2000ApJ...532..700A

We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources that were detected with the SIS in the 2-7 keV band above 3.5 σ. The flux limit corresponds to ∼ 1x10^–13 ergs.cm^–2.s^–1 in the 2-10 keV band. The sources are identified with 30 active galactic nuclei (AGNs), two clusters of galaxies, and one Galactic star. Only one source is still unidentified.

All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7-10 keV band are identified with narrow-line or weak-broad-line AGNs at redshifts smaller than 0.5. This fact supports the idea that absorbed X-ray spectra of narrow-line and weak-broad-line AGNs make the cosmic X-ray background (CXB) spectrum harder in the hard X-ray band than that of a broad-line AGN, which is the main contributor in the soft X-ray band. Assuming their intrinsic spectra are same as a broad-line AGN (a power-law model with a photon index of 1.7), their X-ray spectra are fitted with hydrogen column densities of logN_H(cm^–2)=22-23 at the object's redshift. On the other hand, X-ray spectra of the other AGNs are consistent with that of a nearby type 1 Seyfert galaxy. In the sample, four high-redshift luminous broad-line AGNs show a hard X-ray spectrum with an apparent photon index of 1.3±0.3. The hardness may be explained by the reflection component of a type 1 Seyfert galaxy. The hard X-ray spectra may also be explained by absorption with logN_H(cm^–2)=22-23 at the object's redshift, if we assume an intrinsic photon index of 1.7. The origin of the hardness is not clear yet.

Based on the logN-logS relations of each population, contributions to the CXB in the 2-10 keV band are estimated to be 9% for less-absorbed AGNs (logN_H(cm^–2)<22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22<logN_H(cm^–2)<23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the flux range from 3x10^–11 ergs.cm^–2.s^–1 to 2x10^–13 ergs.cm^–2.s^–1. If the four hard broad-line AGNs are included in the absorbed AGNs, the contribution of the absorbed AGNs to the CXB is estimated to be 6%.

In optical spectra, there is no high-redshift luminous cousin of a narrow-line AGN in our sample. The redshift distribution of the absorbed AGNs is limited below z=0.5 excluding the four hard broad-line AGNs, in contrast to the existence of 15 less-absorbed AGNs above z=0.5. The redshift distribution of the absorbed AGNs suggests a deficiency of AGNs with column densities of logN_H(cm^–2)=22-23 in the redshift range 0.5-2, or in the X-ray luminosity range larger than 10⁴⁴ ergs.s^–1, or both. If the large column densities of the four hard broad-line AGNs are real, they could complement the deficiency of X-ray absorbed luminous high-redshift AGNs.