A neutron star with a massive progenitor in Westerlund 1

Michael P. Muno, J. Simon Clark, Paul A. Crowther, Sean M. Dougherty, Richard de Grijs, Casey Law, Stephen L. W. McMillan, Mark R. Morris, Ignacio Negueruela, David Pooley, Simon Portegies Zwart, Farhad Yusef-Zadeh

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192 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)L41-L44
Number of pages4
JournalAstrophysical Journal Letters
Issue number1
Publication statusPublished - 1 Jan 2006
Externally publishedYes


  • open clusters and associations : individual (Westerlund 1)
  • stars : neutron
  • X-rays : stars


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