The pulse-phase-dependent spectrum of the anomalous X-ray pulsar 1RXS J170849-400910

We report on the results of a 50 ks BeppoSAX observation of 1RXS J170849-400910, one of the five (plus a candidate) known anomalous X- ray pulsars (AXPs). The BeppoSAX data are consistent with a power- law + blackbody spectral decomposition, making 1RXS J170849-400910 the fourth source of this class for which such a spectral decomposition was found. The inferred power- law slope and blackbody temperature are Gamma similar to 2.6 and kT(BB) similar to 0.46 keV, respectively. We find that the pulse profile is significantly energy- dependent, a remarkable feature for an AXP. By using the power- law + blackbody decomposition, we detect a significant variation in at least one spectral parameter, the power- law photon index, as a function of the pulse phase. This is the first significant detection of a spectral parameter variation in an AXP. The implications of these results are briefly discussed.