Washington, DC-A type of X-ray explosion found on neutron stars does not occur near black holes, scientists announced here today. The lack of explosions is strong evidence for the existence of a black hole event horizon, a theoretical boundary into which matter vanishes and cannot escape.

The explosions are brief thermonuclear eruptions called type I X-ray bursts and last about one minute. The bursts occur every several hours on the surface of very small, dense stars called neutron stars. They are fueled by gas that a neutron star siphons off a companion star.

The gas accumulates on the neutron star's surface and when enough builds up, the gas erupts in an X-ray burst.

Scientists examined neutron stars and black holes detected by NASA's Rossi X-ray Timing Explorer during the past nine years. They found X-ray bursts from 13 sources believed to be neutron stars, but none from 18 suspected black holes.

"By looking at objects that pull in gas, we can infer whether that gas crashes and accumulates onto a hard surface or just quietly vanishes," said study leader Ron Remillard, an astronomer at MIT's Kavli Institute. "For the group of suspected black holes we studied, there is a complete absence of X-ray bursts. The gas that would fuel such bursts appears to vanish."

The findings were presented at the 207th meeting of the American Astronomical Society.

"Proving that something has an event horizon is probably an impossible test, but we can test our thinking about how they form and how they lie in space and curve space time," Remillard said.

One of the ways to test whether event horizons exist is to show that black holes don't have conventional surfaces made up of normal matter, said Kimberly Weaver, a scientist from NASA's Goddard Space Flight Center who was not involved in the study. While the current findings on their own do not add up to definitive proof for the existence of event horizons, they do strongly support previous findings, including one that found similar phenomena occurring with ultraviolet light, she added.

A neutron stars forms when a star 10 to 25 times more massive than our Sun runs out of fuel and expels most of itself into space. The remains, typically one or two solar masses, collapses into a compact sphere about 10 miles across.

When stars with more than 25 solar masses collapse, they're thought to become black holes with infinite densities and no surfaces. A black hole is thought to be surrounded by an event horizon, a spherical region of space that extends about 50 miles from its center. Within the event horizon, the pull of gravity is so strong that nothing, not even light, can break free.

Like neutron stars, black holes can siphon off another star's gas if the pair are close enough, but because they don't have surfaces for the gas to collect upon, black holes can't produce X-ray bursts.

The idea of using the lack of X-ray bursts to confirm the existence of event horizons has been proposed before, but the current study improves upon earlier research by giving a better account of the conditions that give birth to such explosions. It also allows scientists to calculate how many X-ray bursts should occur when the amount of gas accumulating on the neutron star's surface is known.

Weaver said that knowledge about event horizons could prove important for future NASA missions that plan to map black holes using electromagnetic radiation like X-rays.

"In one case we want to image an event horizon and the idea is to use X-rays to do that," Weaver said. "So if there's a problem with using typical electromagnetic radiation to study black holes, we want to know that now."

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