CAMBRIDGE, Mass. – Gamma-ray bursts, extremely powerful explosions
occurring in distant parts of the universe, may be the energetic
offspring of a cosmic dance between black holes and their
dance-partner stars, said scientists from the Massachusetts Institute
of Technology and Tel Aviv University in the Feb. 21 issue of
Science. But they show that the bursts are only a small portion of
the total energy released during this cosmic tango.

As a black hole and its partner-really the remnants of a star that
has become a donut-like ring (or torus) spinning around the black
hole-rotate faster and faster in a cosmic tango, the black hole
eventually draws its partner in so close that the star is destroyed,
and the black hole gasps for its own final breath.

During this dance, the torus operates much like a passive partner,
receiving enormous amounts of energy from the black hole and
radiating that energy out in all directions through the magnetic
field lines entangling the two cosmic bodies like entwined arms.

In their paper, the scientists present a model for the energy
released during this dance. The model describes how thÿÐæ™ergy later
detected as gamma-ray bursts is actually released in beamed jets
along the black hole’s rotational axis, while the majority of the
black hole’s energy, about 100 times more, radiates out from the
torus as gravitational waves.

The model solves a paradox involving recent observational evidence
that gamma-ray bursts are focused beams of moderate energy. But the
rotating black holes (known as Kerr black holes), which are
considered a possible source of gamma-ray bursts, release large
amounts of energy omnidirectionally, the scientists assert in their
paper. They have accounted for both phenomena in the model.

“Of all astrophysical transient events, cosmological gamma-ray bursts
are potentially the most prominent laboratory for studying Kerr black
holes,” said Maurice van Putten, assistant professor of mathematics
at MIT and lead author of the paper.

“The observed time-variability shows that the source of these
gamma-ray bursts is very compact, perhaps 10 to 30 kilometers in
diameter, yet highly energetic,” said van Putten. “All else equal, a
Kerr black hole, which spins so fast that one-third of its mass is
stored in rotational energy, is considered by many a strong

According to the inferences of Einstein’s theory of general
relativity, Kerr black holes have no detailed texture on their
surface. Van Putten believes that, if active, Kerr black holes would
be luminous in all directions. While their powerful “dark” emissions
of gravitational waves have not been picked up yet by the detection
equipment presently in use, they could be detected in the next decade
using LIGO, the Laser Interferometer Gravitational-wave Observatory,
a new observatory that went into operation about two years ago.

The torus, which is being pulled around by the black hole’s
exceedingly strong gravitational grasp, rotates around the black hole
for about 20 seconds before being consumed. During that time, jets of
energy are emitted along the black hole’s axis of rotation when the
black hole reaches a rotational speed of more than twice that of the
torus. These jets burst into a firework of gamma rays as they
dissipate their kinetic energy some 10 billion miles away from the
black hole. The bursts were discovered serendipitously by American
and Russian satellites some three decades ago.

Because 20 seconds is a long time for a torus to survive, the
scientists assert that it must be accepting energy from the black
hole and passing it along as gravitational waves radiating out in all
directions. If the torus rotated without receiving energy from the
black hole, it would be sucked into the hole. If it received all that
energy without passing it into the atmosphere, it would blow apart.

“How can a torus around a black hole as small as 20 kilometers
survive for such a long time? We propose that in some 10,000 orbits
around the black hole, the torus forms an efficient catalytic
converter of black hole spin energy before its final destruction,”
said van Putten.

If the proposed model is correct, the gravitational waves emitted
from around the torus of the Kerr black hole will be detected
eventually by LIGO. This could lead to identification of the first
known Kerr black hole, an object which was found as an exact solution
to the Einstein equations in 1963 by Roy P. Kerr, but whose existence
as a cosmic body has not been proved yet by observation.

But until LIGO-the first instrument able to detect gravitational
waves-reaches its highest level of sensitivity in 2008, scientists
have no way of actually measuring the gravitational waves.

In 2000, LIGO was inaugurated in Livingston, La. with a twin facility
in Hanford, Wash. The project is a joint effort of the California
Institute of Technology, MIT and about 20 other institutions
represented in the LIGO Scientific Collaboration supported by the
National Science Foundation.

The facility represents years of effort on the parts of MIT Professor
of Physics Emeritus Ranier Weiss and many other scientists. David
Shoemaker, of MIT’s Center for Space Research, now directs the MIT
LIGO Laboratory. A similar facility, VIRGO, jointly owned by France
and Italy, was recently inaugurated in Pisa, Italy.

“Association of gravitational-wave bursts with the radio afterglows
that should appear several months after the explosion may provide
additional tests for this model, and may help in analyzing future
LIGO data,” said Amir Levinson of the School of Physics and Astronomy
at Tel Aviv University, second author of the paper.

Other collaborators on the research are Eve C. Ostriker of the
University of Maryland, Hyun-Kyu Lee of Hanyang University in Korea
and Abhinanda Sarkar of GE, India. Van Putten’s research is funded by
NASA, MIT’s C.E. Reed Fund and NATO.

“Making predictions of what LIGO will find is like walking into a
dark room. Before you turn on the light, you don’t know exactly what
you’ll see, but you have some good ideas of what will be there,” said
van Putten. “In 2008 it will get serious. We will see what’s really
in the room. Right now, we’re trying to guess which source is out
there and which will be the best candidate for study.”