18238( 4) - 29-Apr-2026 01:23:56 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18238 Version: 4 Check-in Time: 29-Apr-2026 01:23:56 STScI Edit Number: 0 Title AT2018cow: A Unique Laboratory for Super-Eddington Accretion and Black-hole Growth ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Anna Yen Qin Ho Cornell University Y CoI: Prof. Andrew James Levan Radboud University Nijmegen N CoI: Dr. Daniel Perley Liverpool John Moores University N CoI: Prof. Eliot Quataert Princeton University N CoI: Michael Camilo Cornell University N CoI: Dr. Rahul Jayaraman Cornell University N CoI: Dr. Anne Inkenhaag University of Bath N CoI: Ms. Mila Winter-Granic Princeton University N CoI: Dr. Stephen Bradley Cenko NASA Goddard Space Flight Center N ------------------------------------------------------------------------------------ Abstract Black-hole accretion via a disk plays a fundamental role in astronomy. In the past decade, a number of accretion-powered systems have been discovered that defy standard models, motivating new frameworks for disk evolution that accurately treat the poorly understood regime of super-Eddington accretion. An ideal test case for new disk-evolution models is AT2018cow, an accretion-powered transient in a galaxy only 60 Mpc away. We propose to observe AT2018cow's accretion disk using JWST and HST. We will model the newly observed UVOIR SED, together with existing X-ray observations, using a new framework for the long-term evolution of accretion disks that takes super-Eddington accretion into account. The observations and modeling will enable us to both refine the modeling framework and infer the black-hole mass (a key degeneracy between competing progenitor models). JWST/NIRCam is essential because its angular resolution will enable AT2018cow to be separated from the background much better than the existing HST measurements. NIRCam's NIR sensitivity also has the potential to detect an underlying compact cluster, which would further help distinguish between progenitor scenarios and enable the cluster contribution to be jointly modeled with the accretion disk emission. A newly formed accretion disk in a very nearby system that underwent super-Eddington accretion is a rare opportunity: AT2018cow is a unique probe of a poorly understood regime of accretion physics that is likely critical for understanding massive BH growth at high redshift, as well as for determining the progenitors of new transient classes. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request 2613s of exposures in F225W and 1527s in F336W spread across two orbits. To do this, we observe with F225W for all of one orbit and a short portion of the second, and the rest of the second orbit is used for F336W. We use the UVIS2-C1K1C-SUB as we only need to image the compact source, not the extended emission from the galaxy. This also increases possible exposure time by making the readout faster. 18238( 4) - 29-Apr-2026 01:23:56 - [ 2] We use a FLASH level of 21 electrons to maintain charge transfer efficiency, and this threshold would set the lamp at a higher setting and reduce overheads. ------------------------------------------------------------------------------------ 18238( 4) - 29-Apr-2026 01:23:56 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SN-2018COW UNIDENTIFIED, ACCRETION RA=16H16M0.2200S, J2000 V = 25 DISK, OPTICAL EMITTER, DEC=+22D16'4.91" ULTRAVIOLET EMITTER Reference Frame: ICRS Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18238( 4) - 29-Apr-2026 01:23:56 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 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 ------------------------------------------------------------------------------------------------------------------------------------ 18238( 4) - 29-Apr-2026 01:23:56 - [ 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 SN-2018COW WFC3/UVI ACCUM UVIS2-C1K F225W FLASH=21 1 581 S PATTERN 1 1 S 1C-SUB (2324 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2018COW WFC3/UVI ACCUM UVIS2-C1K F225W FLASH=21 1 501 S (501 S 1C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SN-2018COW WFC3/UVI ACCUM UVIS2-C1K F336W FLASH=21 1 422 S PATTERN 1 3 S 1C-SUB (1688 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN-2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F225W 1 none none none 1 N/A S K1C-SUB SN-2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F225W 2 none none none 1 N/A S K1C-SUB SN-2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F225W 3 none none none 1 N/A S K1C-SUB SN-2018COW 1 WFC3/UVI ACCUM UVIS2-C1 F225W 4 none none none 1 N/A S K1C-SUB SN-2018COW 2 WFC3/UVI ACCUM UVIS2-C1 F225W none none none none 2 N/A S K1C-SUB SN-2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F336W 1 none none none 2 N/A S K1C-SUB SN-2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F336W 2 none none none 2 N/A S K1C-SUB SN-2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F336W 3 none none none 2 N/A S K1C-SUB SN-2018COW 3 WFC3/UVI ACCUM UVIS2-C1 F336W 4 none none none 2 N/A S K1C-SUB 18238( 4) - 29-Apr-2026 01:23:56 - [ 6] Summary Form for Proposal 18238 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=21 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F225W, F336W ------------------------------------------------------------------------------------------------------------------------------------ Target Names SN-2018COW ------------------------------------------------------------------------------------------------------------------------------------