Scientists watch neutron star explosion in real time
NASA-GSFC NEWS RELEASE
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.
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."
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.
"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.