The optical-UV spectral energy distribution of the unabsorbed AGN population in theXMM-NewtonBright Serendipitous Survey

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, 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 .

Black-hole masses of type 1 AGN in theXMM-Newtonbright serendipitous survey

“…In this paper we want to derive the mass of the central SMBH for the type 1 AGNs. In total, the XBS contains 276 type 1 AGN but we have computed the M BH only for the sub-sample of sources that was studied by Marchese et al (2012) in order to have a reliable estimate of the bolometric luminosity. The sub-sample considered by Marchese et al contains the type 1 AGN that fall in the area of sky surveyed by GALEX (Martin et al 2005;Morrissey et al 2007), therefore it can be considered as representative of the entire XBS sample of type 1 AGN.…”

“…To account for these points, we used the result of a systematic study of the optical/UV spectral energy distribution (SED) of the type 1 AGN of the XBS survey, described in Marchese et al (2012). In this work we have collected photometric points, both in the optical (most from the SDSS) and in the UV band (from GALEX) and built the SED for each source.…”

“…Using the methods described in the previous sections, we computed the black-hole masses for all the 154 type 1 AGNs of the XBS for which we analysed the SED, as described in Marchese et al (2012) and for which we acquired an optical spectrum. For 32 objects we only covered the Hβ emission line while for 70 objects we have covered only the MgIIλ2798 Å line.…”

“…Similarly, the errors on the monochromatic luminosities are computed from the SED fitting procedure by again imposing Δχ 2 = ±1 from the best-fit value. As described in Marchese et al (2012), the SED fitting procedure takes the errors on the photometric points into account. These 1σ uncertainities include both the errors on photometry and additional sources of error due to the correction for the intrinsic extinction and the long term variability (since the used photometric data are not simultaneous).…”

“…For most of the type 1 AGN contained in this sample, the optical/UV spectral energy distribution has been studied and a reliable estimate (i.e. not based on a bolometric correction) of the bolometric luminosity has already been published (Marchese et al 2012). In this paper we present the estimate of the black-hole masses, using the single-epoch method.…”