Boom! A layer of carbon (yellow) may have exploded on the surface of this neutron star (center of blue disk). In this artist's conception, the dwarf companion star is a few times larger than Earth, while the entire binary is much smaller than our

Astronomers Find a Carbon Bomb

HONOLULU--Astronomers love supernovae and other explosions, but they've never witnessed a blast quite like the one described here yesterday. Carbon on the surface of an ultradense star detonated in a 3-hour thermonuclear burst, according to a report at a divisional meeting of the American Astronomical Society. If confirmed, the burst would be the first known cosmic explosion fueled solely by carbon.

The blast came from a waltzing pair of stars called a low-mass x-ray binary. In such a system, a dwarf star orbits closely around a neutron star, a stellar corpse that packs the mass of one or two suns into a dense ball just 20 kilometers wide. Gas from the dwarf flows into a hot, spiraling disk around the neutron star. Some gas hits the star's surface, forming a layer of mostly hydrogen and helium. When pressures get high enough within the thickening layer, the elements can flash-fuse in a thermonuclear explosion. Satellites see the explosions as bright x-ray flares that last 10 or 20 seconds.

But astronomer Tod Strohmayer of NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, has found a burst that broke the mold. On 9 September 1999, the Rossi X-ray Timing Explorer satellite picked up a powerful flare from 4U 1820-30, the tightest known low-mass x-ray binary, with an orbital period of just 11 minutes. The dwarf star dumps nearly pure helium onto the neutron star, as its hydrogen gas was stripped long ago.

That helium builds the bomb. The helium layer accumulates 20 or 30 meters thick before it explodes, which can happen a few times a day at 4U 1820-30. Each blast leaves some carbon, one of the "ashes" of helium fusion. After a year or so, those ashes are several hundred meters deep. The pressure may heat the base of the layer to nearly a billion degrees, detonating a carbon bomb that explodes for hours. "This thing is 1000 times more powerful than the helium bursts," Strohmayer says. "It may blow apart the entire accretion disk."

Astrophysicist Lars Bildsten of the University of California, Santa Barbara, agrees that a carbon blast is the most likely explanation, but he was still surprised. His models indicate that temperatures within a carbon layer that thick should not get nearly high enough to trigger the explosion. The puzzle will be fun to solve, he says: "This is very exciting from a nuclear physics standpoint."

Related sites
Technical information on the blast from NASA
Background on x-ray binaries
Rossi X-ray Timing Explorer