August 2003 NUI Galway Astronomers discover the secrets of a Neutron Star

Astronomers at NUI Galway have discovered and measured for the first time, a link between the intensity of optical light and the intensity of radio waves from a Pulsar. Their work, published in the latest edition of the journal Science, has important implications for our understanding of how these enigmatic objects work. Pulsars, which were first discovered by the Irish astronomer Jocelyn Bell in 1967, have defied a full theoretical understanding despite more than thirty years of study.

What is a pulsar? Dr Andy Shearer who led the NUI Galway research team explains that when a large star dies its life ends with a large explosion - a supernova - one of the most energetic events in the universe. "Some supernovae can result in the formation of what is known as a neutron star," he says.

When a neutron star is young (e.g., 100,000 years), it emits a flash or 'pulse of radiation every time it rotates - it is now known as a pulsar. According to Dr Shearer, explaining the pulse and hence the conditions around a neutron star has baffled astronomers for the past thirty years. "Understanding the pulsar phenomena remains one of the unsolved problems in astrophysics," he says.

The focus of the NUI Galway team was the 'Crab' Pulsar, which rotates 33 times a second. The team took simultaneous observations of the Crab pulsar at both radio and optical wavelengths. The NUI Galway-built TRIFFID camera, using Global Positioning Satellite (GPS) technology, recorded the optical signals at the William Herschel Telescope (WHT) in La Palma and the radio signals were recorded using the Dutch radio observatory at Westerbork, Holland.

"We observed over 10,000 Giant Radio Pulses and discovered, for the first time, that there is a link between the radio and optical signals from pulsars," said Dr. Shearer. In post analysis, the radio and optical arrival times were linked to better than a 10 millionth of a second despite the radio and optical observatories being over 2000 kilometres apart.

The importance of this discovery lies in the fact that to date no convincing explanation describes all the possible observations of pulsars. "Our limited knowledge of the workings of a plasma in the extreme conditions around a pulsar has meant we do not know what causes the brief flashes of radio waves, light, X-ray and gamma ray signals that are characteristic of these enigmatic objects", says Dr. Shearer. Indeed most theoretical studies have looked at either the radio waves or the optical but not both.

"Our observations have, for the first time, linked emission from these two parts of the electromagnetic spectrum - and in doing so ruled out some of the competing models," said Dr Shearer. "We hope that future observations - particularly of the polarisation of the radio and optical radiation will lead, finally, to a complete understanding of how pulsars work."