Fastest Pulsar Speeding Out of Galaxy, Astronomers Discover (Forwarded)

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Dave Finley, NRAO
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David Aguilar, CfA
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August 31, 2005

Fastest Pulsar Speeding Out of Galaxy, Astronomers Discover

A speeding, superdense neutron star somehow got a powerful "kick" that
is propelling it completely out of our Milky Way Galaxy into the cold
vastness of intergalactic space. Its discovery is puzzling astronomers
who used the National Science Foundation's Very Long Baseline Array
(VLBA) radio telescope to directly measure the fastest speed yet found
in a neutron star.

The neutron star is the remnant of a massive star born in the
constellation Cygnus that exploded about two and a half million years
ago in a titanic explosion known as a supernova. Ultra-precise VLBA
measurements of its distance and motion show that it is on course to
inevitably leave our Galaxy.

"We know that supernova explosions can give a kick to the resulting
neutron star, but the tremendous speed of this object pushes the limits
of our current understanding," said Shami Chatterjee, of the National
Radio Astronomy Observatory (NRAO) and the Harvard-Smithsonian Center
for Astrophysics. "This discovery is very difficult for the latest
models of supernova core collapse to explain," he added.

Chatterjee and his colleagues used the VLBA to study the pulsar
B1508+55, about 7700 light-years from Earth. With the ultrasharp radio
"vision" of the continent-wide VLBA, they were able to precisely measure
both the distance and the speed of the pulsar, a spinning neutron star
emitting powerful beams of radio waves. Plotting its motion backward
pointed to a birthplace among groups of giant stars in the constellation
Cygnus -- stars so massive that they inevitably explode as supernovae.

"This is the first direct measurement of a neutron star's speed that
exceeds 1,000 kilometers per second," said Walter Brisken, an NRAO
astronomer. "Most earlier estimates of neutron-star speeds depended on
educated guesses about their distances. With this one, we have a
precise, direct measurement of the distance, so we can measure the speed
directly," Brisken said. The VLBA measurements show the pulsar moving at
nearly 1100 kilometers (more than 670 miles) per second -- about 150
times faster than an orbiting Space Shuttle. At this speed, it could
travel from London to New York in five seconds.

In order to measure the pulsar's distance, the astronomers had to detect
a "wobble" in its position caused by the Earth's motion around the Sun.
That "wobble" was roughly the length of a baseball bat as seen from the
Moon. Then, with the distance determined, the scientists could calculate
the pulsar's speed by measuring its motion across the sky.

"The motion we measured with the VLBA was about equal to watching a home
run ball in Boston's Fenway Park from a seat on the Moon," Chatterjee
explained. "However, the pulsar took nearly 22 months to show that much
apparent motion. The VLBA is the best possible telescope for tracking
such tiny apparent motions."

The star's presumed birthplace among giant stars in the constellation
Cygnus lies within the plane of the Milky Way, a spiral galaxy. The new
VLBA observations indicate that the neutron star now is headed away from
the Milky Way's plane with enough speed to take it completely out of the
Galaxy. Since the supernova explosion nearly 2 and a half million years
ago, the pulsar has moved across about a third of the night sky as seen
from Earth.

"We've thought for some time that supernova explosions can give a kick
to the resulting neutron star, but the latest computer models of this
process have not produced speeds anywhere near what we see in this
object," Chatterjee said. "This means that the models need to be
checked, and possibly corrected, to account for our observations," he said.

"There also are some other processes that may be able to add to the
speed produced by the supernova kick, but we'll have to investigate more
thoroughly to draw any firm conclusions," said Wouter Vlemmings of the
Jodrell Bank Observatory in the UK and Cornell University in the U.S.

The observations of B1508+55 were part of a larger project to use the
VLBA to measure the distances and motions of numerous pulsars. "This is
the first result of this long-term project, and it's pretty exciting to
have something so spectacular come this early," Brisken said. The VLBA
observations were made at radio frequencies between 1.4 and 1.7 GigaHertz.

Chatterjee, Vlemmings and Brisken worked with Joseph Lazio of the Naval
Research Laboratory, James Cordes of Cornell University, Miller Goss of
NRAO, Stephen Thorsett of the University of California, Santa Cruz,
Edward Fomalont of NRAO, Andrew Lyne and Michael Kramer, both of Jodrell
Bank Observatory. The scientists presented their findings in the
September 1 issue of the Astrophysical Journal Letters.

The VLBA is a system of ten radio-telescope antennas, each with a dish
25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on
the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the
VLBA spans more than 5,000 miles, providing astronomers with the
sharpest vision of any telescope on Earth or in space.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

IMAGE CAPTION:
[http://www.nrao.edu/pr/2005/fastpulsar/pulsarpath.jpg (67KB)]
Over about 2.5 million years, Pulsar B1508+55 has moved across about a
third of the night sky as seen from Earth. CREDIT: Bill Saxton, NRAO/AUI/NSF