Nature of Hard X-ray Source from Optical Identification of the ASCA   Large Sky Survey
  We present results of optical identification of the ASCA Large Sky Survey. X-ray sources which have hard X-ray spectra were identified with type-2 AGN at redshifts smaller than 0.5. It is supported that the absorbed X-ray spectrum of type-2 AGN makes the Cosmic X-ray Background harder in the hard X-ray band than type-1 AGN, which is main contributer in the soft X-ray band. Absence of type-2 AGN at redshift larger than 1 in the identified sample, which contrasts to the existence of 6 broad-line QSOs, suggests a deficiency of so-called ``type-2 QSO'' at high redshift. 
M. Akiyama, K. Ohta, T. Yamada, Y. Ueda, T. Takahashi, M. Sakano, T.   Tsuru, I. Lehmann, G. Hasinger