Optical Identification of theASCALarge Sky Survey

Abstract

We present results of optical identiÐcation 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 2È7 keV band above 3.5 p. The Ñux limit corresponds to D1 ] 10~13 ergs cm~2 s~1 in the 2È10 keV band. The sources are identiÐed with 30 active galactic nuclei (AGNs), two clusters of galaxies, and one Galactic star. Only one source is still unidentiÐed.All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7È10 keV band are identiÐed 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-broadline 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 Ðtted with hydrogen column densities of at the objectÏs redshift. On log N H (cm~2) \ 22È23 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 reÑection component of a type 1 Seyfert galaxy. The hard X-ray spectra may also be explained by absorption with at the objectÏs redshift, if we assume an intrinsic photon index of 1.7. The origin log N H (cm~2) \ 22È23 of the hardness is not clear yet.Based on the log NÈlog S relations of each population, contributions to the CXB in the 2È10 keV band are estimated to be 9% for less-absorbed AGNs including the four high-(log N H (cm~2) \ 22) redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 \ log N H (cm~2) \ 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the Ñux 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 deÐciency of AGNs with column densities of in log N H (cm~2) \ 22È23 the redshift range 0.5È2, 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 deÐciency of X-ray absorbed luminous high-redshift AGNs.

TheBeppoSAXHigh‐Energy Large‐Area Survey. V. The Nature of the Hard X‐Ray Source Population and Its Evolution

“…This distribution has been assumed to be representative of the whole population of AGN 1s at any redshift and luminosity and is used to compute the fraction of AGN 1s fainter than R ¼ 21 missed from the optical spectroscopic identifications of the HELLAS sample. Grossan (1992), 12 from Boyle et al (1998), and 25 from Akiyama et al (2000). The excess of faint AGN 1s at high redshift is even more evident than in the previous figure.…”

“…We have estimated the relationship between optical and hard X-ray luminosity for AGN 1s as follows. We collected a mixed sample of both 25 optically selected and 25 hard X-ray selected AGN 1s, mostly in the redshift range 0 < z < 2, from Mineo et al (2000), George et al (2000), and Akiyama et al (2000). The 25 optically selected AGN 1s have an average logðL 5 keV =L R Þ ratio of À4.78 with a standard deviation of 0.59, while the 25 X-rayselected AGN 1s have an average logðL 5 keV =L R Þ ratio of À4.38 with a standard deviation of 0.58.…”

“…In order to increase the statistical significance of our analysis, the HELLAS data have been combined with other hard X-ray samples of AGN 1s identified by Grossan (1992), Boyle et al (1998), and Akiyama et al (2000).…”

“…The sample of Akiyama et al (2000) consists of 25 AGN 1s, five AGN 2s, two clusters, one star, and one unidentified source. The sources have been detected by ASCA.…”

Supermassive Black Holes in Galaxies

“…We supplement the CLASXS, CDF-N, and CDF-S surveys with the very high-luminosity ASCA sample of Akiyama et al (2000). Figure 2 shows the useful flux ranges of the hard X-ray samples in these fields.…”