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

¹ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (Lc), Italy e-mail: campana@merate.mi.astro.it
² SRON – Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
⁴ Department of Physics, University of Padua, via Marzolo 8, 35131, Padova, Italy
⁵ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking Surrey, RH5 6NT, UK

Received: 24 April 2006
Accepted: 27 October 2006

Abstract

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.