1998ApJ...508..621H

We have detected X-ray emission from the primary candidate for a ``failed cluster'' of galaxies, 0806+20, with ASCA. The hypothetical ``failed cluster'' is described by Tucker, Tananbaum, & Remillard as a large cloud of X-ray-emitting hot gas without any visible galaxies. The ASCA X-ray image of 0806+20 shows no significant extension. The time variation of X-ray flux is not significant either. Although we cannot determine whether the spectra of 0806+20 are thermal or nonthermal, we do detect an emission-line feature around 4 keV at a 95% significance level. If we identify it with an Fe K line, which should contradict the ``failed cluster'' hypothesis, the redshift of 0806+20 is around 0.6. Moreover, we made deep optical follow-up observations, but we could not find any clear counterparts of this X-ray source down to a magnitude limit of R ∼ 24 mag. We thus conclude that 0806+20 is either a very distant cluster of galaxies (so that the individual galaxies could not be detected) or an extremely optically underluminous X-ray quasar.