The Crab Nebula, housing the Crab Pulsar at its center, is a supernova remnant of a stellar explosion that took place somewhere around 1054. It is located in the Taurus constellation about 6,500 light years away and at the time of the explosion it was allegedly visible on the sky in midday for as much as three weeks, at some point having had a brightness level higher than that of the Moon.
An analysis carried out by the Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration and focused on the pulsar in its center reveals that as much as 4 percent of the energy it radiates into the surrounding space is lost to gravitational waves. Pulsars are neutron stars with fast rotation rates around their axis which emit lighthouse-like radio pulses that sweep across the sky as they rotate - therefore the name pulsar.
"The Crab Pulsar is spinning at a rate of 30 times per second. However, its rotation rate is decreasing rapidly relative to most pulsars, indicating that it is radiating energy at a prodigious rate", says Graham Woan from the University of Glasgow, part of the team conducting the investigations.
Pulsars are believed to lose energy either by asymmetric particle emissions, by radiating magnetic dipoles or by gravitational-wave emissions. Einstein's Theory of General Relativity says that gravitational waves are ripples in the fabric of space-time, generated by the movement of massive objects. In the case of neutron stars with perfect spherical shapes gravitational waves would not be generated. However, imperfections can arise as the surface layers of the star are being strained or distorted by powerful magnetic fields.
"The Crab neutron star is relatively young and therefore expected to be less symmetrical than most, which means it could generate more gravitational waves", said Woan.
Although gravitational waves don't seem to work as a significant braking mechanism for the Crab pulsar, the result itself provides information about the pulsar and its structure.
"We can now say something definite about the role gravitational waves play in the dynamics of the Crab Pulsar based on our observations. This is the first time the spin-down limit has been broken for any pulsar, and this result is an important milestone for LIGO", says David Reitze, a professor of physics from the University of Florida.
Michael Landry from the LIGO Hanford Observatory, one of the co-authors of the study, says that most part of the energy lost by the Crab pulsar is radiated through mechanisms such as electromagnetic emissions, the rotating magnetic field of the pulsar and the emission of high-velocity particles in the supernova remnant, while gravitational waves make up less than 4 percent of the energy released in the surrounding space.
The new results provided by the LIGO experiment demonstrated its capability to produce real scientific data and are probably some of the most important results the LIGO detector has obtained in its two year period of observations, says Jay Marx from the California Institute of Technology while revealing his hopes that LIGO will provide similar contributions in the near future.
Bernard F. Schutz, director of the Albert Einstein Institute, says that neutron stars are born as extremely hot objects, although as time passes they cool rapidly to create a semisolid crust. The results revealed by LIGO show that the Crab pulsar might have developed surface irregularities when it was younger, but by now they have evened out and the energy loss through gravitational waves is quite small.
"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", said Nobel Prize-winning radio astronomer from the Princeton University, Joseph Taylor.