Is the dependence of spectral index on luminosity real in optically selected AGN samples?

Abstract

We critically examine the dependence of spectral index on luminosity in optically selected AGN samples. An analysis of optically selected high-redshift quasars showed an anti-correlation of ${\alpha }_{OX}$, the spectral index between the rest-frame 2500 A and 2 keV, with optical luminosity (Miyaji et al. 2006). We examine this relationship by means of Monte Carlo simulations and conclude that a constant spectral index independent of optical luminosity is still consistent with this high-z sample. We further find that that contributions of large dispersions and narrow range of optical luminosity are most important for the apparent, yet artificial, ${\alpha }_{OX}-l_o$ correlation reported. We also examine another, but more complete low-z optical selected AGN sub-sample from Steffen et al. (2006), and our analysis shows that a constant spectral index independent of optical luminosity is also consistent with the data. By comparing X-ray and optical luminosity functions, we find that a luminosity independent spectral index is in fact more preferred than the luminosity dependent spectral index model. We also discuss the selection effects caused by flux limits, which might systematically bias the $l_x-l_o$ relation and cause discrepancy in optically selected and X-ray selected AGN samples. To correctly establish a dependence of spectral index of AGNs on their luminosity, a larger and more complete sample is needed and consequences of luminosity dispersions and selection effects in flux limited samples must be taken into account properly.

arXiv comment10 pages, 11 figures, accepted for publication in MNRAS

References in corpus (16)

• Cosmological Evolution of the Hard X-ray AGN Luminosity Function and the Origin of the Hard X-ray Background
• The SDSS Quasar Survey: Quasar Luminosity Function from Data Release Three
• Luminosity-dependent evolution of soft X-ray selected AGN: New Chandra and XMM-Newton surveys
• The X-ray-to-Optical Properties of Optically-Selected Active Galaxies Over Wide Luminosity and Redshift Ranges
• The Cosmic Evolution of Hard X-ray Selected Active Galactic Nuclei
• X-ray emission from radio-quiet quasars in the SDSS Early Data Release. The alpha_ox dependence upon UV luminosity
• Soft X-ray and Ultraviolet Emission Relations in Optically Selected AGN Samples
• The 2dF-SDSS LRG and QSO Survey: The z<2.1 Quasar Luminosity Function from 5645 Quasars to g=21.85
• The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infra-Red to X-Ray Spectral Energy Distribution
• X-ray Lighthouses of the High-Redshift Universe. II. Further Snapshot Observations of the Most Luminous z>4 Quasars with Chandra
• Chandra Survey of Radio-quiet, High-redshift Quasars
• The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei
• Chandra and XMM-Newton observations of the first quasars: X-rays from the age of cosmic enlightenment
• Differences in the AGN Populations of Groups and Clusters: Clues to AGN Evolution
• The soft X-ray properties of quasars in the Sloan Digital Sky Survey
• Chandra Observations of Six QSOs at z $\approx$ 3

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