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

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.

Note: This release, with image, is available on the NRAO
Web site, at: