The Chandra space telescope has spied a black hole that is far less voracious than expected.
Using four NASA space observatories, astronomers have shown that the gravitational, matter-sucking reach of a flaring black hole fails to extend as far as predicted.
Many black holes are fed by companion stars from which black holes strip matter to form a flat, pancake-like structure called in accretion disk. As matter spirals toward the inner edge of the accretion disk, it is heated by the immense gravity of the black hole that causes it to radiate in X-rays.
By examining the X-rays, researchers gauge how close the accretion disk can approach the black hole before it is assimilated.
Most astronomers agree that when material is transferred onto the black hole at a high rate, the accretion disk will reach to within about 25 miles (40 kilometers) 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.
Observations of an object known as XTE J1118+480, a black hole roughly seven times the mass of the Sun, surprised astronomers.
"The Chandra data indicate that this accretion disk gets no closer to the event horizon than about 600 miles (965 kilometers), a far cry from the 25 miles that some had expected," said Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics who led the Chandra observations.
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.
This provides a better understanding of how energy is released when matter spirals into a black hole.
How the discovery was made
In March 2000, XTE J1118+480, a black hole roughly seven times the mass of the Sun and locked in a close binary orbit with a Sun-like star, experienced a sudden eruption in X-rays that led to the discovery of the object by the Rossi X-ray Timing Explorer telescope.
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 wavelengths.
On April 18, 2000, the Hubble Space Telescope and the Extreme Ultraviolet Explorer observed ultraviolet radiation from the object.
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.
"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."
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