The X-ray sky is not as well known as is sometimes thought .
We report on our search of minority populations ( Kim & Elvis 1999 ) .
One of the most intriguing is that of ‘ blank field sources ’ , i.e .
bright ROSAT sources ( F _ { X } > 10 ^ { -13 } erg cm ^ { -2 } s ^ { -1 } ) with no optical counterpart on the Palomar Sky Survey ( to O=21.5 ) within their 39 ^ { \prime \prime } ( 99 % ) radius error circle . The nature of Blank Field sources is unknown : no known extragalactic population has such extreme X-ray to optical ratios ( f _ { X } / f _ { V } > 60 ) .
Moreover blank field source tend to have much flatter PSPC spectra compared to radio-quiet AGN .
Both properties suggest obscuration . ‘ Blank field sources ’ could be : Quasar-2s , low-mass AGNs , isolated neutron stars , high redshift clusters of galaxies , failed clusters , AGNs with no big blue bump .
Identification with any of these populations would be an interesting discovery .