2003-2005 INTEGRAL and XMM-Newton observations of 3C 273

Aims:The aim of this paper is to study the evolution of the broadband spectrum of one of the brightest and nearest quasars 3C 273.
Methods: We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM-Newton monitoring of the blazar 3C 273 in 2003-2005 in the UV, X-ray and soft γ-ray bands and study the results in the context of the long-term evolution of the source.
Results: The 0.2-100 keV spectrum of the source is well fitted by a combination of a soft cut-off power law and a hard power law. No improvement of the fit is achieved if one replaces the soft cut-off power law by either a blackbody, or a disk reflection model. During the observation period the source has reached the historically softest state in the hard X-ray domain with a photon index Γ = 1.82 ± 0.01. Comparing our data with available archived X-ray data from previous years, we find a secular evolution of the source toward softer X-ray emission (the photon index has increased by ΔΓ≃ 0.3-0.4 over the last thirty years). We argue that existing theoretical models have to be significantly modified to account for the observed spectral evolution of the source.