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All in the Timing

A satellite observes the lightning-fast flickerings of cosmic objects
Over the years, astronomers have gained new perspectives on the universe by exploring sections of the electromagnetic spectrum invisible to human eyes. More subtly, they have also learned to broaden their perspective on time, looking for events that happen so quickly that we might never notice them. The National Aeronautics and Space Administration's orbiting Rossi X-ray Timing Explorer (RXTE) has a clever talent for both kinds of insight (see sidebar). It focuses on the energetic X-rays that originate in violent processes occurring around hyperdense objects such as neutron stars and black holes. And unlike previous x-ray observatories, RXTE can observe lightning-fast flickerings--thousands of times per second, in some cases--that reveal unprecedented details of their underlying phenomena.

The x-ray sky looks very different than the familiar one. When seen though RXTE' s eyes, the sky flares with radiation from a class of variable stars known as x-ray binaries. In these misfit duos, one member has evolved into a neutron star, a dense stellar corpse just 20 kilometers in diameter- -or into an even smaller yet more massive black hole. In either case, the collapsed star's powerful gravity snatches material from its partner, a more sedate star like the sun. Gas spiraling inward grows fiercely hot, emitting the observed X rays. In some instances, the gas collects on the star's surface until it reaches a critical mass, touching off a tremendous thermonuclear detonation.

Or so the theory goes--nobody understands the exact details of what happens around a neutron star. But using RXTE, people like Tod E. Strohmayer of the NASA Goddard Space Flight Center are starting to find out. In a recent paper in the September 20, 1996 Astrophysical Journal Letters, Strohmayer and his colleagues report that the emissions from one X- ray binary fluctuates an astounding 1,100 times per second , much faster than any variation ever before observed. "The first thing you say when you see something like that is, this can't be!" he exclaims. M. Coleman Miller of the University of Chicago thinks the X-ray stuttering is a kind of beat pattern that results from the overlapping periods of the neutron star's rotation and the cyclic orbiting of hot gas about to crash onto the star's surface. For the first time, it seems, we can actually watch a neutron star feeding.

Related RXTE studies may finally settle the mystery concerning the origin of a group of astronomical speedsters called millisecond pulsars. About 15 years ago, radio astronomers discovered that some pulsars (spinning neutron stars that emit pulses of radiation) have rotation periods of just a few thousandths of a second, a billion times more rapid than the sun's rotation. Startled theorists proposed that these pulsars might be born in x-ray binaries, where the disk of gas crashing onto the neutron star could give it a powerful kick of angular momentum.

RXTE observations of three star systems containing pulsars that emit brilliant bursts of X rays bolster the speculation. Those bursts are thought to result from the episodic nuclear explosions taking place on the surfaces of the neutron stars in these systems; the resulting hot spots act as beacons that, for a short time, allow astronomers to observe directly each neutron star's rotation, even through the overlying clouds of infalling gas. Strohmayer reports that the oscillation period (and so presumably the rotation period of the underlying star) during bursts is just 1/600th of a second--much shorter than the spin rate of known newborn pulsars and a strong sign that these stars are in fact in fact being sped up until centrifugal effects nearly tear them to pieces.

But the process is far from cut and dried. Jean H. Swank of Goddard, the project scientist for RXTE, notes that neutron stars in some other x-ray binaries appear to slow down at times. This paradoxical phenomenon may be caused by magnetic interactions between the star and the surrounding accretion disk, though slipping and sliding between the layers of nuclear material that make up the star may also play a role.

These findings are only the beginning. Swank hopes that RXTE will eventually detect x-ray variations caused by oscillations of the surfaces of neutron stars. Such observations would permit astronomers to perform a kind of remote seismology, tracing out the stars' internal structure. That information would offer powerful insight into the nature and behavior of neutrons and quarks, the building blocks of the centers of atoms.

Swank also notes that RXTE is looking far beyond our galaxy to study the emissions from quasars, object hardly larger than our solar system that outshine entire galaxies. Most astrophysicists believe that quasars are in some ways like giant X-ray binaries, except that the central object is a black hole having as much as a billion times the mass of the sun--a beast capable of swallowing entire stars.

Herein lies a beautiful irony. The rays we see from quasars have been traveling earthward for hundreds of millions of years or more--and yet their deepest secrets might be resolved, literally, in the blink of an eye.

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