SIMBAD references

In the ROSAT All-Sky Survey, an 80.7 deg² region near the north ecliptic pole (NEP) constitutes the deepest contiguous observation of the X-ray sky. Four hundred and forty-five unique sources have been detected above a flux of ∼2x10^–14 ergs.s^–1.cm^–2 (0.5-2.0 keV). The X-ray data for each source provide a likelihood of existence, position and position error, count rate and error, background count rate at the source position, exposure time, two hardness ratios and their errors, and a measure of extent and likelihood of nonzero extent. We describe the range and statistics of X-ray properties exhibited by the NEP sources and the accuracy with which they have been determined, concentrating on the flux measurement of extended galaxy cluster X-ray sources. We show two examples of sources whose X-ray morphology is useful in their optical identification.