Optical Identification of the ASCA Medium Sensitivity Survey in the Northern Sky: Nature of Hard X-ray-selected Luminous Active Galactic Nuclei

Abstract

We present the results of optical spectroscopic identifications of a bright subsample of 2-10keV hard X-ray selected sources from the ASCA Medium Sensitivity Survey in the northern sky. The flux limit of the subsample is 3*10^-13 erg s^-1 cm^-2 in the 2-10keV band. All but one of the 87 hard X-ray selected sources are optically identified, with AGNs, 7 clusters of galaxies, and 1 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*10^23 cm^-2 from their X-ray spectra. Optical properties of X-ray absorbed AGNs with N_H > 1*10^22 cm^-2 indicate the effects of dust absorption: at redshifts, z<0.6, AGNs without broad Hβemission lines have significantly larger N_H value than AGNs with broad Hβemission lines. At z>0.6, the X-ray absorbed AGNs have a large hard X-ray to optical flux ratio (log f2-10keV/fR > +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 and less-absorbed AGNs are compared.