17290( 4) - 19-Sep-2023 15:51:56 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17290 Version: 4 Check-in Time: 19-Sep-2023 15:51:56 STScI Edit Number: 0 Title Probing the nature of the UV-bright transient emission from AT2018cow more than 4 years post-explosion ------------------------------------------------------------------------------------ Type Cycle GO 30 ------------------------------------------------------------------------------------ Investigators Contact? PI: Yuyang Chen University of Toronto Y CoI: Maria Drout University of Toronto N CoI: Prof. Ryan Foley University of California - Santa Cruz N CoI: Dr. Anthony Piro Carnegie Institution of Washington N CoI: Dr. Charles Kilpatrick Northwestern University N ------------------------------------------------------------------------------------ Abstract Recent advances in high-cadence surveys have revealed a new class of luminous and rapidly-evolving transients, which are not intrinsically rare but pose significant challenges to conventional supernova models. In 2018, an extreme member of this class, AT2018cow, was identified. AT2018cow was the first rapidly-evolving transient discovered promptly at a small distance, facilitating intensive multiwavelength follow-up. However, despite being the most extensively studied transient of its class, its true nature is still debated, with possibilities ranging from tidal disruption events to the failed explosion of a massive star. Late-time observations of rapidly-evolving transients offer a means to break this degeneracy, and HST images taken during 2020 (more than 700 days post-explosion) revealed a UV-bright point source at the precise position of AT2018cow. The nature of this source was initially ambiguous, as it could be interpreted as either emission from the transient or an underlying stellar population. However, new observations obtained in June 2022 now definitively show that the source is fading in the UV---and hence that the emission is due to AT2018cow itself. Here we request a single epoch of HST/UVIS imaging in order to further trace the decline rate and change in spectral shape of the late-time emission, necessary to constrain the power source. In particular, extrapolations based on the 2020 and 2022 data indicate the peak of the SED may shift into the HST band during Mid-Cycle 30. We emphasize that with its current UV-optical brightness of 25-27 mag (AB), HST is the only current instrument capable of probing this source. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request five orbits of observations with WFC3/UVIS to constrain the evolution of a transient underlying source at the position of AT2018cow more than 4 years post-explosion. For the observations, we request the use of two UV filters (F225W and F336W), three optical filters (F438W, F555W, and F814W), and one narrow Halpha filter (F665N). Two orbits are used to observe in the F814W and F665N filters with a box dither: each orbit contains four exposures (two for each filter). Three orbits are used to observe in the F225W, F336W, F438W, and F555W filters with a 17290( 4) - 19-Sep-2023 15:51:56 - [ 2] 3-point line dither: each orbit contains four exposures (one for each filter). The UVIS2-C1K1C-SUB aperture is used for all exposures to reduce overheads. We are aware of the recommendation of 1-3 orbits per visit for Cycle 30, but for our proposal, we do request a single visit that includes all five orbits. The reason is that our objective is to obtain an accurate spectral energy distribution of an evolving/transient source at a single epoch, which can only be achieved if all filters are observed at approximately the same time (i.e., consecutively). Finally, as indicated in the phase I proposal, we request the observation to be taken during the second half of Cycle 30 (after the start of April) when the predicted spectral evolution becomes significant enough to be discernible. ------------------------------------------------------------------------------------ 17290( 4) - 19-Sep-2023 15:51:56 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AT2018COW EXT-STAR, SUPERNOVA RA=16H16M0.2290S, J2000 26.5 AB mag in F555W band DEC=+22D16'4.83" Reference Frame: Other: SIMBAD Comments: Magnitude is a rough estimate from extrapolating previous HST observations. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2015.5 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 17290( 4) - 19-Sep-2023 15:51:56 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE-3PT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.135 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 3 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.145 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 17290( 4) - 19-Sep-2023 15:51:56 - [ 5] Visit: 01 Visit Requirements: AFTER 01-APR-2023:00:00:00 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 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F814W FLASH=3 1 609 S PATTERN 2 1-2 S 1C-SUB (2436 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F665N FLASH=16 1 597 S S 1C-SUB (2388 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F225W FLASH=17 1 673 S PATTERN 1 3-6 S 1C-SUB (2019 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F336W FLASH=17 1 648 S S 1C-SUB (1944 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F438W FLASH=14 1 602 S S 1C-SUB (1806 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F555W FLASH=4 1 479 S S 1C-SUB (1437 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F814W 1 none none none 1 N/A S K1C-SUB AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F814W 2 none none none 1 N/A S K1C-SUB AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F814W 3 none none none 2 N/A S K1C-SUB AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F814W 4 none none none 2 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F665N 1 none none none 1 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F665N 2 none none none 1 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F665N 3 none none none 2 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F665N 4 none none none 2 N/A S K1C-SUB AT2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F225W 1 none none none 3 N/A S K1C-SUB 17290( 4) - 19-Sep-2023 15:51:56 - [ 6] 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F225W 2 none none none 4 N/A S K1C-SUB AT2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F225W 3 none none none 5 N/A S K1C-SUB AT2018COW 4 WFC3/UVI ACCUM UVIS2-C1 F336W 1 none none none 3 N/A S K1C-SUB AT2018COW 4 WFC3/UVI ACCUM UVIS2-C1 F336W 2 none none none 4 N/A S K1C-SUB AT2018COW 4 WFC3/UVI ACCUM UVIS2-C1 F336W 3 none none none 5 N/A S K1C-SUB AT2018COW 5 WFC3/UVI ACCUM UVIS2-C1 F438W 1 none none none 3 N/A S K1C-SUB AT2018COW 5 WFC3/UVI ACCUM UVIS2-C1 F438W 2 none none none 4 N/A S K1C-SUB AT2018COW 5 WFC3/UVI ACCUM UVIS2-C1 F438W 3 none none none 5 N/A S K1C-SUB AT2018COW 6 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none none none 3 N/A S K1C-SUB AT2018COW 6 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none none none 4 N/A S K1C-SUB AT2018COW 6 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none none none 5 N/A S K1C-SUB 17290( 4) - 19-Sep-2023 15:51:56 - [ 7] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: Repeat of failed second orbit of visit 01. 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 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F814W FLASH=3.0 1 607 S PATTERN 3 1-2 S 1C-SUB (1214 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F665N FLASH=16.0 1 595 S S 1C-SUB (1190 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F814W 1 none none none 1 N/A S K1C-SUB AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F814W 2 none none none 1 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F665N 1 none none none 1 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F665N 2 none none none 1 N/A S K1C-SUB 17290( 4) - 19-Sep-2023 15:51:56 - [ 8] Summary Form for Proposal 17290 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=14, FLASH=16, FLASH=16.0, FLASH=17, FLASH=3, FLASH=3.0, FLASH=4 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01-APR-2023:00:00:00, PATTERN 1 3-6, PATTERN 2 1-2, PATTERN 3 1-2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F225W, F336W, F438W, F555W, F665N, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2018COW ------------------------------------------------------------------------------------------------------------------------------------