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 × 10^-10 s s^-1, which implies that the spin-down luminosity is E ≤ 3 × 10³³ ergs s-1. The X-ray luminosity of the pulsar is L_X ≈ 3⁺¹⁰-2 × 10³³(D/5 kpc)² 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 K < 18.5, which rules out a binary companion with M > 1 M_☉. 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_i ≈ 35 M_☉ and >50 post-main-sequence stars that indicate the cluster is 4 ± 1 Myr old. Therefore, the progenitor to this pulsar had an initial mass M_i > 40 M_☉. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.