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

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 10(37) erg (15-150 keV), and (ii) a rapid spin-down (glitch) with Delta P/P similar to -10(-4). We find that the luminosity of the pulsar increased by a factor of 100 in the interval between observations, from 1 x 10(33) to 1 x 10(35) 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.