Volume 539, March 2012
|Number of page(s)||41|
|Published online||23 February 2012|
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: firstname.lastname@example.org; email@example.com
2 Università Milano Bicocca, Dipartimento di Fisica G. Occhialini, Piazza della Scienza 3, 20126 Milano, Italy
Received: 24 June 2011
Accepted: 17 November 2011
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, kbol, 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 Ldisc and the hard X-ray luminosity L[2−10] keV, in the form Ldisc ∝ Lβ[2-10] keV, where β = 1.18 ± 0.05. We find a very shallow dependence of kbol on the X-ray luminosity with respect to the broad distribution of values of kbol. We find a correlation between kbol and the hard X-ray photon index Γ2−10 keV and a tight correlation between the optical-to-X-ray spectral index αox and kbol, so we conclude that both Γ2−10 keV and αox can be used as a proxy for kbol.
Key words: galaxies: active / galaxies: nuclei / accretion, accretion disks / cosmology: observations / methods: statistical
© ESO, 2012
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