A neutron star with a massive progenitor in Westerlund 1

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2 - 455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107( 1) s. We place an upper limit to the period derivative of P < 2 x 10(-10) s s(-1), which implies that the spin-down luminosity is E <= 3 x 10(33) ergs s(-1). The X-ray luminosity of the pulsar is L-X approximate to 3(-2)(+10) x 10(33) ergs s(-1), and the spectrum can be described by a kT = 0.61(-0.02)(+0.02) keV blackbody with a radius of R-bb = 0.27 +/- 0.03(D/5 kpc) km. Deep infrared observations reveal no counterpart with, which rules out a binary companion with M > 1 M-circle dot. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2 - 455216 only 1.'6 from the core of Westerlund 1 indicates that it is a member of the cluster with > 99.97% confidence. Westerlund 1 contains 07 V stars with initial masses M-t approximate to 35 M-circle dot and > 50 post-main-sequence stars that indicate the cluster is 4 +/- Myr old. Therefore, the progenitor to this pulsar had an initial mass M-i > 40 M-circle dot. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.