SIMBAD references

2000ApJ...532..700A - Astrophys. J., 532, 700-727 (2000/April-1)

Optical identification of the ASCA large sky survey.

AKIYAMA M., OHTA K., YAMADA T., KASHIKAWA N., YAGI M., KAWASAKI W., SAKANO M., TSURU T., UEDA Y., TAKAHASHI T., LEHMANN I., HASINGER G. and VOGES W.

Abstract (from CDS):

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 logNH(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 logNH(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 (logNH(cm–2)<22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22<logNH(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 logNH(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.


Abstract Copyright:

Journal keyword(s): Cosmology: Diffuse Radiation - Galaxies: Active - Galaxies: Quasars: General - Surveys - X-Rays: Galaxies

Nomenclature: Table 2: [AOY2000] AX JHHMMSS+DDMM A N=48. Table 6: [AOY2000] LSS1-NN (Nos 1-01 to 1-11), [AOY2000] LSS2-NN (Nos 2-01 to 2-14).

CDS comments: Several 1RXS J... do not exist in the RASS catalogue. Some are replaced by existing 1RXS, the others are renamed RX J...

Simbad objects: 76

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2021.09.26-23:18:02

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