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In this paper we analyse five observations of the BL Lac object AO 0235+16performed with the Chandra and XMM-Newton satellites during the years2000-2005. In the February 2002 observation the source is found in a brightstate and presents a steep X-ray spectrum, while in all the other epochs it isfaint and the spectrum is hard. The soft X-ray spectrum appears to be stronglyabsorbed, likely by the intervening system at z=0.524, which also absorbs theoptical-UV radiation. We find that models that consider spectral curvature aresuperior to single power law ones in fitting the X-ray spectrum. In particular,we favour a double power law model, which agrees with the assumption of asuperposition of two different components in the X-ray domain. Both in theChandra and in one of the XMM-Newton observations, a tentative detection of theredshifted Fe Kalpha emission line may suggest its origin from the inner partof an accretion disc. Thermal emission from this accretion disc might explainthe UV-soft-X-ray bump that appears in the spectral energy distributions, whenthe X-ray spectra are complemented with the optical-UV data from the OpticalMonitor onboard XMM-Newton. More likely, the bump can be interpreted in termsof an additional synchrotron component emitted from an inner region of the jetwith respect to that where the lower-energy emission comes from. An inspectionof the X-ray light curves reveals that intraday variability occurs only whenthe source is in a bright state.Comment: In press for A&

X-ray emission from the blazar AO 0235+16: the XMM-Newton and Chandra point of view