black hole Using four NASA space observatories, astronomers have shown that a flaring
black hole source has an accretion disk that stops much farther out than
some theories predict. This provides a better understanding of how energy is
released when matter spirals into a black hole.

On April 18, 2000, the Hubble Space Telescope and the Extreme Ultraviolet
Explorer observed ultraviolet radiation from the object known as XTE
J1118+480, a black hole roughly seven times the mass of the Sun, locked in a
close binary orbit with a Sun-like star. Simultaneously, the Rossi X-ray
Timing Explorer observed high-energy X-rays from matter plunging toward the
black hole, while the Chandra X-ray Observatory focused on the critical
energy band between the ultraviolet and high-energy X-rays, providing the
link that tied all the data together.

“By combining the observations of XTE J1118+480 at many different
wavelengths, we have found the first clear evidence that the accretion disk
can stop farther out,” said Jeffrey McClintock of the Harvard-Smithsonian
Center for Astrophysics who led the Chandra observations. “The Chandra data
indicate that this accretion disk gets no closer to the event horizon than
about 600 miles, a far cry from the 25 miles that some had expected.”
Scientists theorize that the accretion disk is truncated there because the
material erupts into a hot bubble of gas before taking its final plunge into
the black hole.

Matter stripped from a companion star by a black hole can form a flat,
pancake-like structure, called an “accretion disk.” As material spirals
toward the inner edge of the accretion disk, it is heated by the immense
gravity of the black hole, which causes it to radiate in X-rays. By
examining the X-rays, researchers can gauge how far inward the accretion
disk extends.

Most astronomers agree that when material is transferred onto the black hole
at a high rate, then the accretion disk will reach to within about 25 miles
of the event horizon — the surface of “no return” for matter or light
falling into a black hole. However, scientists disagree on how close the
accretion disk comes when the rate of transfer is much less.

“The breakthrough came when Chandra did not detect the X-ray signature one
would expect if the accretion disk came as near as 25 miles,” said Ann Esin,
a Caltech theoretical astrophysicist who led a group that explored the
implications of the observations. “This presents a fundamental problem for
models in which the disk extends close to the event horizon.”

In March 2000, XTE J1118+480 experienced a sudden eruption in X-rays that
led to the discovery of the object by RXTE. The X-ray source was in a
direction where absorption by gas and dust was minimal, allowing ultraviolet
and low-energy X-rays to be observed. In the following month, an
international team organized observations of XTE J1118+480 in other

Chandra observed XTE J1118+480 for 27,000 seconds with its Low-Energy
Transmission Grating (LETG) and the Advanced CCD Imaging Spectrometer
(ACIS). The research team for this investigation also included scientists
from both the United States (CfA, MIT, University of Notre Dame, Lawrence
Livermore National Laboratory, NASA Goddard Space Flight Center) and the
United Kingdom (The Open University, University of Southampton, Mullard
Radio Astronomy Observatory).

The LETG was built by the SRON and the Max Planck Institute, and the ACIS
instrument by the Massachusetts Institute of Technology, Cambridge, Mass.,
and Penn State University, University Park. NASA’s Marshall Space Flight
Center in Huntsville, Ala., manages the Chandra program. TRW,Inc., Redondo
Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian’s
Chandra X-ray Center controls science and flight operations from Cambridge,

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