Astronomers Discover Fastest-Spinning Pulsar (Forwarded)

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Embargoed for Release: 2:00 p.m., EST, Thursday, January 12, 2006

Astronomers Discover Fastest-Spinning Pulsar

Astronomers using the National Science Foundation's Robert C. Byrd Green
Bank Telescope have discovered the fastest-spinning neutron star ever
found, a 20-mile-diameter superdense pulsar whirling faster than the
blades of a kitchen blender. Their work yields important new information
about the nature of one of the most exotic forms of matter known in the
Universe.

"We believe that the matter in neutron stars is denser than an atomic
nucleus, but it is unclear by how much. Our observations of such a
rapidly rotating star set a hard upper limit on its size, and hence on
how dense the star can be.," said Jason Hessels, a graduate student at
McGill University in Montreal. Hessels and his colleagues presented
their findings to the American Astronomical Society's meeting in
Washington, DC.

Pulsars are spinning neutron stars that sling "lighthouse beams" of
radio waves or light around as they spin. A neutron star is what is left
after a massive star explodes at the end of its "normal" life. With no
nuclear fuel left to produce energy to offset the stellar remnant's
weight, its material is compressed to extreme densities. The pressure
squeezes together most of its protons and electrons to form neutrons;
hence, the name "neutron star."

"Neutron stars are incredible laboratories for learning about the
physics of the fundamental particles of nature, and this pulsar has
given us an important new limit," explained Scott Ransom, an astronomer
at the National Radio Astronomy Observatory and one of Hessels'
collaborators on this work.

The scientists discovered the pulsar, named PSR J1748-2446ad, in a
globular cluster of stars called Terzan 5, located some 28,000
light-years from Earth in the constellation Sagittarius. The
newly-discovered pulsar is spinning 716 times per second, or at 716
Hertz (Hz), readily beating the previous record of 642 Hz from a pulsar
discovered in 1982. For reference, the fastest speeds of common kitchen
blenders are 250-500 Hz.

The scientists say the object's fast rotation speed means that it cannot
be any larger than about 20 miles across. According to Hessels, "If it
were any larger, material from the surface would be flung into orbit
around the star." The scientists' calculation assumed that the neutron
star contains less than two times the mass of the Sun, an assumption
that is consistent with the masses of all known neutron stars.

The spinning pulsar has a companion star that orbits it once every 26
hours. The companion passes in front of the pulsar, eclipsing the pulsar
about 40 percent of the time. The long eclipse period, probably due to
bloating of the companion, makes it difficult for the astronomers to
learn details of the orbital configuration that would allow them to
precisely measure the masses of the pulsar and its companion.

"If we could pin down these masses more precisely, we could then get a
better limit on the size of the pulsar. That, in turn, would then give
us a better figure for the true density inside the neutron star,"
explained Ingrid Stairs, an assistant professor at the University of
British Columbia and another collaborator on the work.

Competing theoretical models for the types and distributions of
elementary particles inside neutron stars make widely different
predictions about the pressure and density of such an object.

"We want observational data that shows which models fit the reality of
nature," Hessels said.

If the scientists can't use PSR J1748-2446ad to do that, they are
hopeful some of its near neighbors will yield the data they seek. Using
the GBT, the astronomers so far have found 30 new fast "millisecond
pulsars" in the cluster Terzan 5, making 33 pulsars known in the cluster
in total. This is the largest number of such pulsars ever found in a
single globular cluster.

Dense globular clusters of stars are excellent places to find
fast-rotating millisecond pulsars. Giant stars explode as supernovae and
leave rotating pulsars which gradually slow down. However, if a pulsar
has a companion star from which it can draw material, that incoming
material imparts its spin, or angular momentum, to the pulsar. As a
result, the pulsar spins faster. "In a dense cluster, interactions
between the stars will create more binary pairs that can yield more
fast-rotating pulsars," Ransom said.

The great sensitivity of the giant, 100-meter diameter GBT, along with a
special signal processor, called the Pulsar Spigot, made possible the
discovery of so many millisecond pulsars in Terzan 5. "We think there
are many more pulsars to be found in Terzan 5 and other clusters, and
given that the fast ones are often hidden by eclipses, some of them may
be spinning even faster than this new one," Ransom said.

"We're excited about using this outstanding new telescope to answer some
important questions about fundamental physics," he said.

In addition to Hessels, Ransom and Stairs, the research team includes
Paulo Freire of Arecibo Observatory in Puerto Rico, Victoria Kaspi, of
McGill University, and Fernando Camilo, of Columbia University. Their
report is being published in Science Express, the online version of the
journal Science.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc. The pulsar research also was supported by the Canada
Foundation for Innovation, the Natural Sciences and Engineering Research
Council of Canada, the Quebec Foundation for Research on Nature and
Technology, the Canadian Institute for Advanced Research, the Canada
Research Chairs Program, and the National Science Foundation.

Graphics: How Are Millisecond Pulsars Formed?
http://www.nrao.edu/pr/2006/mspulsar...graphics.shtml