SIMBAD references

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

Optical identification of the ASCA large sky survey.


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 ∼ 1x10–13–2.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 3x10–11–2.s–1 to 2x10–13–2.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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