An international team of researchers using an enormous radio telescope in Australia has just found the 1,000th pulsar known to science.
A pulsar is the collapsed core of a massive star, only 20 kilometers across, born when the original star explodes at the end of its life.
The team of researchers, comprised of astronomers from England, Australia, Italy and MIT, has been surveying the plane of the Milky Way for new radio pulsars using the Commonwealth Scientific and Industrial Research Organization's (CSIRO) radio telescope in Parkes, New South Wales.
The MIT contingent -- Victoria M. Kaspi, assistant professor of physics in the Center for Space Research and physics graduate student Fronefield Craw-ford -- are working on significant portions of the software analysis of the search data, as well as much of the data processing.
Thanks to a new "multibeam" system, a powerful new instrument on the telescope that has slashed the time it takes to scan the sky, the new survey is clocking up pulsars more than 10 times faster than any previous search -- about one for each hour the telescope is used. It already has found more than 200.
The multibeam system, built in a joint venture between researchers at the University of Manchester in England and the Australia Telescope National Facility, allows astronomers to observe many different spots in the sky simultaneously. "It's like having more than a dozen radio telescopes operating at once," said Professor Andrew Lyne of the University of Manchester's Jodrell Bank Observatory.
The MIT team has found new ways to rid the data of terrestrial sources of radio interference, such as signals from satellites and cellular phones that masquerade as "fake" pulsars. "Interference from human sources is a growing problem in radio astronomy today," Professor Kaspi said.
Professor Kaspi and Mr. Crawford travel to Australia several times a year to help with data collection at the Parkes telescope, which holds the international record for having discovered the largest number of these small spinning stars since the first was found in 1967.
Even surveys like this can find only 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 toward us," said the head of the Australian contingent of the pulsar team, CSIRO's Dr. Dick Manchester.
A STAR'S AFTERLIFE
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 tons. 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 pulsars, and we have only a few examples of some types," Dr. Manchester said. "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."
This survey has been so successful because "we're observing at an unusually high radio frequency -- 15 times that used in FM radio transmissions. At such frequencies, pulsar signals 'pass through' interstellar space relatively unimpeded," Professor Lyne said. "The natural 'hiss' produced by the Milky Way itself, a nuisance for pulsar hunters, is reduced at these frequencies," he said.
The MIT group is working on the detailed follow-up of perhaps the most interesting of the newly discovered pulsars -- the rare young ones. "We are particularly interested in young pulsars," Professor Kaspi said. "Their signals tend to glitch -- show sudden changes -- which is a sign of a 'star-quake' 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," she said.
The MIT team's research includes observations at X-ray energies using X-ray satellites, as well as radio imaging, in search of remnants of supernova explosions that created the pulsar, or evidence of elusive "pulsar wind nebulae" and bow shocks that can sometimes indicate the pulsar's direction of motion.
SEARCH FOR UNDERSTANDING
The more pulsars scientists find, the better we can understand how pulsars are born and how they 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," Dr. Manchester said.
Studying a large population of pulsars also leads to a better understanding of what makes them tick. "Like people, pulsars are all individuals -- they have different signal characteristics," he said. "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 center of a pulsar is denser than an atomic nucleus," said Nichi D'Amico, a team member from the Osservatorio Astronomico di Bologna in Italy. "The equations that describe pulsar matter put a limit on how fast a pulsar can spin without breaking apart. The fastest pulsar we know of spins around 600 times a second. If we found one spinning faster -- say, at 1,200 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 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."
An array of particularly fast-rotating pulsars could help detect a background of gravitational waves, a hypothetical relic of circumstances in the early universe near the time of the Big Bang. "Setting limits on the energy in a gravitational wave background using pulsars as 'buoys' in the gravitational wave 'sea' constrains how the universe evolved from the Big Bang era to the universe we see today," Professor Kaspi said.
A version of this article appeared in MIT Tech Talk on November 18, 1998.
