the U N I V E R S E T O D A Y
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August 3, 2000 - Issue #284
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JODRELL BANK OBSERVATORY,
UNIVERSITY OF MANCHESTER
The world-famous Lovell Telescope at Jodrell Bank Observatory, University of Manchester, has discovered a pulsar that is wobbling, giving astronomers a glimpse into the interior of a neutron star.
A pulsar is a neutron star, the extremely dense remnant of a massive normal star that has undergone a supernova explosion. A neutron star is typically 20 km in diameter, about the size of a city, weighs a million times the mass of the Earth, and spins as fast as a top, with predictable regularity. A pulsar produces beams of radio emission above its magnetic poles, and these sweep like lighthouse beams across the sky. When a beam crosses the Earth, radio telescopes receive a periodic "pulse" of radiation with a characteristic shape.
The Jodrell Bank scientists (Ingrid Stairs, Andrew Lyne and Setnam Shemar) have been studying 13 years' worth of data from the pulsar PSR B1828-11. This pulsar rotates 2.5 times per second, but, unlike any other, wobbles regularly with a period of about 1000 days. The motion is very much like the wobble of a top or gyroscope; its effects are shown in an illustration and animation. This wobble, or precession, has two manifestations: it causes the observed pulse to change its shape, and causes the time between pulses to vary, becoming sometimes shorter, sometimes longer.
In an article to appear in the August 3rd issue of Nature, the Manchester astronomers argue these variations imply that the neutron star, instead of being perfectly spherical, is slightly squashed. Stairs explains: "The bulge in the neutron star causes the angle between the pulsar's rotation axis and its radio beam to change with time, creating the wobbling effect that we measure." Lyne emphasizes that the bulge is incredibly small: "This star departs from being a perfect sphere by only 0.1 mm in 20 km. On Earth this would mean that no mountain could be higher than 3 cm!"
The surprising aspect to the discovery is not the small size of the wobble, but that fact that it is seen at all. Astronomers know from other long-term observations, mostly done at Jodrell Bank, that a pulsar is made up largely of a neutron superfluid, with a solid crust. Current theories predict that the interaction between the superfluid and the crust should cause any precession to die out extremely quickly. "But this pulsar is one hundred thousand years old, and it's still wobbling!" exclaims Lyne. "We really don't understand how this precession can be happening, and theorists are going to have to do some work to explain it," adds Stairs.
It is remarkable that by observing emission received from these objects thousands of light years away, radio astronomers can study the internal structure of these tiny objects and learn about the fundamental physics of matter at pressures and densities far greater than anything that can be achieved in a laboratory on Earth.
The results were obtained by using the University of Manchester's giant 76-metre (250ft) Lovell radio telescope at Jodrell Bank, which is still the second largest fully-steerable radio telescope in the world. For more than 40 years it has played a major role in astronomical research due to its large collecting area and great flexibility. Equipped with state-of-the-art receiver systems, the telescope is now 30 times more sensitive than when it was first built. In recent years it has played a leading role in many fields of astronomy, including the detection and study of a new population of pulsars and the discovery of the first gravitational lens. It is also attracting great public interest through its participation in the most sensitive search ever for signals from extra-terrestrial intelligence.
Currently, the Lovell telescope is undergoing a major upgrade funded by the Joint Infrastructure Fund (JIF). The improvements in both sensitivity and frequency range will expand the useable frequency range by a factor of four, increasing the sensitivity at the key operational frequency of 5 GHz by a factor of five. A wide range of new science can be carried out with the upgraded telescope, taking it into a second half-century at the forefront of astronomical research with as much promise and potential as when it was first built.
Pulsars are rapidly rotating neutron stars emitting radio waves. They are the collapsed cores of supergiant stars that have been exploded as supernovae. Thanks to the great sensitivity of the Lovell Telescope, Jodrell Bank has been at the forefront of pulsar research since their discovery by Cambridge astronomers in 1967. Over three quarters of the more than 1000 pulsars now known have been discovered by Jodrell Bank astronomers.
The wobbling (or precession) causes the rotation axis of the pulsar to follow a circle-like motion in time (see yellow and green axes at different epochs). The motion is very much like the wobble of a top or gyroscope. As a result, we see the cone-like lighthouse beam of the radio pulsar under different angles, resulting in the observed changes in pulse shape and arrival times. (Image by M. Kramer)