Scientists watch neutron star explosion in real time
Posted: February 23, 2004

A neutron star halfway across the Milky Way galaxy is ready for its close-up. A rare and massive explosion on this star illuminated the region and allowed scientists to view details never seen before, virtually bringing the scientists to the action occurring just a few miles above the star's surface.

An artist's concept shows the rare explosion on a neutron star, which is the dead core of a massive star. Credit: NASA/Dana Berry
Scientists at NASA and the Canadian Institute for Theoretical Astrophysics (CITA) report their findings in the current issue of Astrophysical Journal Letters. The action was captured second-by-second in movie-like fashion through a process called spectroscopy with NASA's Rossi X-ray Timing Explorer.

A neutron star is the dense, core remains of an exploded star at least eight times more massive than the sun. The neutron star contains about a sun's worth of mass packed in a sphere only about 10 miles (16 kilometers) in diameter.

Often neutron stars are in binary (two-star) systems. Gas from the nearby companion star can funnel towards the neutron star, attracted by the star's strong gravity. The gas spirals toward the neutron star like water going down a drain, forming what scientists call an accretion disk.

"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," said Dr. David Ballantyne of CITA at the University of Toronto. "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."

An artist's concept of the neutron star surface before the explosion. Credit: NASA/Dana Berry
Under normal conditions, accretion disks appear far too minute to resolve with even the most powerful telescopes. The explosion occurred on a neutron star named 4U 1820-30, 25,000 light years from Earth. It poured out more energy in three hours than the sun does in 100 years. The region was illuminated in such a way, the scientists could see details as fine as the accretion disk buckling from the explosion and then slowly recovering its original form after approximately 1,000 seconds.

Such explosions are the result of accretion. As matter from the companion star crashes down on the neutron star, it builds up a 10 to 100 yard layer of material comprised mostly of helium. The fusion of the helium into carbon and other heavier elements releases enormous energy and powers a strong burst of X-ray light, far more energetic than visible light. Such bursts can occur several times a day on a neutron star and last for about 10 seconds.

Ballantyne and his colleague, Dr. Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Md., observed a "superburst." These are much more rare than ordinary, helium-powered bursts and release 1000 times more energy. Scientists say superbursts are caused by a buildup of nuclear ash in the form of carbon from the helium fusion. Current thinking suggests it takes several years for the carbon ash to build up to such an extent that it begins to fuse.

An artist's concept reveals what the surface might look like during the explosion. Credit: NASA/Dana Berry
The superburst was so bright and long, it acted like a spotlight beamed from the neutron star surface onto the innermost region of the accretion disk. The X-ray light from the burst illuminated iron atoms in the accretion disk, a process called fluorescence. The Rossi Explorer captured the characteristic signature of the iron fluorescence, its spectrum. This, in turn, provided information about the iron's temperature, velocity and location around the neutron star.

"The Rossi Explorer can get a good measurement of the fluorescence spectrum of the iron atoms every few seconds," Strohmayer said. "Adding up all this information, we get a picture of how this accretion disk is being deformed by the thermonuclear blast. This is the best look we can hope to get, because the resolution needed to actually see this action as an image, instead of spectra, would be a billion times greater than what the Hubble Space Telescope offers."

The scientists said the bursting neutron stars serve as a laboratory to study accretion disks, which are seen (but in less detail) throughout the universe around nearby stellar black holes and exceedingly distant quasar galaxies. Stellar black holes with accretion disks do not produce X-ray bursts.

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