0 Assigned ID: 3245 ----- Attempting Submission 1 (Fri Mar 25 14:34:16 GMT 2022) ----- The nature of extragalactic ultraluminous X-ray sources - exceeding the Eddington limit for a 10 solar mass compact object - remains enigmatic. Historically, the debate around what powers ULXs has revolved around genuinely massive black holes or super-Eddington accretion onto stellar-sized black hole. However, the recent discovery that some of these systems host pulsating neutron stars has shuttered long-held views. Following the release of a comprehensive ULX catalog, we propose a systematic search for optical counterparts to over 300 ULXs in 120 host galaxies within 20 Mpc; these galaxies have sufficiently deep, multi-band ACS/WFC3 imaging data to pinpoint the stellar counterparts. This is a key first step in the attempt to determine ULX binary parameters, and identify the first dynamically confirmed black hole ULX. PI Gallo is an expert in black hole accretion and X-ray binaries. She will advise co-I Hunt in the data analysis, interpretation and paper writing. co-I Hunt has worked extensively on optical counterparts to X-ray sources using Hubble data; she will lead the analysis, interpretation and paper writing under Gallo's supervision. co-I Chandar has vast experiencd with HST data; her research focuses on stellar populations. She will co-advise Hunt and aid in the interpretation and paper writing. co-I Walton is the author of the ULX catalog at the basis of this AR request, and an expert in X-ray binaries. He will aid in the interpretation and paper writing. Through Michigan, the team has access to the Magellan telescope in Chile, for follow-up spectroscopy of the most promisng targets. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 0 Regular false false false false false false SUPPORTED