Exciting the magnetosphere of the magnetar CXOU J164710.2–455216 in Westerlund 1

Abstract

Abstract We describe XMM–Newton observations taken 4.3 d prior to and 1.5 d subsequent to two remarkable events that were detected with Swift on 2006 September 21 from the candidate magnetar CXOU J164710.2–455216: (i) a 20-ms burst with an energy of 1037 erg (15–150 keV), and (ii) a rapid spin-down (glitch) with ΔP/P∼−10−4. We find that the luminosity of the pulsar increased by a factor of 100 in the interval between observations, from 1 × 1033 to 1 × 1035 erg s−1 (0.5–8.0 keV), and that its spectrum hardened. The pulsed count rate increased by a factor of 10 (0.5–8.0 keV), but the fractional rms amplitude of the pulses decreased from 65 to 11 per cent, and their profile changed from being single-peaked to exhibiting three distinct peaks. Similar changes have been observed from other magnetars in response to outbursts, such as that of 1E 2259+586 in 2002 June. We suggest that a plastic deformation of the neutron star crust induced a very slight twist in the external magnetic field, which in turn generated currents in the magnetosphere that were the direct cause of the X-ray outburst.