An international team of researchers using a giant radio telescope in Australia, equipped with a new "multibeam" receiver system, has just discovered the 1000th pulsar to be found within our Galaxy since the first few were discovered in Cambridge in 1967.
The team of researchers, comprised of astronomers from the UK, Australia, Italy and the USA, have been surveying the plane of our Galaxy, the Milky Way, for new radio pulsars using the 64-metre Parkes Radio Telescope in New South Wales, Australia. The powerful new "multibeam" receiver was built as a joint venture between engineers at the Australia Telescope National Facility and the University of Manchester's Nuffield Radio Astronomy Laboratories, Jodrell Bank, funded by the Particle Physics and Astronomy Research Council.
The receiver gives the telescope 13 beams capable of scanning the sky simultaneously and, as Professor Andrew Lyne of the University of Manchester, explained, "It's like having over a dozen giant radio telescopes operating at once". As a result, the system requires 13 sets of sophisticated data acquisition systems, one for each beam, which were largely developed and built by the UK group. Following initial detection at Parkes, confirmation and follow-up observations for many of the new pulsars are being made with the 76-metre Lovell Radio Telescope at Jodrell Bank.
Thanks to this new, state-of-the-art system, the survey is discovering new pulsars at a rate more than 10 times greater than any previous search has achieved - about one for every hour of observing time. It has already added more than 200 new pulsars to the nearly 800 known when the survey began about a year ago. By the end of the survey, in around a year or so's time, it is expected that over 600 additional pulsars will have been discovered.
A pulsar is the collapsed core of a massive star that has ended its life in a supernova explosion. Weighing more than our Sun, yet only 20 kilometres across, these incredibly dense objects produce beams of radio waves which sweep round the sky like a lighthouse, often hundreds of times a second. Radio telescopes receive a regular train of pulses as the beam repeatedly crosses the Earth so the objects are observed as a pulsating radio signal.
Pulsars make exceptional clocks, which enable a number of unique astronomical experiments. Some very old pulsars, which have been "spun up" to speeds of over 600 rotations per second by material flowing onto them from a companion star, appear to be rotating so smoothly that they may be even "keep time" more accurately than the best atomic clocks here on Earth. Very precise timing observations of systems in which a pulsar is in orbit around another neutron star have been able to prove the existence of gravitational radiation as predicted by Albert Einstein and have provided very sensitive tests of his theory of General Relativity - the theory of gravitation which supplanted that of Isaac Newton.
The team are hoping that they might soon discover a neutron star in orbit around a black hole. Dr Dick Manchester, leader of the Australian group, explains that "theories predict that around one in a thousand pulsars may be orbiting a black hole. If such a pair were to be found, it would give us the ability to learn far more about black holes, which are such elusive and enigmatic objects."
Nevertheless, even surveys like this can only find a fraction of the 300,000 pulsars thought to exist in our galaxy. "Many have signals that are too weak to pick up, or their beams are not pointing towards us" points out Dr Dick Manchester, the leader of the Australian group within the pulsar team. "Like people, pulsars are all individual and have their own characteristic signals. We want to get beyond this and understand how they actually emit their signals."
The team members at the Massachusetts Institute of Technology have developed methods of eliminating sources of radio interference, such as satellite signals, from the data which can masquerade as "fake" pulsars. As Professor Victoria M. Kaspi, points out, "Interference from human sources is a growing problem in radio astronomy and we are having to strive ever harder to observe through it."
Pulsars are also called "neutron stars" as their interior is composed of neutrons covered by a crust of iron and nickel. Dr Nichi D'Amico, team member from Bologna, Italy, points out that "the centre of a pulsar is denser than an atomic nucleus - a lump the size of a sugar cube would weigh 100 million tons!"
"Signals from distant pulsars can be also used to probe the conditions in the depths of our Galaxy" explains Dr Fernando Camilo of the University of Manchester. "The space between the stars contains giant, invisible clouds of electrons threaded with magnetic fields, which blur the pulsar signals that travel through them. By unravelling this blurring, the conditions in space can be reconstructed. The survey has, so far, doubled the number of really distant pulsars known so enabling us to probe the galaxy out to more than 20,000 light years."
Pulsars could even help increase our understanding of the evolution of the Universe. As Professor Kaspi explains, "observations of rapidly rotating pulsars spread across the galaxy may enable us to detect a background of gravitational waves. The pulsar signals are affected as a gravity wave passes by them in much the same way a buoy in the sea responds to a passing wave. Precise measurements can then shed light on how the Universe has evolved from the Big Bang era to the Universe that we see today."
The research team members are:
Professor Andrew Lyne, Dr Fernando Camilo, Ms Nuria McKay, Mr Dominic Morris and Mr Dan Shepard, University of Manchester, Nuffield Radio Astronomy Laboratories, Jodrell Bank.
Dr Dick Manchester and Dr Jon Bell, CSIRO Australia Telescope National Facility.
Dr Nichi D'Amico, Observatorio Astronomico di Bologna.
Professor Victoria Kaspi and Mr Froney Crawford, Massachusetts Institute of Technology .
Background information about Pulsars may be found on the Jodrell Bank Pulsar web site
Commonwealth Scientific and Industrial Research Organisation
5 November 1998
The telescope holds the international record for having discovered the largest number of these small spinning stars since the first was found in 1967. The new survey is clocking them up more than ten times faster than any previous search, anywhere -- about one for each hour the telescope is used -- and has already found more than 200.
"This is thanks to the power of a new instrument on the telescope, the multibeam system, which has slashed the time it takes to scan the sky," said co-leader of the pulsar team, CSIRO's Dr Dick Manchester.
Even surveys like this can find only a fraction of the 300,000 pulsars thought to live in our Galaxy. "Many have signals that are too weak to pick up, or their beams are not pointing towards us," explained Dr Manchester.
The survey is an international collaboration between astronomers from the University of Manchester, UK; the CSIRO Australia Telescope National Facility; the Massachusetts Institute of Technology, USA; and the Osservatorio Astronomico di Bologna, Italy.
A pulsar is the collapsed core of a massive star, only 20 kilometres across, born when the original star explodes at the end of its life.
Like an egg, a pulsar has a hard external crust covering a fluid interior. This fluid 'neutron matter' is so dense that a piece the size of a sugar cube has a mass of 100 million tonnes. Deep in the pulsar's innards the density is so great that matter may exist only as exotic subatomic particles.
A pulsar is ringed by a strong magnetic field. Electrons flung around by the field put out a beam of radio waves. As a pulsar spins, its beam sweeps repeatedly over the Earth and is seen as a pulsating radio signal.
Just as biologists hunt for new species to build up a picture of the Earth's biodiversity, astronomers hunt for new pulsars to understand 'astrodiversity'.
"There are many different types of pulsar, and we have only a few examples of some types," said Dr Manchester. "One of the main aims of the survey is to find more examples of these rare types and perhaps other types not even known or anticipated at present."
"In this survey's first hundred pulsars we found one orbiting another neutron star -- this is only the sixth such object known."
"Most of all we'd like to find a pulsar orbiting a black hole, to test ideas about black-hole physics. Theories predict that one pulsar in a thousand should be in such a system," he said. "We are particularly interested in young pulsars," said team member Professor Vicky Kaspi of Massachusetts Institute of Technology. "Their signals tend to glitch -- show sudden changes -- which is a sign of a 'starquake' taking place, and we can use this to study their interiors."
"As well, some young pulsars could be counterparts of high-energy X-ray and gamma-ray sources. We've detected many such sources but can't identify them with any particular objects."
The more pulsars we find, the better we can understand how they are born and evolve. "We think most of the pulsars in the Galaxy are weak. Not many of these have been found, and so our current estimates of how many pulsars exist and how often they are born are rather uncertain," said Dr Manchester.
Studying a large population of pulsars also means we can better understand what makes them 'tick'. "Like people, pulsars are all individuals -- they have different signal characteristics," said Dr Manchester. "We want to get beyond those idiosyncrasies to understand how pulsars actually emit their signals."
And beyond this is the very question of what pulsars are. "The centre of a pulsar is denser than an atomic nucleus," said Dr Manchester. The equations that describe pulsar matter put a limit on how fast a pulsar can spin without it breaking apart. The fastest pulsar we know of spins around 600 times a second. If we found one spinning faster -- say, at 1200 times a second -- that would better pin down what pulsars are made of."
Signals from distant pulsars also reveal the conditions in the depths of the Galaxy, said Dr Fernando Camilo of the University of Manchester. "The space between the stars is threaded through with magnetic fields and invisible giant clouds of electrons," he explained. "These blur pulsar signals that travel through them. From the nature of the blur we can reconstruct the conditions in space. Already our survey has doubled the known number of really distant pulsars -- those more than 20 000 light-years from the Sun -- which are going to allow us to probe out to those distances."
A network of particularly 'fast-ticking' pulsars could even help us to 'see' gravity waves, says Professor Matthew Bailes of Swinburne University of Technology, who is doing another pulsar search with the Parkes telescope.
The pulsars would be like ocean buoys that rise and fall as a wave passes by. "A passing gravity wave would slightly alter the time the pulsar's signal takes to reach us," said Professor Bailes.
The team members of the Parkes multibeam pulsar survey are: Professor Andrew Lyne, Dr Fernando Camilo, Ms Nuria McKay and Mr Dan Sheppard, Jodrell Bank Observatory, University of Manchester; Professor Vicky Kaspi and Mr Froney Crawford, Massachusetts Institute of Technology; Dr Nichi D'Amico, Osservatorio Astronomico di Bologna; and Dr Dick Manchester and Dr Jon Bell, CSIRO Australia Telescope National Facility.
The Parkes radio telescope is operated by the CSIRO Australia Telescope National Facility.