Caption: Infrared Image of Supernova Remnant; Dashed Line and Arrow Indicate Pulsar’s Motion Detected by VLA [enlarge]

Astronomers using the National Science Foundation’s

Very Large Array (VLA) radio telescope have found a
pulsar — a spinning, superdense neutron star — that
apparently is considerably younger than previously
thought. This finding, combined with the discovery
in 2000 of a pulsar that was older than previously
thought, means that many assumptions astronomers have
made about how pulsars are born and age must be
reexamined, according to the researchers.

“We are learning that each individual pulsar is a very
complicated object, and we should assume nothing about
it,” said Bryan Gaensler, of the Harvard-Smithsonian
Center for Astrophysics in Cambridge, MA. “Our work
makes it more difficult to put pulsars into neat
categories, but ultimately will yield new insights
into how pulsars are born,” he added. The research
is reported in the March 10 edition of the Astrophysical
Journal Letters.

The astronomers studied a pulsar called B1951+32 and
a supernova remnant called CTB 80, both nearly
8,000 light-years from Earth. The supernova
remnant is the shell of debris from the explosion
of a giant star. The explosion resulted from the
giant star’s catastrophic collapse into the superdense
neutron star. By observing the pulsar and the supernova
remnant from 1989 to 2000 with the VLA, the scientists
were able to measure the movement of the pulsar,
which, they found, is moving directly outward from
the center of the shell of explosion debris.

“We’ve always felt that, if you see a pulsar and a
supernova remnant close together, the pulsar had
been born in an explosion at the center of the
supernova remnant, but this is the first time that
actual observational measurement shows a pulsar moving
away from the center of the supernova remnant. It’s nice
to finally have such an example,” said Joshua Migliazzo
of the Center for Space Research at the Massachusetts
Institute of Technology, another one of the researchers.

By tracking the pulsar’s motion for more than a decade,
the astronomers were able to calculate that it is
traveling through space at more than 500,000 miles per
hour. At that speed, the pulsar required about 64,000
years to travel from its birthplace — the site of
the supernova explosion — to its present location.
That means, the astronomers say, that the pulsar is
about 64,000 years old.

This age, however, differs significantly from the age
estimated by another method which has been used by
astronomers for decades. This method uses measurements of
the rotation rate of the neutron star and the tiny amount
by which that rotation slows over time to arrive at an
estimate called the pulsar’s “characteristic age.” For
B1951+32, that method produced an estimated age of
107,000 years.

“Now we have a pulsar that is much younger than we thought.
In 2000, a different pulsar was shown to be significantly
older than we thought. That means that some of the
assumptions that have gone into estimating the ages of
these objects are unjustified,” Migliazzo said.

The pulsar’s rotation is thought to slow because the
neutron star’s powerful magnetic field acts as a giant
dynamo, emitting light, radio waves and other electromagnetic
radiation as the star rotates. The energy lost by emitting
the radiation results in the star’s rotation slowing down.

Previous estimates of pulsar ages have assumed that all
pulsars are born spinning much faster than we see them now,
that the physical characteristics of the pulsar such as its
mass and magnetic-field strength do not change with time,
and that the slowdown rate can be estimated by applying
the physics of a magnet spinning in a vacuum.

“With one pulsar older than the estimates and one younger,
we now realize that we have to question all three of these
assumptions,” said Gaensler.

Further research, the scientists say, should help them
understand more about the conditions under which pulsars
form and just how they get their spin in the first place.
Neutron stars are formed in a fraction of a second as
a massive star collapses onto itself, compressing its
matter to the density of an atomic nucleus. During the
collapse, the neutron star is thought to receive numerous
“kicks” that spin it up.

The measurements of B1951+32’s position were made in
1989, 1991, 1993 and 2000, with the VLA. The 2000
observations also used the Pie Town station of NSF’s

Very Long Baseline Array (VLBA), which improved the
precision of the measurements.
The other pulsar,
 which was found to be older than its estimated age, is
called B1757-24 or “the duck.” The report on its
motion and age was published in Nature in July of
2000.

In addition to Gaensler and Migliazzo, the researchers
are: Donald Backer of the University of California-Berkeley;
Benjamin Stappers of ASTRON in the Netherlands;
Eric Van Der Swaluw of the Dublin Institute for Advanced
Studies in Ireland; and Richard Strom of ASTRON and the
University of Amsterdam in the Netherlands.

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