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A Slow X-ray Pulsar in the Young, Massive Star Cluster Westerlund 1

A Slow X-ray Pulsar in the Young, Massive Star Cluster Westerlund 1. 2MASS Atlas Image. A Galactic Super Star Cluster. Distance: 5kpc Mass: 10 5 M sun Core radius: 0.6 pc Extent: ~6 pc across Core density:~10 6 pc -3 Age: 4 +/- 1 Myr Supernova rate: 1 every 10,000 years.

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A Slow X-ray Pulsar in the Young, Massive Star Cluster Westerlund 1

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  1. A Slow X-ray Pulsar in the Young, Massive Star Cluster Westerlund 1 2MASS Atlas Image

  2. A Galactic Super Star Cluster • Distance: 5kpc • Mass: 105 Msun • Core radius: 0.6 pc • Extent: ~6 pc across • Core density:~106 pc-3 • Age: 4 +/- 1 Myr • Supernova rate: 1 every 10,000 years 2MASS Atlas Image

  3. Chandra Observations Two exposures: 2005 May, 18 ks 2005 June, 38 ks WR/O star binaries, plus unresolved pre-MS stars This is a pulsar!

  4. Pulsar CXO J164710.2-455216 • 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.

  5. The Pulsar is within Westerlund 1(99.95% conf.) • A search of 300 archival Chandra and XMM fields reveals no new 5-30 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)

  6. The Progenitor to the Pulsar had an Initial Mass of >40 Msun • Westerlund 1 contains 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 under-gone supernovae.

  7. 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 1048.1-5397(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).

  8. 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 rapidly-rotating cores, in stars that evolved too quickly to dissipate their angular momentum (e.g., Duncan & Thomas 1992; Heger et al. 2005).

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