Measuring a Pulsar's Smoothness
(Originally published by the Massachusetts Institute of Technology)
June 3, 2008
In one of the first significant scientific findings from a huge collaborative effort to detect gravitational waves, the team operating the Laser Interferometer Gravity-wave Observatory (LIGO) is reporting this week that the pulsar at the center of the Crab Nebula must have an extremely smooth surface.
"This is one of the very first findings where the sensitivity of the instrument and the kind of analysis we've done is of more scientific interest," says David Shoemaker, senior research scientist in the MIT Kavli Institute for Astrophysics and Space Science and director of the MIT LIGO Laboratory. The report was posted online this week, and will be submitted for publication in Astrophysical Journal Letters.
The Crab pulsar is a rapidly spinning ball of ultra-dense matter, called a neutron star, created when a star died in a massive explosion called a supernova. The remains of the star collapsed so that its atoms were squeezed into subatomic particles called neutrons, and the mass of the star -- once a sphere about a half-million miles across -- was compressed into a ball only about 6 miles (or 10 km) across.
The explosion that produced the Crab pulsar occurred in 1054 A.D., and was recorded as a "guest star" that could be seen in broad daylight. The rapidly spinning remnant, called a pulsar, emits twin beams of radio waves like the beams of light from a lighthouse, whose blinking on and off 30 times each second provides a precise measurement of the pulsar's rotation rate.
Astronomers observed years ago that the pulsar's rotation has been slowing down. "There are a number of theories as to how it can lose energy" to put the brakes on its rotation, Shoemaker says: by emitting particles, magnetism, or gravitational waves, which are disturbances in the very )structure of space.
But observations with LIGO in 2005 and 2006 found no such gravitational waves, up to the level the instrument could detect. That means at most, only 4 percent of the energy from the pulsar could be in the form of gravitational waves.
Any irregularities on its surface would produce gravity waves, which were predicted by Einstein's theory of general relativity. The fact that none were seen shows there cannot be any bumps more than a few meters high on the pulsar.
MIT shares responsibility for LIGO with the California Institute of Technology, which has a grant from the National Science Foundation to operate the project. It includes about 600 researchers from dozens of institutions around the world. Besides Shoemaker, MIT associate professor of physics Erotokritos Katsavounidis leads the MIT LIGO data analysis group and worked on the Crab pulsar analysis.
The LIGO instrument is just beginning a major upgrade, led by Shoemaker, to increase its power by tenfold and allow it to monitor a thousand-times-greater volume of space for the presence of gravity waves. That means that it will be able to accomplish as much in a few hours as the present version can in a year, Shoemaker says. The new version, called Advanced LIGO, is expected to begin operation in 2013. MIT Kavli Institute principal research scientist Peter Fritschel is the systems scientist for the Advanced LIGO.
"If there's no signal after a year with Advanced LIGO," Shoemaker says, "then there's something wrong with the theory of relativity."