SIMBAD references

We describe a possible new class of X-ray sources that have robust detections in ultradeep Chandra data yet have no detections at all in our deep multiband Great Observatories Origins Deep Survey Hubble Space Telescope Advanced Camera for Surveys (ACS) images, which represent the highest quality optical imaging obtained to date on these fields. These extreme X-ray/optical ratio sources (EXOs) have values of F_X/F_optat least an order of magnitude above those generally found for other active galactic nuclei (AGNs), even those that are harbored by reddened hosts. We thus infer two possible scenarios: (1) if these sources lie at redshifts z≲6, then their hosts need to be exceedingly underluminous or more reddened, compared with other known sources, or (2) if these sources lie above z∼6-7, such that even their Lyα emission is redshifted out of the bandpass of our ACS z₈₅₀filter, then their optical and X-ray fluxes can be accounted for in terms of relatively normal ~L_*hosts and moderate-luminosity AGNs.