Exploring X-ray and radio emission of type 1 AGN up to z ~ 2.3

Context. X-ray emission from active galactic nuclei (AGN) is dominated by the accretion disk around a supermassive black hole. The radio luminosity, however, has not such a clear origin except in the most powerful sources where jets are evident. The origin (and even the very existence) of the local bi-modal distribution in radio-loudness is also a debated issue. Aims. By analysing X-ray, optical and radio properties of a large sample of type 1 AGN and quasars (QSOs) up to z > 2, where the bulk of this population resides, we aim to explore the interplay between radio and X-ray emission in AGN, in order to further our knowledge on the origin of radio emission, and its relation to accretion. Methods. We analyse a large (~800 sources) sample of type 1 AGN and QSOs selected from the 2XMMi XMM-Newton X-ray source catalogue, cross-correlated with the SDSS DR7 spectroscopic catalogue, covering a redshift range from z ~ 0.3 to z ~ 2.3. Supermassive black hole masses are estimated from the Mg II emission line, bolometric luminosities from the X-ray data, and radio emission or upper limits from the FIRST catalogue. Results. Most of the sources accrete close to the Eddington limit and the distribution in radio-loudness does not appear to have a bi-modal behaviour. We confirm that radio-loud AGN are also X-ray loud, with an X-ray-to-optical ratio up to twice that of radio-quiet objects, even excluding the most extreme strongly jetted sources. By analysing complementary radio-selected control samples, we find evidence that these conclusions are not an effect of the X-ray selection, but are likely a property of the dominant QSO population. Conclusions. Our findings are best interpreted in a context where radio emission in AGN, with the exception of a minority of beamed sources, arises from very close to the accretion disk and is therefore heavily linked to X-ray emission. We also speculate that the radio-loud/radio-quiet dichotomy might either be an evolutionary effect that developed well after the QSO peak epoch, or an effect of incompleteness in small samples.Based on observations obtained with XMM-Newton (an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA, NASA). Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. This research has made use of NASA’s Astrophysics Data System. The research uses the interactive graphical viewer and editor for tabular data TOPCAT (http://www.starlink.ac.uk/topcat/) and its command-line counterpart STILTS (http://www.starlink.ac.uk/stilts/). We warmly thank the referee for her/his suggestions that significantly improved the paper. L.B. acknowledges support from the Spanish Ministry of Science and Innovation through a “Juan de la Cierva” fellowship. Financial support for this work was provided by the Spanish Ministry of Economy and Competitiveness through research grant AYA2010-21490-C02-01 FJHH acknowledges support from CSIC through the undergraduate research programme “JAE-Introducción a la investigación”