Detecting Stellar Counterparts of Hyperluminous X-ray Sources with HST

We propose a project to identify stellar counterparts for 60 hyperluminous X-ray sources (HLXs) using archival HST imaging. HLXs are luminous, off-nuclear X-ray sources and represent candidate intermediate-mass black holes (IMBHs) in the nuclei of dwarf galaxies that have merged with a larger galaxy. They provide a unique avenue for studying the role of mergers for growing primordial black hole seeds and constraining seed formation mechanisms. HST is the only observational resource with the imaging resolution and sensitivity to detect the faint stellar counterparts associated with most HLXs. This project will yield the currently largest sample of HLXs with stellar counterpart detections, and we will use it to address the following primary science goals: constraining the fraction of contaminating (background or foreground) X-ray sources, measuring stellar counterpart masses and putting constraints on BH masses, and examining how mergers affect IMBH growth. The archival approach of this project will eliminate the burden on HST time while maximizing its contribution to the science of HLXs and IMBH candidates. Based on our preliminary results, the science yielded by this project will include stellar masses of 60 HLX candidates, 40 strong IMBH candidates, and unprecedented studies of merger-driven BH growth, including in mergers with large mass ratios that are difficult to otherwise identify.