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 LX ≈ 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 Rbb = 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 Mi ≈ 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 Mi > 40 M☉. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.
- open clusters and associations : individual (Westerlund 1)
- stars : neutron
- X-rays : stars