XMM-Newton spectroscopy of an X-ray selected sample of RL AGNs ^*,**

¹ INAF – Osservatorio Astronomico di Brera, via Brera 28 – 20121 Milano, Italy e-mail: [galbiati;caccia;tommaso;braito;rdc;paola]@brera.mi.astro.it
² Dipartimento di Fisica, Università di Milano, Via Celoria 16, 20133 Milan, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany e-mail: hbrunner@mpe.mpg.de; ile@mpe.mpg.de
⁴ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK e-mail: mjp@mssl.ucl.ac.uk

Received: 22 July 2004
Accepted: 11 October 2004

Abstract

This paper presents the X-ray spectroscopy of an X-ray selected sample of 25 radio-loud (RL) AGNs extracted from the XMM-Newton Bright Serendipitous Survey (XBSS). The main goal of the work is to assess and study the origin of the X-ray spectral differences usually observed between radio-loud and radio-quiet (RQ) AGNs. To this end, a comparison sample of 53 RQ AGNs has been also extracted from the same XBSS sample and studied together with the sample of RL AGNs. Since there are many claims in the literature that RL AGNs have, on average, a flatter spectral index when compared to the RQ AGNs, we have focused the analysis on the distribution of the X-ray spectral indices of the power-law component that models the large majority of the spectra in both samples. We find that the mean X-ray energy spectral index is very similar in the 2 samples and close to . However, the intrinsic distribution of the spectral indices is significantly broader in the sample of RL AGNs. In order to investigate the origin of this difference, we have divided the RL AGNs into blazars (i.e. BL Lac objects and FSRQs) and “non-blazars” (i.e. radiogalaxies and SSRQs), on the basis of the available optical and radio information. Although the number of sources is small, we find strong evidence that the broad distribution observed in the RL AGN sample is mainly due to the presence of the blazars. Furthermore, within the blazar class we have found a link between the X-ray spectral index and the value of the radio-to-X-ray spectral index () suggesting that the observed X-ray emission is directly connected to the emission of the relativistic jet. This trend is not observed among the “non-blazars” RL AGNs. This favours the hypothesis that, in these latter sources, the X-ray emission is not significantly influenced by the jet emission and it has probably an origin similar to the RQ AGNs. Overall, the results presented here indicate that the observed distribution of the X-ray spectral indices in a given sample of RL AGNs is strongly dependent on the amount of relativistic beaming present in the selected sources, i.e. on the relative fraction of blazars and “non-blazars”.