Pulsar discovery at NUI Galway

 

Astronomers have made an important new discovery about the mysterious pulsar. Dick Ahlstrom reports

The Irish climate was no impediment to a group of Irish astronomers which for the first time has answered important questions about the enigmatic astral body, the pulsar. These objects have defied attempts to understand them since their discovery in 1967, but the NUI Galway group of astronomers has revealed at least some of their secrets.

Pulsars are a most unusual kind of star that emits powerful pulses of radio waves. Ironically, the Irish were involved in pulsar study from the very beginning via the great Irish astronomer, Jocelyn Bell, who discovered them.

The NUI team, which is working in collaboration with a group in Amsterdam, measured for the first time a link between the intensity of optical light coming from the pulsar and the intensity of its radio wave emissions, publishing its results two weeks ago in the journal, Science.

The finding could reveal significant new information about how these mysterious bodies work, explains NUI Galway lecturer, Dr Andy Shearer, who led the Irish research group.

"We had been working on observations of pulsars for the last 10 years. One of the unanswered questions was whether there was a link between the optical signal and the radio signal."

Pulsars only arise at the end of a star's lifetime. Large stars "die" in a massive supernova explosion and then collapse inwards to form what is called a neutron star.

They become extremely dense in this state with all the mass of a star similar to our sun compacting into a space no more than 10 km across, less than the distance across Dublin. A mere teaspoon of this super-massive material would weight more than a billion tonnes, and neutron stars rotate very rapidly, up to 600 revolutions per second.

Young neutron stars, pulsars, emit a powerful radio frequency signal that beams out like the light beacon of a lighthouse, with the signal linked to how fast it rotates, explains Shearer. A very few however also give off visible light.

"About 1,400 pulsars are known, most of them seen as radio objects," he explains. "Only eight are observed to pulse optically. Understanding the pulsar phenomena remains one of the unsolved problems in astrophysics."

The team decided to answer at least one question, whether there was a link between the intensity of the radio signal and visible signal coming from the handful of pulsars that gave off observable light. They turned their attentions to the "Crab" pulsar in the Crab Nebula which rotates 33 times a second.

The Galway group over the past decade had developed a unique world-class camera system known as TRIFFID. Built using funding from Enterprise Ireland, TRIFFID was able to capture the weak visual signal coming from the pulsar when attached to the William Herschel Telescope in La Palma. The matched radio signals were recorded using the National Radio Observatory at Westerbork run by the University of Amsterdam.

"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," says Shearer.

"The Crab is very unusual in that the pulsed radio emissions vary. About once every second the radio pulse jumps in intensity to 1,000 times its usual strength. The team was able to match this with a 3 per cent increase in optical signal intensity.

"This is the first time these types of observations have been seen," he said. "When we did the calculations, we found the amount of energy released is about the same," this despite the change in intensity, he added. "That is what is difficult to explain."

NUI Galway and other groups around the world are now trying to understand the meaning of the new findings and to discover where on the neutron star the radio and optical signals come from, for example the surface rather than the interior.

While the discovery raises as many questions as it answers, it is extremely valuable, he believes. "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 for pulsars."