ASCA observations of the "failed cluster" of galaxies candidate 0806+20

We have detected X-ray emission from the primary candidate for a "failed cl uster" of galaxies, 0806 + 20, with ASCA. The hypothetical "failed cluster" is described by Tucker, Tananbaum, & Remillard as a large cloud of X-ray-e mitting 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 no t significant either. Although we cannot determine whether the spectra of 0 806 + 20 are thermal or nonthermal, we do detect an emission-line feature a round 4 keV at a 95% significance level. If we identify it with an Fe K lin e, which should contradict the "failed cluster" hypothesis, the redshift of 0806 + 20 is around 0.6. Moreover, we made deep optical follow-up observat ions, but we could not find any clear counterparts of this X-ray source dow n to a magnitude limit of R similar to 24 mag. We thus conclude that 0806 20 is either a very distant cluster of galaxies (so that the individual ga laxies could not be detected) or an extremely optically underluminous X-ray quasar.