A neutron star with a massive progenitor in Westerlund

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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

189 Scopus citations


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 Ṗ < 2 × 10-10 s s-1, which implies that the spin-down luminosity is Ė ≤ 3 × 1033 ergs s-1. The X-ray luminosity of the pulsar is Lx ≈ 3-2+10 × 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 Abb = 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 Ml ≈ 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 Ml > 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 (US)
Pages (from-to)L41-L44
JournalAstrophysical Journal
Issue number1 II
StatePublished - Jan 1 2006


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

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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