A Slow X-ray Pulsar in the Young, Massive Star Cluster Westerlund 1 M. Muno S. Dougherty J. S. Clark R. De Grijs P. Crowther C. Law S. McMillan D. Pooley M. Morris S. Portegies Zwart I. Negueruela F. Yusef Zadeh 2MASS Atlas Image A Galactic Super Star Cluster • • • • • Distance: 5kpc Mass: 105 Msun Core radius: 0.6 pc Extent: ~6 pc across Core density:~106 pc3 2MASS Atlas Image • Age: 4 +/- 1 Myr • Supernova rate: 1 every 10,000 years Chandra Observations WR/O star binaries, plus unresolved pre-MS stars Two exposures: 2005 May, 18 ks 2005 June, 38 ks This is a pulsar! Pulsar CXO J164710.2455216 • Period: 10.6107(1) s • Spin-down: <2x10-10 s s-1 • LX = 3x1033 erg s-1 (not a radio pulsar) • Spectrum: kT = 0.6 keV blackbody (not a cooling NS) • No IR counterpart, so K>18.5 (Mcount. < 1Msun; not an X-ray binary) This pulsar is almost certainly a magnetar. The Pulsar is within Westerlund 1(99.95% conf.) • A search of 300 archival Chandra and XMM fields reveals no new 530 s pulsars, so there is a <0.5% chance of finding a magnetar in any field (Nechita, Gaensler, Muno, et al. in prep). • The pulsar is well within the cluster, with a <10% chance of being an unrelated X-ray source. Position of pulsar Expected density of interlopers (dashed line, very small number) • The Progenitor to the Pulsar had an Initial Mass of >40 M Westerlund 1 contains sun O6V and O7V stars with initial masses of 35-37 Msun (Clark et al., in prep). • Its age is <5 Myr. • At this age, only stars more massive than 40 Msun would have undergone supernovae. Massive Stars and Magnetars • The Westerlund 1 pulsar is the third example of a magnetar with a >30 Msun progenitor. – An HI shell has been interpreted as a bubble blown by a 30-40 Msun progenitor to 1E 1048.15397 (Gaensler et al 2005). – SGR 1806-20 is thought to belong to a <4.5 Myr old star cluster, so its progenitor was > 50 Msun (e.g., Figer et al. 2005). Note that SGR 1900+15 may belong to a <10 Myr old cluster, so its progenitor was > 20 Msun (Vrba et al. 2000). Conclusions • Massive stars can lose 95% of their mass either through winds or during supernovae (e.g., Heger et al 2003, Akiyama & Wheeler 2005). • Magnetars probably form from rapidlyrotating cores, in stars that evolved too quickly to dissipate their angular momentum (e.g., Duncan & Thomas 1992; Heger et al. 2005).