June 15, 1998 SAN DIEGO -- Twinkle, twinkle little pulsar is much more than a nursery rhyme to radio astronomers. They have found a way to use the twinkling to measure the velocity and distance of these speeding neutron stars that are up above the world so high that they have escaped from the galaxy.
In the hope of finding new pulsars and calculating just how fast they can travel, Cornell University professor of astronomy James Cordes, and Barney Rickett, an astronomer at the University of California, San Diego, have devised a method that combines computer modeling with two of the world's largest radio telescopeS, the Very Long Baseline Array (VLBA) and the Arecibo Observatory, to measure the speed and distance of these incredibly dense, spinning objects well above the galactic plane.
Cordes reported on this new hybrid method here today (June 9) at the 192nd meeting of the American Astronomical Society.
Five years ago, Cordes and Joseph Taylor, professor of physics at Princeton University, developed a mathematical model for tracking the path of radio waves as they travel through the ionized gas that fills the void between stars. This gave them a way to calculate the distance to most pulsars.
Pulsars are neutron stars, the highly dense, collapsed cores of stars that are thrown out in stellar explosions called supernovas. "There are probably rocketlike effects that occur during the few seconds it takes for the explosion to occur," says Cordes. The result is incredibly energetic objects with very intense magnetic fields. The fastest pulsar recorded to date is B2224+65 in the Guitar Nebula, which is moving at 1,600 kilometers (994 miles) a second. Cordes estimates that a neutron star is born about once every 100 years in the galaxy and that about one in four of these will eventually escape. Over the age of our galaxy, the Milky Way, this means that about 25 million neutron stars have escaped.
The problem is that Cordes' and Taylor's model cannot estimate distances to pulsars that have escaped from the Milky Way. Now, Cordes and Rickett have found a way to calculate both the distance and the speed of these rocketing stars by measuring the rate at which they twinkle and combining this with their angular motion in the sky as measured by the VLBA.
In more technical terms, the astronomers are measuring the interstellar scintillation (ISS) of pulsars. ISS is analogous to the twinkling of stars but occurs in the radio signals from celestial sources rather than in optical light. The twinkling of light from distant stars as seen from Earth is caused by the atmosphere. But radio twinkling, or scintillation, results as radio waves travel through interstellar gas and the turbulence that resides in it. Cordes likens the effect to looking down into a pool and seeing the shimmering of sunlight across the bottom. The distortion, he says, "becomes a signal you can study."
To calculate the speed and distance of very faint pulsars will require measurements from both Arecibo Observatory in Puerto Rico and the VLBA. The VLBA is a radio interferometer, consisting of 10, 25-meter (27 yards) dishes spread across the United States from the Virgin Islands to Hawaii. Recorded data from the 10 dishes are played back into a central computer to mimic a single, giant telescope. It produces radio images of compact radio sources with great resolution and quality.
Notes Cordes, "We would like to understand the physics of supernova explosions better. One way to do this is to find new pulsars and measure their velocities in order to identify the fastest speed that can be produced."
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