The Crab Pulsar is Probed By LIGO. Is it Really a Smooth Neutron Star?

Scientists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) have announced their first land-mark discovery. LIGO was built to detect gravitational waves (as predicted by Einstein’s general relativity), but this discovery is actually about not detecting gravitational waves. Hold on, what’s all the fuss about then? This sounds like a null result, and in some ways it is. But on the other hand it may be one of the most important neutron star observations ever. So what has LIGO (not) seen?

The Crab Nebula has a rapidly spinning neutron star called the Crab Pulsar hiding inside. The nebula can be found 6,500 light years away in the constellation Taurus, formed after a large supernova observed in the year 1054 AD. Astronomers discovered the left-over neutron star in 1969 and have been observing the young, 900 year old pulsar ever since. However, little is known about the shape or structure of this 10 km-diameter massive stellar object. All we can see are the pulses of gamma- and X-ray radiation beaming into space, flashing at a rate of 30 pulses per second.

However, Einstein’s general relativity predicts that any massive body disturbing space-time will generate gravitational waves which propagate throughout the Universe. LIGO was built with this in mind, by using highly accurate laser interferometry to detect the passage of gravitational waves through space-time surrounding Earth. It is predicted that the direction of gravitational wave propagation may also be derived, heralding the beginning of gravitational wave astronomy. But LIGO will take a long time to collect the data, and it will need a lot more “exposure time” before we can say for certain that we can detect gravitational waves.

In today’s announcement, it is reported that the LIGO facility had monitored the neutron star from November 2005 to August 2006 by using all three LIGO interferometers. This created a very sensitive detector. By comparing Crab Pulsar rotation rate data from the Jodrell Bank Observatory with the LIGO data for this period, they watched for a synchronous gravitational-wave signal.

No signal was observed.

So what does this mean? Does it mean that gravitational waves do not exist after all? No. Although gravitational waves are still in the realms of “theory”, there is strong evidence to suggest they are out there, and that they can be observed. More time is required to collect more data so the LIGO signal-to-noise ratio can be improved.

If gravitational waves are out there, why was there no gravitational wave signal from the rapidly spinning neutron star in the Crab Nebula? This is where it gets really interesting. This is by far the most significant result to come from LIGO, it means that the neutron star is smooth. If the neutron star had any surface features, as the body rotated it would cause ripples in space-time. It is smooth, so no ripples can be generated.

A good analogy is to imagine a spinning buoyant sphere (like a volleyball) on the surface of a swimming pool – there will be very little disturbance on the water’s surface. Now spin a rugby ball on the surface – ripples will be swept out by the long ends of the ball. This analogy is also important when we consider the loss of energy in the spinning ball. Spinning the volleyball will cause minimum loss in energy (as there is little drag, and little wave production), spinning the rugby ball will cause a huge loss in energy (lots of drag, lots of waves). The volleyball will spin for much longer than the rugby ball.

This is another important observation, the LIGO group observe a slow-down in rotation rate, but less than 4 % of the energy loss can be attributed to gravitational wave production. Therefore the neutron star in the Crab Nebula has a smooth surface with very little variation in surface topography causing drag. After all, LIGO should be able to observe gravitational waves produced by a surface deformation only a few meters high.

“The physics world has been waiting eagerly for scientific results from LIGO. It is exciting that we now know something concrete about how nearly spherical a neutron star must be, and we have definite limits on the strength of its internal magnetic field.” – Nobel Prize-winning radio astronomer and professor at Princeton University, Joseph Taylor.