Swift and Chandra confirm the intensity-hardness correlation of the AXP 1RXS J170849.0-400910

Convincing evidence of long-term variations in the emission properties of the anomalous X-ray pulsar 1RXS J170849.0-400910 has been gathered in the past few years. In particular, and following the pulsar glitches of 1999 and 2001, XMM-Newton witnessed a decline in 2003 of the X-ray flux accompanied by a definite spectral softening. This suggested the existence of a correlation between the luminosity and the spectral hardness in this source, similar to what was seen in the soft γ-repeater SGR 1806-20. Here we report on new Chandra and Swift observations of 1RXS J170849.0-400910 performed in 2004 and 2005, respectively. These observations confirm and strengthen the proposed correlation. The trend appears to have now reversed: the flux has increased and the spectrum is now harder. We briefly discuss the consequences of these observations for the twisted magnetosphere scenario for anomalous X-ray pulsars.