CXOU J164710.2−455216 is an anomalous X-ray pulsar and magnetar in the massive galactic open cluster Westerlund 1.

[1][2] The Westerlund 1 cluster is believed to have formed in a single burst of star formation,[3] implying that the progenitor star must have had a mass in excess of 40 solar masses.

The fact that a neutron star was formed instead of a black hole implies that more than 95% of the star's original mass must have been lost before or during the supernova that produced the magnetar.

[4][5] On 21 September 2006 the Swift satellite detected a 20ms soft gamma-ray burst in Westerlund 1.

Fortuitously, XMM-Newton observations had been made four days earlier, and repeat observations 1.5 days after the burst revealed the magnetar to be the source of the burst, with the X-ray luminosity increasing by a factor of 100 during the outburst.