A Neutron Star with a Massive Progenitor in Westerlund 1
Abstract
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 Msolar. 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 arcmin 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 Msolar 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 Msolar. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2006
- DOI:
- 10.1086/499776
- arXiv:
- arXiv:astro-ph/0509408
- Bibcode:
- 2006ApJ...636L..41M
- Keywords:
-
- open clusters and associations: individual (Westerlund 1);
- Stars: Neutron;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 4 pages, 5 figures. Final version to match ApJL (added one figure since v2)