Image: Neutron star blast
Dana Berry  /  NASA
An artist's conception shows a neutron star at the height of its explosion, blasting the inner material from its accretion disk outward. The star's larger companion can be seen as a blue disk in the background. The neutron star draws a steady diet of hydrogen and helium from the companion.
By Senior Science Writer
updated 2/24/2004 6:22:11 PM ET 2004-02-24T23:22:11

Scientists have obtained a rare glimpse of the chaotic environment just miles from the surface of an explosive corpse of a star that is slowly consuming its companion.

An eruption from the neutron star illuminated material flowing onto it, providing lucky astronomers with an unprecedented peek at the activity near the surface of the ultra-dense object. The eruption poured out more energy in three hours than the sun does in a century.

The results were announced Monday and will be detailed in the Astrophysical Journal Letters.

Not dead yet
The neutron star is the exploded remains of a star that was once at least eight times as weighty as our sun. It now packs a mass equal to the sun into an area only about 6 miles (10 kilometers) in diameter. Its innards are almost entirely neutrons, which can huddle together far more densely than atoms or molecules.

A comparatively normal companion star is gravitationally bound to feed the neutron star with a steady diet of hydrogen and helium, which spirals toward the dense object in a somewhat flat "accretion" disk.

The geometry of the neutron star and its disk can be envisioned as a small Saturn with a thick version of its rings, explained David Ballantyne of the Canadian Institute for Theoretical Astrophysics at the University of Toronto. Add to that mental picture a larger, balloonlike shape representing the normal star whose material is being sucked into the disk by the gravity of the neutron star.

All the while, the two stars orbit one another.

The incoming hydrogen and helium pile up on the neutron star. In a pull of gravity 300,000 times that at Earth's surface, the gas is pressurized.

Every few hours, the layer ignites in a reaction of thermonuclear fusion like what powers the cores of all stars, Ballantyne said in a telephone interview. These bursts last just a few seconds or minutes, but they precipitate grander events.

Ash from the regular eruptions builds up on the entire surface of the neutron star.

"An ocean of heavy elements," mostly carbon, develops, Ballantyne said. Every year or two, when the carbon gets as deep as a football field is long, the pressure causes the whole layer to ignite in a "superburst," which is a thousand times stronger than a regular burst and lasts for hours.

Lucky break
The neutron star, named 4U 1820-30, is about 25,000 light-years away. It is on a list of those that erupt hourly, and so it was being monitored by NASA's Rossi X-ray Timing Explorer. There was no indication when the next superburst would occur, but the satellite serendipitously recorded the whole event.

The explosion illuminated the accretion disk from above and below, as radiation shot out from the neutron star in all directions.

Tod Strohmayer of NASA's Goddard Space Flight Center reported in 2000 the first observation of one of these neutron star superbursts. This time he and Ballantyne learned a little about the resulting dynamics.

There are no photographs of the explosion, but by examining how light interacted with iron atoms in the disk, the scientists were able to sort out pure burst radiation from radiation that was absorbed and re-emitted by the accretion disk.

They found that the inner portion of the disk was pushed outward.

Radiation — light, X-rays and other photons — can serve to push matter around subtly. But Ballantyne said that in this case the radiation probably heated the gas in the accretion disk, causing it to expand.

After a few minutes, the disk began to settle back, ultimately resuming its former shape.

Structure and movement
The observations revealed structure and movement in the accretion disk down to about 12 to 13 miles (20 kilometers) from the surface of the neutron star.

"This is the first time we have been able to watch the inner regions of an accretion disk, in this case literally a few miles from the neutron star's surface, change its structure in real time," Ballantyne said. "Accretion disks are known to flow around many objects in the universe, from newly forming stars to the giant black holes in distant quasars. Details of how such a disk flows could only be inferred up to now."

For now, the observations do not shed direct light on black holes, however. While accretion disks around black holes do generate bursts of energy, those events are not thought to originate in the same manner as with neutron stars, Ballantyne said, because black holes should not have solid surfaces on which material can settle.

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