The Lockman Hole
The deepest and longest observation with the ROSAT PSPC was pointed in the center of the Lockman Hole in the constellation of the Ursa Major (the Big Dipper). This is the direction of the minimum column density of Galactic hydrogen (roughly between the pointer stars of the Big Dipper), which allows the most sensitive search for extragalactic objects. More than 60% of the extragalactic background above 1 keV can be resolved into over 100 discrete X-ray sources.
HEASARC Guest Observer Facility
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