  We studied the temporal and spectral evolution of the synchrotron emission
from the high energy peaked BL Lac object 1E 1207.9+3945. 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. The light curve shows a large flux increasing, 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 points out 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. New X-ray observations are important to monitor
the temporal and spectral evolution of the source; new generation gamma-ray
telescopes like AGILE and GLAST could for the first time detect its inverse
Compton emission.
