  We report the results of a long observation of the anomalous X-ray pulsar
1RXS J170849-400910 obtained with the BeppoSAX satellite in August 2001. The
best-fit phase-averaged spectrum was an absorbed power law plus blackbody
model, with photon index Gamma~2.4 and a blackbody temperature of kT_bb~0.4
keV. We confirm the presence of significant spectral variations with the
rotational phase of the pulsar. In the spectrum corresponding to the rising
part of the pulse we found an absorption-like feature at ~8.1 keV (a
significance of 4 sigma), most likely due to cyclotron resonant scattering. The
centroid energy converts to a magnetic field of 9x10^11 G and 1.6x10^15 G in
the case of electrons and protons, respectively. If confirmed, this would be
the first detection of a cyclotron feature in the spectrum of an anomalous
X-ray pulsar.
