Optical identification of the ASCA large sky survey

Masayuki Akiyama, Kouji Ohta, Toru Yamada, Nobunari Kashikawa, Masafumi Yagi, Wataru Kawasaki, Masaaki Sakano, Takeshi Tsuru, Yoshihiro Ueda, Tadayuki Takahashi, Ingo Lehmann, Günther Hasinger, Wolfgang Voges

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116 Citations (Scopus)


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 ∼1 × 10-13 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 log NH(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 log NH(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 log N-log S relations of each population, contributions to the CXB in the 2-10 keV band are estimated to be 9% for less-absorbed AGNs (log NH(cm-2) < 22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 < log NH(cm-2) < 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the flux range from 3 × 10-11 ergs cm-2 s-1 to 2 × 10-13 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 log NH(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.

Original languageEnglish
Pages (from-to)700-727
Number of pages28
JournalAstrophysical Journal
Issue number2 PART 1
Publication statusPublished - 2000 Apr 1


  • Diffuse radiation
  • Galaxies: active
  • Quasars: general
  • Surveys
  • X-rays: galaxies


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