2011MNRAS.411.2137G

The aim of this work is a broad-band study, using the INTEGRAL, Swift and XMM-Newton satellites, of a sample of nine blazars (seven flat-spectrum radio quasars and two BL Lacertae objects) with redshift (z) up to about 4. The spectral analysis has shown clear evidence of a flattening of the continuum towards low energies (E < 3 keV observer frame). This behaviour is well reproduced, with both an absorbed power-law model (N_H∼ 10²⁰-10²³/cm2 in the rest frame of the sources) and a broken power-law continuum model (with an energy break below 3 keV in the observer frame). No Compton reflection features, Fe Kα line or hump at high energies have been detected, with the exception of the source IGR J22517+2218, which shows the presence of a weak iron line. In this paper, we also investigate a possible correlation between the absorption column density N_H and the redshift. We confirm the existence of an N_H-z trend, with the higher absorption at z > 2 for a larger sample compared to previous results. A distribution of N_H and the photon index Γ is also presented. The hard X-ray data allow us to detect highly absorbed sources (with N_H≥ 10²³/cm2 in the rest frame of the source) characterized by a photon-index distribution peaking at harder values (Γ∼ 1.4) with respect to those obtained with XMM data only (Γ∼ 2).