Optical Counterparts to Ultraluminous X-ray Sources

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.