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

E. Lusso, A. Comastri, C. Vignali, G. Zamorani, M. Brusa, R. Gilli, K. Iwasawa, M. Salvato, F. Civano, M. Elvis, A. Merloni, A. Bongiorno, J. R. Trump, A. M. Koekemoer, E. Schinnerer, E. Le Floc'H, N. Cappelluti, K. Jahnke, M. Sargent, J. SilvermanV. Mainieri, F. Fiore, M. Bolzonella, O. Le Fèvre, B. Garilli, A. Iovino, J. P. Kneib, F. Lamareille, S. Lilly, M. Mignoli, M. Scodeggio, D. Vergani

Research output: Contribution to journalArticlepeer-review

225 Scopus citations


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 L [2-10] keV = 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 αox 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 αox and L2500 Å is found, in agreement with previous investigations of optically selected samples. We calculate bolometric corrections, kbol, for the whole sample using hard X-ray luminosities (L[2-10] keV), 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 αox and κbol, which can be used to estimate accurate bolometric corrections using only optical and X-ray data. We find a significant correlation between aox and Eddington ratio, in which the ratio between X-ray and optical flux decreases with increasing Eddington ratio.

Original languageEnglish (US)
Article numberA34
JournalAstronomy and Astrophysics
Issue number5
StatePublished - 2010
Externally publishedYes


  • Galaxies: active
  • Galaxy: evolution
  • Methods: statistical
  • Quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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