Staff Writer - Space.com: Vanishing Gas Confirms Black Hole Event Horizons
Vanishing Gas Confirms Black Hole Event Horizons
http://space.com/scienceastronomy/060109_event_horizon.html
By Ker Than
Staff Writer
09 January 2006 ET
Washington, DC-- 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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