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I. Cagnoni (SISSA, Italy), M. Elvis, D.W. Kim, F. Nicastro (CfA, USA), A. Celotti (SISSA, Italy)
The ROSAT survey contains a surprising number of ``blank field" sources i.e. bright X-ray sources (fX>10-13) with no optical counterpart on the Palomar Sky survey (O>21.5) within their 99% radius error circle. For comparison a typical type 1 AGN at these X-ray fluxes would have (O ~18).
The faintness of any optical counterpart means that ROSAT blank field sources have extreme X-ray to optical ratios (fX/fV >60), incompatible with any known population. They also have unusual X-ray colors (flatter ROSAT spectra compared to radio-quiet AGNs). Without further constraints these sources might be any of a number of possible objects, from isolated old neutron stars, to reddened AGN, to `failed clusters'.
We discuss the possibilities on the basis of both the ROSAT PSPC data, and newly aquired Chandra and infrared IRTF data.