Astrophysicists Using Space Observatories Catch Magnetar in Gigantic Stellar Belch
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Embargoed until 8:00 AM PDT, Wednesday, April 4, 2007
Astrophysicists Using Space Observatories Catch Magnetar in Gigantic
Stellar Belch
PASADENA, Calif.--When it comes to eerie astrophysical effects, the
neutron stars commonly known as magnetars are hard to beat. The
massive remnants of exploded stars, magnetars are the size of
mountains but weigh as much as the sun, and have magnetic fields
hundreds of trillions of times more powerful than the earthly field
that turns our compass needles north.
Now astrophysicists have managed to catch a recently discovered
magnetar in a sort of giant cosmic hiccup that still has them
puzzled. In multiple reports in the Astrophysical Journal and Monthly
Notices of the Royal Astronomical Society, the researchers describe
the behavior of the body located in a star cluster about 15,000
light-years away in the Ara constellation in the southern hemisphere.
The magnetar goes by the unwieldy official name CXOU
J164710.2-455216, or more informally, the "Westerlund 1 magnetar."
"We only know of about a dozen magnetars," says Michael Muno, a
scientist at the California Institute of Technology's Space Radiation
Laboratory, and the original discoverer of the magnetar. "In brief,
what we observed was a seismic event on the magnetar, which tells us
a lot about the stresses these objects endure."
In September 2005, about a year after Muno found the magnetar, the
object produced a burst that luckily came at a time when it was being
heavily observed by several satellites, including NASA's Swift X-ray
and gamma-ray observatory, and the European Space Agency's X-ray
satellite, XMM-Newton. Just five days before the burst, Muno and
collaborators had been looking at the magnetar with the XMM-Newton
and seen it in the relatively calm state in which he had originally
found it.
As most magnetars do, it produced a beam of X-ray light that, like
the beam from a lighthouse, swept across Earth once every 10 seconds.
This allowed its rotational rate to be determined very precisely. The
event that produced the burst also caused the magnetar to shine 100
times more brightly, created three separate beams to sweep past Earth
where previously only one had existed, and sped up its rotation rate
by about a thousandth of a second.
Muno says more work is required to understand what happened with the
magnetar, because it is built of matter far denser than anything on
Earth, and its composition is still a mystery.
However, it is possible to make educated guesses by extending
theories developed to explain other neutron stars. The magnetic
fields inside the neutron star are probably wound up, like a twisted
spring. In a manner somewhat similar to plate tectonics here on
Earth, as the magnetic fields unwind, they put stress on the outer
crust. The crust would resist these stresses for a while, but would
eventually fracture, producing a seismic event. The fractures would
cause the magnetar's surface to shine brightly from multiple sources.
Also, there is reason to think that part of the interior of the
neutron star is liquid and may be rotating faster than the crust. The
seismic event could cause this fluid to become attached to the crust,
so that the outer crust would speed up slightly.
"So we think the crust cracked," Muno says, adding that the
observations are important for two reasons. "First, we have now seen
another way in which these exotic objects dissipate their internal
fields as they age.
"Second, this event was only spotted because a team of us were
concentrating hard on this newly discovered object," he adds. "The
fact that we saw the event only a year after we discovered the
magnetar implies that dozens more could be lurking in our galaxy."
"If we find many more of these magnetars, we will have to reevaluate
our understanding of what happens when stars die," says GianLuca
Israel, an Italian astronomer who is publishing a separate paper on
the magnetar with his collaborators, appearing this week in the
Astrophysical Journal.
Muno is lead author of a paper appearing this week in Monthly Notices
of the Royal Astronomical Society.
Contact:
Robert Tindol
tin...@caltech.edu
(626) 395-3631