Optical identification of the ASCA medium sensitivity survey in the northern sky: Nature of hard X-ray-selected luminous active galactic nuclei

We present the results of optical spectroscopic identifications of a bright subsample of 2-10 keV hard X-ray-selected sources from the ASCA Medium Sensitivity Survey in the northern sky (AMSSn). The flux limit of the subsample is 3 × 10^-13 erg s^-1 cm^-2 in the 2-10 keV band. All but one of the 87 hard X-ray-selected sources are optically identified, with AGNs (including broad-line AGNs, narrow-line AGNs, and three BL Lac objects), seven clusters of galaxies, and one galactic star. It is the largest complete sample of hard X-ray-selected AGNs at the bright flux limit. Amounts of absorption to their nuclei are estimated to be hydrogen column densities (N_H) of up to ∼ 3 × 10²³ cm ^-2 from their X-ray spectra. Optical properties of X-ray absorbed AGNs with N_H > 1 × 10²² cm^-2 indicate the effects of dust absorption: at redshifts z < 0.6, AGNs without broad Hβ emission lines have significantly larger N_H-values than AGNs with broad Hβ emission lines. At z > 0.6, the X-ray absorbed AGNs have a large hard X-ray to optical flux ratio (logf_{2-10 keV}/f_R > +1). However, three X-ray absorbed z > 0.6 AGNs show strong broad lines. In combination With hard X-ray-selected AGN samples from the ASCA Large Sky Survey, the ASCA Deep Survey in the Lockman Hole, and Chandra Deep Field North, the luminosity distributions of absorbed (N_H > 1 × 10 ²² cm^-2) and less-absorbed (N_H < 1 × 10²² cm^-2) AGNs are compared.