First evidence of a cyclotron feature in an anomalous X-ray pulsar

Abstract We report on the results of the longest uninterrupted 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 found to be an absorbed power law plus blackbody model, with photon index Γ∼2.4 and a black body 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 (significance level of 4ς), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9 × 1011G and 1.6 × 1015G 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.