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We present the first X-ray census of fast radio burst (FRB) host galaxies toconduct the deepest search for AGN and X-ray counterparts to date. Our sampleincludes seven well-localized FRBs with unambiguous host associations andexisting deep Chandra observations, including two events for which we presentnew observations. We find evidence for AGN in two FRB host galaxies based onthe presence of X-ray emission coincident with their centers, including thedetection of a luminous ($L_X\approx\,5\times\,10^{42}\,\rm\,erg\,s^{-1}$)X-ray source at the nucleus of FRB20190608B's host, for which we infer an SMBHmass of $\rm{M_{BH}\sim\,10^{8}\,M_{\odot}}$ and an Eddington ratio$\rm{L_{bol}/L_{Edd}\approx\,0.02}$, characteristic of geometrically thin disksin Seyfert galaxies. We also report nebular emission line fluxes for 24 highlysecure FRB hosts (including 10 hosts for the first time), and assess theirplacement on a BPT diagram, finding that FRB hosts trace the underlying galaxypopulation. We further find that the hosts of repeating FRBs are not confinedto the star-forming locus, contrary to previous findings. Finally, we placeconstraints on associated X-ray counterparts to FRBs in the context ofultraluminous X-ray sources (ULXs), and find that existing X-ray limits forFRBs rule out ULXs brighter than $L_X\gtrsim\,10^{40}\,\rm\,erg\,s^{-1}$.Leveraging the CHIME/FRB catalog and existing ULX catalogs, we search forspatially coincident ULX-FRB pairs. We identify a total of 28 ULXs spatiallycoincident with the localization regions for 17 FRBs, but find that theDM-inferred redshifts for the FRBs are inconsistent with the ULX redshifts,disfavoring an association between these specific ULX-FRB pairs.

An X-ray Census of Fast Radio Burst Host Galaxies: Constraints on AGN and X-ray Counterparts