The X-ray to optical-UV luminosity ratio of X-ray selected type 1 AGN in XMM-COSMOS^*

¹ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: elisabeta.lusso2@unibo.it
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Max Planck Institut für extraterrestische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
⁷ Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822-1839, USA
¹¹ Department of Physics, ETH Zurich, 8093 Zurich, Switzerland
¹² ESO, Karl-Schwarschild-Strasse 2, 85748 Garching, Germany
¹³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio (Rm) 00040, Italy
¹⁴ Laboratoire d'Astrophysique de Marseille, OAMP, CNRS-Université Aix-Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁵ INAF – IASF, via Bassini 15, 20133 Milano, Italy
¹⁶ INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy
¹⁷ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France

Received: 15 September 2009
Accepted: 4 January 2010

Abstract

We present a study of the X-ray to optical properties of a sample of 545 X-ray selected type 1 AGN, from the XMM-COSMOS survey, over a wide range of redshifts (0.04 < z < 4.25) and X-ray luminosities (40.6 ≤ Log ≤ 45.3). About 60% of them are spectroscopically identified type 1 AGN, while the others have a reliable photometric redshift and are classified as type 1 AGN on the basis of their multi-band Spectral Energy Distributions. We discuss the relationship between UV and X-ray luminosity, as parameterized by the  spectral slope, and its dependence on redshift and luminosity. We compare our findings with previous investigations of optically selected broad-line AGN (mostly from SDSS). A highly significant correlation between and is found, in agreement with previous investigations of optically selected samples. We calculate bolometric corrections, k_bol, for the whole sample using hard X-ray luminosities (), and the Eddington ratios for a subsample of 150 objects for which black hole mass estimates are available. We confirm the trend of increasing bolometric correction with increasing Eddington ratio as proposed in previous works. A tight correlation is found between  and k_bol, which can be used to estimate accurate bolometric corrections using only optical and X-ray data. We find a significant correlation between and Eddington ratio, in which the ratio between X-ray and optical flux decreases with increasing Eddington ratio.