observations of a sample of γ − ray loud active galactic nuclei ⋆

Aims. To understand the nature of γ−ray loud active galactic nuclei (AGN) and the mechanisms for the generation of high-energy γ−rays. Methods. We performed a homogeneous and systematic analysis of simultaneous X-ray and optical/UV properties of a group of 15 γ−ray loud AGN, using observations performed with XMM-Newton. The sample is composed of 13 blazars (6 BL Lac and 7 Flat-Spectrum Radio Quasar) and 2 radio galaxies that are associated with detections at energies > 100 MeV. The data for 7 of them are analyzed here for the first time, including the first X-ray observation of PKS 1406 − 706. The spectral characteristics of the sources in the present sample were compared with those in previous catalogs of blazars and other AGN, to search for difference or long term changes. Results. All the selected sources appear to follow the classic " blazar sequence " and the spectral energy distributions (SED) built with the present X-ray and optical/UV data and completed with historical data, confirm the findings of previous studies on this type of source. Some sources display interesting features: four of them, namely AO 0235 + 164, PKS 1127 − 145, S5 0836 + 710 and PKS 1830 − 211 show the presence of an intervening absorption system along the line of sight, but only the last is known to be gravitationally lensed. AO 0235 + 164 was detected during an outburst and its SED shows a clear shift of the synchrotron peak. 3C 273 shows a change in state with respect to the previous BeppoSAX observations that can be interpreted as an increase of the Seyfert-like component and a corresponding decline of the jet emission. This is consistent with the monitoring at radio wavelengths performed during the same period. PKS 1406 − 706 is detected with a flux higher than in the past, but with a corresponding low optical flux. Although it is classified as FSRQ, the SED can be modelled with a simple synchrotron self-Compton model.