16925( 2) - 22-Mar-2022 12:39:18 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16925 Version: 2 Check-in Time: 22-Mar-2022 12:39:18 STScI Edit Number: 0 Title Probing the nature of a UV-bright point source at the precise location of AT 2018cow ------------------------------------------------------------------------------------ Type Cycle GO 29 ------------------------------------------------------------------------------------ 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) reveal a UV-bright point source at the precise position of AT 2018cow. However, the nature of this underlying source is ambiguous as it can be interpreted as either the emission from the transient or an underlying stellar population--both of which would place significant constraints on the origin of AT2018cow. Recently, a shallow HST SNAP observation in 2021 showed tentative (2-sigma) evidence for fading in the optical (where the source is faint). Here we request a single epoch of HST/UVIS imaging to confirm the transient nature of the source and measure its rate-of-decline, necessary to constrain the power source. Extrapolations based on physically and observationally motivated models show that the source may be detected during this Mid-Cycle at 25.5-26.0 mag (AB). We emphasize that HST is the only current instrument with the resolution and depth capable of probing this source. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request three orbits of observations with WFC3/UVIS to measure the properties of the source identified at the position of AT2018cow and determine whether it is constant or evolving with time. For the observations, we request the use of two UV filters (F225W and F336W) and two optical filters (F555W and F814W). Each filter will have three exposures with a 3-point line dither. Each orbit will contain four exposures - one exposure for each filter - and the three orbits will complete exactly the desired twelve exposures for all filters 16925( 2) - 22-Mar-2022 12:39:18 - [ 2] (three exposures x four filters). The UVIS2-C1K1C-SUB aperture is used for all exposures to reduce overheads. ------------------------------------------------------------------------------------ 16925( 2) - 22-Mar-2022 12:39:18 - [ 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 26.0 ABmag in F555W DEC=+22D16'4.83" Reference Frame: ICRS Comments: Magnitude is a rough estimate from extrapolating the brightness assuming the same spectral shape. ------------------------------------------------------------------------------------------------------------------------------------ 16925( 2) - 22-Mar-2022 12:39:18 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 16925( 2) - 22-Mar-2022 12:39:18 - [ 5] Visit: 01 Visit Requirements: 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 F225W FLASH=18 1 615 S PATTERN 1 1-4 S 1C-SUB (1845 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F336W FLASH=18 1 651 S S 1C-SUB (1953 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F555W FLASH=10 1 383 S S 1C-SUB (1149 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 AT2018COW WFC3/UVI ACCUM UVIS2-C1K F814W 1 757 S S 1C-SUB (2271 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 F225W 1 none none none 1 N/A S K1C-SUB AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F225W 2 none none none 2 N/A S K1C-SUB AT2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F225W 3 none none none 3 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F336W 1 none none none 1 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F336W 2 none none none 2 N/A S K1C-SUB AT2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F336W 3 none none none 3 N/A S K1C-SUB AT2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none none none 1 N/A S K1C-SUB AT2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none none none 2 N/A S K1C-SUB AT2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none none none 3 N/A S K1C-SUB AT2018COW 4 WFC3/UVI ACCUM UVIS2-C1 F814W 1 none none none 1 N/A S K1C-SUB AT2018COW 4 WFC3/UVI ACCUM UVIS2-C1 F814W 2 none none none 2 N/A S K1C-SUB AT2018COW 4 WFC3/UVI ACCUM UVIS2-C1 F814W 3 none none none 3 N/A S K1C-SUB 16925( 2) - 22-Mar-2022 12:39:18 - [ 6] Summary Form for Proposal 16925 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10, FLASH=18 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1-4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F225W, F336W, F555W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2018COW ------------------------------------------------------------------------------------------------------------------------------------