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 \alpha _ { X } \sim 1 .
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 ( \alpha _ { RX } ) 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 ” .