A detailed discussion of this survey, together with the results from
Paper II, will be published as a later paper. Here, I will only
briefly summarize the results of the current survey. I found one white
dwarf, several late type stars, M stars with and without emission lines
and a large population of AGNs. I identified no candidate for an
accreting neutron star over the sample area of 64.5deg2.
Because of the reduced detection threshold for X-ray sources I expect
to be complete to the 99 percent level at a count rate of 0.012s-1.
This implies that at most one of the 89 sources might have not been
detected in the X-ray survey.
An upper limit of one source in the 64.5deg2 survey
area translates to 
such sources per steradian
(
).
This upper limit to the number of old accreting neutron stars,
assuming 108 neutron stars, is a factor of 1.6 lower than the lowest
value of the prediction for isotropic accretion, and more than a factor of 35
lower than we expect for polar-cap accretion (Blaes & Madau
1993).
Acknowledgements
This article is based on research conducted during my three year visit to the California Institute of Technology. I want to thank Prof. S.R. Kulkarni for his hospitality, support and countless discussions during this time. I am indebted to J. Trümper for his support of this project and many fruitful suggestions. The ROSAT project is supported by the German Bundesministerium für Bildung und Wissenschaft (BMBW/DARA) and the Max-Planck-Society. This research has made use of the NASA/IPAC extragalactic database NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration, and of the SIMBAD database, operated at CDS, Strasbourg, France. The Digitized Sky Survey (DSS) was produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities, Incorporated, under cooperative agreement with the National Science Foundation.