OXFORD, Miss. – The search for gravitational waves has
revealed new information about the core of one of the most
famous objects in the sky, providing a glimpse of the
spectacular discoveries that may come from the Laser
Interferometer Gravitational-Wave Observatory Scientific
Collaboration.
The LIGO collaboration, which uses huge detectors in
Washington and Louisiana to search for gravitational waves,
studied the Crab Pulsar in the Crab Nebula, a popular
target for amateur astronomers. The analysis has shown that
no more than 4 percent of the pulsar’s energy loss is
caused by the emission of gravitational waves, disproving
one theory of what is slowing the pulsar’s spin rate.
“The Crab Pulsar is spinning at a rate of 30 times per
second,” said Graham Woan of the University of Glasgow.
“However, its rotation rate is decreasing rapidly relative
to most pulsars, indicating that it is radiating energy at
a prodigious rate.”
Woan co-led the science group that used LIGO data to
analyze the Crab Pulsar, along with Michael Landry of the
LIGO Hanford (Wash.) Observatory. Their findings have been
submitted to the journal Astrophysical Journal Letters.
The LIGO Scientific Collaboration is a group of 600
scientists at universities around the United States and in
11 foreign countries. The University of Mississippi is a
member of the collaboration’s Compact Binary Coalescence
Group, which studies the detection of gravitational waves
from the inspiral and merger of binary compact stars and
black holes. The Crab Pulsar analysis was performed by the
collaboration’s Continuous Wave Group, which studies the
detection of gravitational waves from rotating compact
objects, such as pulsars and neutron stars.
The Crab Nebula, located 6,500 light years away in the
constellation Taurus, was formed in a spectacular supernova
explosion in 1054. According to ancient Chinese texts, the
explosion was visible in daylight for more than three weeks
and may briefly have been brighter than the full moon.
At the heart of the nebula remains an incredibly rapidly
spinning neutron star that sweeps two narrow radio beams
across the Earth each time it turns. The lighthouse-like
radio pulses have given the star the name “pulsar.”
Pulsars are almost perfect spheres made up of neutrons and
contain more mass than the sun in an object only 10
kilometers in radius. The physical mechanisms for energy
loss and the accompanying braking of the pulsar spin rate
have been hypothesized to be asymmetric particle emission,
magnetic dipole radiation and gravitational-wave emission.
Gravitational waves are ripples in the fabric of space and
time and are an important consequence of Einstein’s general
theory of relativity. A perfectly smooth neutron star will
not generate gravitational waves as it spins, but the
situation changes if its shape is distorted. Gravitational
waves would have been detectable even if the star were
deformed by only a few meters, which could arise because
its semisolid crust is strained or because its enormous
magnetic field distorts it.
“The Crab neutron star is relatively young and therefore
expected to be less symmetrical than most, which means it
could generate more gravitational waves,” Woan said.
Using published timing data about the pulsar rotation rate
from the Jodrell Bank Observatory, LIGO scientists
monitored the neutron star from November 2005 to August
2006 and looked for a synchronous gravitational-wave signal
using data from the three LIGO interferometers, which were
combined to create a single, highly sensitive detector.
The analysis revealed no signs of gravitational waves. But
this result is itself important because it provides
information about the pulsar and its structure, said David
Reitze, professor of physics at the University of Florida
and spokesperson for the LIGO Scientific Collaboration.
“We can now say something definite about the role
gravitational waves play in the dynamics of the Crab Pulsar
based on our observations,” Reitze said. “This is the first
time the spin-down limit has been broken for any pulsar,
and this result is an important milestone for LIGO.”
Michael Landry added, “These results strongly imply that no
more than 4 percent of the pulsar’s energy loss is due to
gravitational radiation. The remainder of the loss must be
due to other mechanisms, such as a combination of
electromagnetic radiation generated by the rapidly rotating
magnetic field of the pulsar and the emission of
high-velocity particles into the nebula.”
The LIGO project, which is funded by the National Science
Foundation, was designed and is operated by Caltech and the
Massachusetts Institute of Technology for the purpose of
detecting gravitational waves and for the development of
gravitational-wave observations as an astronomical tool.
The collaboration’s interferometer network includes the
LIGO interferometers (including the 2-kilometer and
4-kilometer detectors in Hanford, Wash., and a 4-kilometer
instrument in Livingston, La.) and the GEO600
interferometer, located in Hannover, Germany, and designed
and operated by scientists from the Max Planck Institute
for Gravitational Physics and partners in the United
Kingdom.
Work on LIGO began in the 1970s, and ground was broken on
the first observatory in 1994.
“After so many years of dedicated work by many scientists,
it is now time for scientific payoffs,” said Marco
Cavaglia, UM assistant professor of physics and astronomy
and principal investigator of the Ole Miss LIGO team. “The
latest result by the LIGO Scientific Collaboration is very
important because it will allow us to better understand the
structure of neutron stars.”
LIGO has evolved to its present capability to produce
significant scientific results, said Jay Marx of the
California Institute of Technology, LIGO’s executive
director.
“The limit on the Crab Pulsar’s emission of gravitational
waves is but one of a number of important results obtained
from LIGO’s recent two-year observing period,” Marx said.
“These results only serve to further our anticipation for
the spectacular science that will come from LIGO in the
coming years.”
For more information on LIGO, go to http://www.ligo.caltech.edu/.
For more information on the Ole Miss LIGO team, go to http://www.phy.olemiss.edu/GR/ligoteam or contact Cavaglia at cavaglia@olemiss.edu.