The current Call for Proposals is for HST Cycle 34. 0 Assigned ID: 6949 ----- Attempting Submission 1 (Thu Mar 19 18:41:33 GMT 2026) ----- HST Phase I Proposal 6949 successfully submitted. Receipt: # 6949-1 This is a joint JWST-HST proposal, ID# 12098, and we were told to submit this as under Cycle 33. Fixed typos in the Proposal Description -485506565 ----- Attempting Submission 1 (Tue Apr 14 18:37:22 GMT 2026) ----- ----- Attempting Submission 2 (Thu Apr 16 17:29:51 GMT 2026) ----- HST Phase II Proposal 18238 successfully submitted. Receipt: # 18238-2 ----- Attempting Submission 3 (Mon Apr 27 22:06:00 GMT 2026) ----- 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. 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. 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. Our team includes experts in the analysis of JWST/NIRCam and HST/WFC3 data (Andrew Levan, Anne Inkenhaag). Our team also includes experts in accretion-disk modeling: co-Is Mila Winter-Granic and Eliot Quataert developed the new framework for the evolution of super-Eddington accretion disks employed here. Finally, our team includes experts on AT2018cow itself and on interpreting the multiwavelength emission from accretion-powered transients, including taking their underlying cluster properties into account (Daniel Perley, Andrew Levan, Anne Inkenhaag, Anna Ho, Brad Cenko, Michael Camilo, Rahul Jayaraman). Therefore the team is well prepared to reduce, analyze, model, and interpret the observations that we are proposing. Stellar Physics and Stellar Types Supermassive Black Holes and Active Galaxies 6 false false false false false false false true SECONDARY JWST 12098 false SUPPORTED UNIDENTIFIED ACCRETION DISK OPTICAL EMITTER ULTRAVIOLET EMITTER J2000 J2000 WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F336W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false 1776354797215 /Users/jasonsevilla/Documents/Cornell/2025-2026/JWST_12098/OP-cache/hst_phase_ii-v01.xml The scheduling data was generated by CASM 34.0. CASM was run in Phase II mode. 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