1. Chandra Observation of the Failed Cluster Candidate 0806+20 K. Hayashida, H. Katayama (Osaka University), K. Mori (Penn State University), T.T. Takeuchi (NAOJ) Failed clusters are hypothetical objects of large gas clouds of hot gas with no visible galaxies. Tucker et al. (1995) searched for such objects using the Einstein IPC database of 1435 fields. 0806+20 is the prime candidate remained for which optical counterpart was not identified, though whose X-ray extent was marginal. We observed this source with ASCA and found its X-ray spectrum was fitted with a power law model of index 1.8 or a thermal model of kT about 5keV. In this paper, we describe the results from the Chandra X- ray observation of 0806+20 performed in February 2002. The Chandra observation reveal that 1) this source is point-like in its X-ray image, and 2)the location of the X-ray source coincides an object with some nebulosity of which optical spectrum was taken by us previously. All the X-ray properties so far is consistent with an idea that this source is an AGN. However, the fact that we could not find apparent emission lines in the optical spectrum may imply this source is one of the ultra-luminous X-ray sources recently discovered in nearby galaxies. On the other hand, this source has gone out of the place of failed cluster candidates. abstract What are Failed Clusters ? Hypothetical objects: large gas clouds of hot gas with no visible galaxies. Tucker, Tananbaum, & Remillard (1995) searched for this objects using Einstein IPC database of 1435 IPC fields. 10 candidates (extended X-ray source with no optical counterpart) were selected. Optical follow up observations left one prime candidate 0806+20, though its X-ray spatial extent had not high significance (less than 90%). ASCA X-ray Observation in 1996 Hashimotodani, Hayashida, & Takeuchi (1998) X-ray spectrum is fitted with power law or themal. Emission line feature around 4keV was detected at a 95% confidence level. X-ray surface brightness profile is consistent with point-like, though we cannot ruled out a cluster-like extended profile with core radius smaller than 0.16'(27kpc @ z=0.1 and 70kpc @ z=0.5). ASCA X-ray Observation in 1997 X-ray spectrum is fitted with a power law model. Emission line around 4keV is not visible with upper limit of equivalent width of 100eV. Consistent with constant intensity in 1ks bin light curve. Chandra Observation in 2002 5ks observation with ACIS-S performed in Feb20 2000. Point-like X-ray source was found at the position of (08h09m08.2s,+20d24m20s). The position is about 50'' apart from the original position determined with Einstein IPC. Radial profile of 0.3-8.0keV is fitted with a PSF model + background(  2/ndf = 52.8/98) The X-ray source 0806+20 is a point source Optical/ Radio Counter Part What is the Nature of 0806+20 ? References X-ray spectrum is fitted with a power law model of index ~1.7 No significant variability during the 5ks observation. (Left) Chandra X-ray contour overlayd on DSS2 image. The X-ray source coincides a source with nebulosity. (Right) NVSS contour overlayd on DSS2 image. 30'' (Left) R-band image taken with Kiso 105cm Schmidt on 1996 Dec. Triangle indicates the X-ray source position determined by Chandra. Nebulosity is visible in this image, too. R-band magnitude is 17.2mag. (Right) Optical spectrum taken with spectrograph of OAA 188cm telescope on 1997 Mar. The continuum spectrum resembles that os a G-star, and emission lines are not found. Absorption features are atomspheric ones. Long term history of X-ray flux and photon index. No apparent change is found. ASCA Chandra SIS GIS All the X-ray properties so far is consistent with an idea that this source is an AGN. However, the fact that we could not find apparent emission lines in the optical spectrum may imply this source is one of the ultra-luminous X-ray sources recently discovered in nearby galaxies. If the optical nebulosity of 10'' corresponds to a galaxy size of 10kpc, the distance of the sources is 200Mpc. In this case, the X-ray luminosity of the source is as high as 4x10 42 erg/s, a little higher than other ULXs. On the other hand, this source has gone out of the place of failed cluster candidates. Tucker, W.H., Tananbaum, H., & Remillard, R.A., 1995, ApJ, 444, 532. Hashimotodani, K., Hayashida, K. & Takeuchi, T.T., 1998, ApJ, 508, 621.