The 26 year-long X-ray light curve and the X-ray spectrum of the BL Lacertae object 1E 1207.9+3945 in its brightest state

Aims. We studied the temporal and spectral evolution of the synchrotron emission from the high energy peaked BL Lac object 1E 1207.9+3945. Methods. Two recent observations have been performed by the XMM-Newton and Swift satellites; we carried out X-ray spectral analysis for both of them, and photometry in optical-ultraviolet filters for the Swift one. Combining the results thus obtained with archival data we built the long-term X-ray light curve, spanning a time interval of 26 years, and the Spectral Energy Distribution (SED) of this source. Results. The light curve shows a large flux increasing, by about a factor of six, in a time interval of a few years. After reaching its maximum in coincidence with the XMM-Newton pointing in December 2000 the flux decreased in later years, as revealed by Swift. The very good statistics available in the 0.5−10 keV XMM-Newton X-ray spectrum reveals a highly significant deviation from a single power law. A log-parabolic model with a best fit curvature parameter of 0.25 and a peak energy at ∼1 keV describes well the spectral shape of the synchrotron emission. The simultaneous fit of Swift UVOT and XRT data provides a milder curvature (b ∼ 0.1) and a peak at higher energies (∼15 keV), suggesting a different state of source activity. In both cases UVOT data support the scenario of a single synchrotron emission component extending from the optical/UV to the X-ray band. Conclusions. New X-ray observations are important to monitor the temporal and spectral evolution of the source; new generation γ-ray telescopes like AGILE and GLAST may for the first time detect its inverse Compton emission.