Distance Measurement Solves Astrophysical Mysteries (Forwarded)

National Radio Astronomy Observatory
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Socorro, NM 87801
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Contact:
Dave Finley, Public Information Officer, Socorro, NM
(505) 835-7302,

August 12, 2003

Distance Measurement Solves Astrophysical Mysteries

Location, location, and location. The old real-estate adage about what's really
important proved applicable to astrophysics as astronomers used the sharp radio
"vision" of the National Science Foundation's Very Long Baseline Array (VLBA) to
pinpoint the distance to a pulsar. Their accurate distance measurement then
resolved a dispute over the pulsar's birthplace, allowed the astronomers to
determine the size of its neutron star and possibly solve a mystery about cosmic
rays.

"Getting an accurate distance to this pulsar gave us a real bonanza," said
Walter Brisken, of the National Radio Astronomy Observatory (NRAO) in Socorro, NM.

The pulsar, called PSR B0656+14, is in the constellation Gemini, and appears to
be near the center of a circular supernova remnant that straddles Gemini and its
neighboring constellation, Monoceros, and is thus called the Monogem Ring. Since
pulsars are superdense, spinning neutron stars left over when a massive star
explodes as a supernova, it was logical to assume that the Monogem Ring, the
shell of debris from a supernova explosion, was the remnant of the blast that
created the pulsar.

However, astronomers using indirect methods of determining the distance to the
pulsar had concluded that it was nearly 2500 light-years from Earth. On the
other hand, the supernova remnant was determined to be only about 1000
light-years from Earth. It seemed unlikely that the two were related, but
instead appeared nearby in the sky purely by a chance juxtaposition.

Brisken and his colleagues used the VLBA to make precise measurements of the sky
position of PSR B0656+14 from 2000 to 2002. They were able to detect the slight
offset in the object's apparent position when viewed from opposite sides of
Earth's orbit around the Sun. This effect, called parallax, provides a direct
measurement of distance.

"Our measurements showed that the pulsar is about 950 light-years from Earth,
essentially the same distance as the supernova remnant," said Steve Thorsett, of
the University of California, Santa Cruz. "That means that the two almost
certainly were created by the same supernova blast," he added.

With that problem solved. the astronomers then turned to studying the pulsar's
neutron star itself. Using a variety of data from different telescopes and armed
with the new distance measurement, they determined that the neutron star is
between 16 and 25 miles in diameter. In such a small size, it packs a mass
roughly equal to that of the Sun.

The next result of learning the pulsar's actual distance was to provide a
possible answer to a longstanding question about cosmic rays. Cosmic rays are
subatomic particles or atomic nuclei accelerated to nearly the speed of light.
Shock waves in supernova remnants are thought to be responsible for accelerating
many of these particles.

Scientists can measure the energy of cosmic rays, and had noted an excess of
such rays in a specific energy range. Some researchers had suggested that the
excess could come from a single supernova remnant about 1000 light-years away
whose supernova explosion was about 100,000 years ago. The principal difficulty
with this suggestion was that there was no accepted candidate for such a source.

"Our measurement now puts PSR B0656+14 and the Monogem Ring at exactly the right
place and at exactly the right age to be the source of this excess of cosmic
rays," Brisken said.

With the ability of the VLBA, one of the telescopes of the NRAO, to make
extremely precise position measurements, the astronomers expect to improve the
accuracy of their distance determination even more.

"This pulsar is becoming a fascinating laboratory for studying astrophysics and
nuclear physics," Thorsett said.

In addition to Brisken and Thorsett, the team of astronomers includes Aaron
Golden of the National University of Ireland, Robert Benjamin of the University
of Wisconsin, and Miller Goss of NRAO. The scientists are reporting their
results in papers appearing in the Astrophysical Journal Letters in August.

The VLBA is a continent-wide system of ten radio- telescope antennas, ranging
from Hawaii in the west to the U.S. Virgin Islands in the east, providing the
greatest resolving power, or ability to see fine detail, in astronomy. Dedicated
in 1993, the VLBA is operated from the NRAO's Array Operations Center in
Socorro, New Mexico.

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/2003/monogem/...m.image.shtml]
The Monogem ring supernova remnant. In this false color image, the X-ray flux in
the 0.25-0.75 keV band increases from blue to white. The ring structure is
highlighted with the large 18.4-degree diameter circle. The current location of
the pulsar PSR B0656+14, marked with a cross, is near the center of the ring.
The estimated position of the pulsar at birth is marked with a small black
rectangle. The X-ray data, from the ROSAT All-Sky Survey, were made available by
the Max-Planck-Institute fuerExtraterrestrische Physik. The image is © 2003 by
the American Astronomical Society and is reprinted with permission.