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07/05/95

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Video animation helps explain gamma-ray pulsar geometry

STANFORD -- The video begins like the introduction to an episode of Star Trek, zooming in on the glowing filaments of the Crab Nebula.

But the seven-minute film clip's purpose is scientific, not entertainment. The Crab Nebula contains one of the more exotic objects in the cosmos, a neutron star, and the computer animation, created by assistant professor of physics Roger W. Romani and his graduate student Ion-Alexis Yadigaroglu, shows how this neutron star illuminates the center of the nebula and how it produces beams of high-energy gamma rays.

The scientists developed the animation to help explain how this exotic star is oriented relative to the other features in the nebula. It visually portrays the complex geometry of the intense magnetic fields and radiation beams that surround the neutron star. The animation was completed and shown for the first time at an American Astronomical Society meeting in Pittsburgh on June 15.

"Today, we understand the basic particle physics pretty well. Working on the equations gave us a pretty good idea of the shape of the regions involved," said Romani. "But some details were confusing, and when we tried to explain it to other people, we found it was difficult for them to visualize. So we decided to create a computer animation."

A neutron star is the compact cinder that remains after a stellar explosion, or supernova. When a star heavier than about 10 solar masses runs out of nuclear fuel, it explodes and its core collapses into an asteroid-sized object of incredible density. The gravitational force is so strong that the protons and electrons that normally make up atoms are crushed into neutrons, hence the name neutron star. The result is matter so compressed that a thimbleful would weigh about 100 million tons.

Much like a skater pulling in her arms while spinning, the process of collapse also spins up the neutron star to rotation rates of thousands of revolutions per second. At the same time, it concentrates the star's magnetic fields a billionfold. The strong fields and rapid rotation accelerate charged particles in the gaseous plasma surrounding the star to nearly the speed of light. The resulting plasma streams produce highly directional beams of radio waves and other electromagnetic radiation. As the star rotates, these beams are swept across the sky. Just as the rotating beam from a lighthouse produces flashes of light, these rotating beams produce pulses of radiation, as viewed from Earth. Accordingly, when the first radio pulses from such a neutron star were detected in 1968, astronomers called it a "pulsar."

Because pulsars broadcast so much energy, their rotation rates drop steadily with age. At birth, a pulsar will typically spin thousands of times per second. By its millionth birthday it has slowed down to around one revolution per second. Normally, it is the younger pulsars, those less than 1 million years old, that generate gamma-ray bursts.

The Crab Nebula, which was created by a supernova that occurred in 1054 and was recorded by the Chinese, contains one of first pulsars to be detected. It rotates at about 30 times per second. When the first X-ray telescopes were launched on rockets and deployed as satellites, the Crab's pulsar also was found to be a prominent source of X-rays. More recently, special gamma-ray telescopes, including the orbiting Gamma-Ray Observatory that Peter Michelson, associate professor of physics, is co-principal investigator of, have established that the Crab's neutron star and six other young, rapidly rotating pulsars also produce gamma rays, which are many times more energetic than X-rays.

Flight through a star field

The video sequence starts with a flight through a star field to the Crab, starting with a ground-based telescope view and then zooming into the heart of the nebula, using Hubble Space Telescope images. Clearly visible above and below the point where the neutron star lies are circular features. According to the scientists, these lie at the boundary between the very dense plasma winds that flow equatorially from the neutron star and the less powerful currents that flow outward from its two poles.

Next, the video shows the graphic representation of the neutron star. (See accompanying illustration.) Radio beams shoot out from the whirling star in a narrow beam strongly reminiscent of the light from a lighthouse. Surrounding the tiny star is a large volume of rotating plasma shaped something like a twisted donut (the dark area in the illustration).

In this area, particles are accelerated to energy levels far beyond what is achievable in the largest particle accelerators. If the neutron star is spinning fast enough, the plasma gains enough energy so that the particles convert spontaneously into gamma radiation. The radiation travels through the magnetic field, but doesn't get far. It runs into incoming photons and is converted back into particles in the region depicted in gray. These particles produce the X-ray and gamma-ray beams that travel outward into the surrounding cosmos and produce the observed pulses.

Using their animation, the scientists can look at the neutron star from any orientation and estimate the profile of the radio waves, X-rays and gamma rays that should be visible. By comparing these profiles with the emissions from the seven known gamma-ray pulsars, the scientists have been able to refine their model.

"In effect, we have done some astrophysical 'reverse engineering,' working back from the observed pulses to understand the incredible particle accelerators at work in these stars," Romani said.

But only minor adjustments to their basic model, which postulates the simplest possible shape for the magnetic fields, proved necessary in order to match the output of all seven gamma- ray pulsars quite closely, they said.

(Those interested can download a short MPEG version of the animation from the World Wide Web at <http://geminga.stanford.edu/users/ion>.)

-dfs-

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