Evidence of a Cyclotron Feature in the Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849-400910

We report the results of a long observation of the anomalous X-ray pulsar 1RXS J170849-400910 obtained with the BeppoSAX satellite in 2001 August. The best-fit phase-averaged spectrum was an absorbed power-law plus blackbody model, with a photon index of Γ~2.4 and a blackbody temperature of kTbb~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 σ), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9×1011 and 1.6×1015 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.