The optical-UV spectral energy distribution of the unabsorbed AGN population in the XMM-Newton Bright Serendipitous Survey^⋆,⋆⋆

¹ INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
e-mail: elena.marchese@brera.inaf.it; roberto.dellaceca@brera.inaf.it
² Università Milano Bicocca, Dipartimento di Fisica G. Occhialini, Piazza della Scienza 3, 20126 Milano, Italy

Received: 24 June 2011
Accepted: 17 November 2011

Abstract

Context. Active galactic nuclei (AGN) emit radiation over a wide range of wavelengths, with a peak of emission in the far-UV region of the electromagnetic spectrum, a spectral region that is historically difficult to observe.

Aims. Using optical, GALEX UV, and XMM-Newton data we derive the spectral energy distribution (SED) from the optical/UV to X-ray regime of a sizeable sample of AGN. The principal motivation is to investigate the relationship between the optical/UV emission and the X-ray emission and provide bolometric corrections to the hard X-ray (2–10 keV) energy range, k_bol, the latter being a fundamental parameter in current physical cosmology.

Methods. We construct and study the X-ray to optical SED of a sample of 195 X-ray selected Type 1 AGN belonging to the XMM-Newton bright serendipitous survey (XBS). The optical-UV luminosity was computed using data from the Sloan Digital Sky Survey (SDSS), from our own dedicated optical spectroscopy and the satellite GALaxy evolution EXplorer (GALEX), while the X-ray luminosity was computed using XMM-Newton data. Because it covers a wide range of redshift (0.03 < z ≲ 2.2), X-ray luminosities (41.8 < log L_{[2−10] keV} < 45.5 erg/s) and because it is composed of “bright objects”, this sample is ideal for this kind of investigation.

Results. We confirm a highly significant correlation between the accretion disc luminosity L_disc and the hard X-ray luminosity L_{[2−10] keV}, in the form L_disc ∝ L^β_{[2-10] keV}, where β = 1.18 ± 0.05. We find a very shallow dependence of k_bol on the X-ray luminosity with respect to the broad distribution of values of k_bol. We find a correlation between k_bol and the hard X-ray photon index Γ_2−10 keV and a tight correlation between the optical-to-X-ray spectral index α_ox and k_bol, so we conclude that both Γ_2−10 keV and α_ox can be used as a proxy for k_bol.