have been put forward to explain the excess in the UV and optical emission in TDEs, which is unexpected according to the classical TDE framework. UV spectroscopy is critical to disentangling these scenarios, as most strong atomic features fall in this bandpass, and emission and absorption line diagnostics can shed light on the physical state (velocity, density, composition) of the emitting gas (in analogy with quasars). However, despite its important diagnostic capabilities, UV spectroscopy has only been obtained for 5 TDEs so far, each of which have yielded important insights on the presence of an outflow and/or an intriguing nitrogen-rich chemical composition. Here we propose to obtain an epoch of FUV and NUV HST/STIS spectroscopic observations of the most recent TDE discovered by ZTF: ZTF18abxftqm (AT2018hco) at z=0.088. The slow UV evolution of ZTF18abxftqm confirmed by Swift observations, provides an excellent opportunity to get insights of the physical conditions in a TDE with high S/N UV spectroscopy. The proposed observations will allow us to test whether outflow is ubiquitous amongst TDEs, and if so, whether their velocities are consistent with current models. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request a single epoch (that could be split into multiple visits, if necessary) of UV spectroscopy of the TDE ZTF18abxftqm, 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 (the source is much too faint for the UV grism on Swift). We take the most recent UVW2 (~1928 A) magnitude measured by the Swift-UVOT, 18.3 (AB) on 28 Dec 2018, and the inferred blackbody temperature from the UV+optical photometry (T~20, 000 K) to estimate the expected SNR in dierent instrumental configurations. We find we can achieve the largest SNR in the near-UV using STIS with the NUV-MAMA detector and the G230L (1570-3180A) grating. Assuming an 15679( 9) - 25-Jan-2019 22:18:17 - [ 2] orbital visibility of 53 minutes and including overheads for guide-star acquisition (6 min for the first orbit, 4 min for the subsequent orbits), 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 35 min of exposure in the first orbit (43 in subsequent orbits). According to the exposure time calculator, we would expect a SNR per resolution element > 5 for the range from 1650 to 3150A for the continuum emission for a single orbit. This should allow us to easily detect strong narrow (Wr > 1A) absorption lines, as well as emission lines as were observed from, e.g., ASASSN-14li and iPTF15af. In the far UV, we have compared the throughput possible with both COS (G140L grating) and STIS (G140L grating) and find that they are roughly comparable. To enable isolation of the transient emission from the underlying host (via the greater spatial resolution), we have chosen STIS with the FUV-MAMA detector. Using similar overheads as above, with a two orbits we find a SNR > 5 over the range from 1150 to 1700A for the continuum. To summarize, in total we request a single epoch of STIS spectroscopy of ZTF18abxftqm, for a total of 3 orbits (2 FUV + 1 NUV). ------------------------------------------------------------------------------------ 15679( 9) - 25-Jan-2019 22:18:17 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ZTF18ABXFTQM GALAXY, ACCRETION DISK, RA=01H07M33.6100S, J2000 Z = V = 17.9 +/- 0.2 UVW2 = 18.3 NUCLEUS, WIND DEC=+23D28'34.39" 0.088 Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 15679( 9) - 25-Jan-2019 22:18:17 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.4165 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 5 Point_Spacing 0.4165 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 15679( 9) - 25-Jan-2019 22:18:17 - [ 5] Visit: 01 Visit Requirements: BEFORE 21-FEB-2019:00:00:00 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 45 S (45 GS ACQ SCENARIO SINGLE M S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ZTF18ABXFTQ STIS/FUV ACCUM 52X0.2 G140L 1425 1 850 S PATTERN 2 2 M -MAMA see below (4650 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 none none 1 1050.0 S -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none none none 1 1050.0 S -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 3 none none none 2 N/A -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 4 none none none 2 N/A -MAMA ZTF18ABXFTQM 2 STIS/FUV ACCUM 52X0.2 G140L 1425 5 none none none 2 N/A -MAMA 15679( 9) - 25-Jan-2019 22:18:17 - [ 6] Visit: 02 Visit Requirements: BEFORE 21-FEB-2019:00:00:00 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 45 S (45 GS ACQ SCENARIO SINGLE M S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ZTF18ABXFTQ STIS/NUV ACCUM 52X0.2 G230L 2376 1 680 S PATTERN 1 2 M -MAMA (2040 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/NUV ACCUM 52X0.2 G230L 2376 1 none none none 1 N/A -MAMA ZTF18ABXFTQM 2 STIS/NUV ACCUM 52X0.2 G230L 2376 2 none none none 1 N/A -MAMA ZTF18ABXFTQM 2 STIS/NUV ACCUM 52X0.2 G230L 2376 3 none none none 1 N/A -MAMA 15679( 9) - 25-Jan-2019 22:18:17 - [ 7] Summary Form for Proposal 15679 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 21-FEB-2019:00:00:00, GS ACQ SCENARIO SINGLE, PATTERN 1 2, PATTERN 2 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names ZTF18ABXFTQM ------------------------------------------------------------------------------------------------------------------------------------