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We describe the optical, near-infrared, and radio properties of a sample ofhard (2-7 keV) X-ray sources detected in a deep Chandra observation of thefield surrounding the Abell 370 cluster. We combine these data with similarobservations of the Chandra Deep Field-North and the Hawaii Survey Field SSA13to obtain a sample of 69 hard X-ray sources (45 are spectroscopicallyidentified) with extremely deep 20 cm observations. We find that about 4% ofthe >Lstar galaxy population is X-ray luminous at any time and hence that blackhole accretion has a duration of about half a Gyr. We find that about 30% ofthe summed 2-7 keV flux from our total sample is from sources at z<1. Weestimate the bolometric luminosities of accretion onto supermassive black holesand the mass inflow rates. The time history of the accretion rate density isevaluated, and its integrated value is reasonably consistent with the valueinferred from the local black hole mass to bulge mass ratio.Comment: 17 pages, submitted to The Astronomical Journa

Supermassive Black Hole Accretion History Inferred from a Large Sample of [ITAL]CHANDRA[/ITAL][ITAL]Chandra[/ITAL] Hard X-Ray Sources