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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
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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).