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 < 2 × 10 -10 s s -1 , which implies that the spin-down luminosity is ≤ 3 × 10 33 ergs s -1 . The X-ray luminosity of the pulsar is L X ≈ 3 × 10 33 ( D /5 kpc) 2 ergs s -1 , and the spectrum can be described by a kT = 0.61 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.