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Scientists Spot Doughnut-Shaped Cloud With a Black Hole Filling

20 July 2004

Illustration of a doughnut shaped ring encircling a supermassive black hole.
Illustration of a dark doughnut-shaped ring deep in the core of a galaxy encircling what appears to be a supermassive black hole.
(Credit: ESA.)

An international team of scientists has found more evidence that massive black holes are surrounded by a doughnut-shaped gas cloud which, depending on our line of sight, blocks the view of the black hole in the center.

Using two European Space Agency orbiting observatories, INTEGRAL and XMM-Newton, scientists looked "edge on" into this doughnut (called a torus by scientists) to see features never before revealed in such clarity. They could infer the doughnut structure and its distance from the black hole by virtue of light that was either reflected or completely absorbed. How the doughnut forms, however, remains a mystery.

"By peering right into the torus, we see the black hole phenomenon in a whole new light, or lack of light, as the case may be here," said Dr. Volker Beckmann of NASA Goddard Space Flight Center in Greenbelt, Md., the lead author on an upcoming article in The Astrophysical Journal. "This torus is not as dense as a Krispy Kreme doughnut, but it is far hotter (up to a thousand degrees) and loaded with many more calories."

Black holes are objects so dense and with gravity so strong that not even light can escape from them. Scientists say that "supermassive" black holes are located in the cores of most galaxies, including our Milky Way galaxy, and contain the mass of millions to billions of suns confined within a region no larger than our Solar System.

Supermassive black holes appear to be surrounded by a hot, thin disk of accreting gas and, farther out, the thick doughnut-shaped torus. Astronomers often view black holes that are aligned face-on or at a slight angle in relation to Earth, thus avoiding the dark, enshrouding torus to study the hot accretion disk.

Beckmann's group took the path less trodden and observed a black hole with a theorized torus directly in the line of sight. X-ray and gamma-ray light, as detected by XMM and INTEGRAL, respectively, partially penetrates the torus. The new view through the haze provides valuable insight into the relationship among the black hole, its accretion disk and the doughnut.

The scientists observed a black hole in the spiral galaxy NGC 4388, which is 65 million light years from Earth in the constellation Virgo. This galaxy is a Seyfert 2, referring to the type of black hole in the core -- that is, one that is enshrouded from our vantage point.

An image of NGC 4388 in infrared wavelengths, captured by ground-based Subaru telescope.
An image of NGC 4388 in infrared wavelengths, captured by ground-based Subaru telescope. We see the entire galaxy. The black hole (and its accretion disk and doughnut ring) would be just a dot in the galaxy core. Seeing galaxies in all wavelengths -- that is, with radio, infrared, optical, ultraviolet, X-ray and gamma-ray telescopes -- reveals the entire workings of the the galaxy, from star creation (birth) to black hole activity (death).
(Credit: NAOJ/Subaru.)

Seyfert 2 galaxies are usually faint to optical telescopes. The torus model is one explanation. Another theory is that the central black hole, for reasons unclear, is not actively accreting gas and is therefore faint. (Accretion produces energy, or light.) NGC 4388 is relatively close and therefore an unusually bright Seyfert 2, easy to study.

The new observation supports the torus model in several ways. Gas in the accretion disk close to the black hole reaches high speeds and temperatures (over 100 million degrees, hotter than the Sun) as it races toward the void. The gas radiates predominantly at high energies, in the X-ray wavelengths. This light, which is able to escape the black hole because it is still outside of its border, ultimately collides with matter in the torus. Some of it is absorbed; some of it is reflected at different wavelengths, like sunlight penetrating a cloud; and the very energetic gamma rays pierce through.

Beckmann's group saw how different processes around a black hole produce light at different wavelengths. For example, some of the gamma rays produced close to the black hole get absorbed by iron atoms in the torus and are reemitted at a lower energy. This in fact is how the scientists knew they were seeing "reprocessed" light farther out. Also, because of the line of sight toward NGC 4388, they knew this iron was from a torus on the same plane as the accretion disk, and not from gas clouds "above" or "below" the accretion disk.

Lower-energy X rays (below 2.5 kilo-electron volts) appear to be from a diffuse emission far away from the black hole. Higher-energy X rays (above 2.5 keV) are directly related to black hole activity. The torus itself appears to be several hundred light years from the black hole.

Dr. Beckmann said the observation could not gauge the diameter of the torus, from inside to outside. Other scientists say that the doughnut shape is more intact closer to the accretion disk, but that it cannot maintain structural integrity farther away, perhaps resembling a doughnut with part of its edges eaten away.

The result marks the clearest observation of an obscured black hole in X-ray and gamma-ray "colors," a swatch of energy nearly a million times wider than the window of visible light, from red to violet. Multiwavelength studies are increasingly important to understanding black holes. XMM-Newton was launched in December 1999, and INTEGRAL was launched in October 2002.

Dr. Beckmann is a visiting scientist at NASA Goddard through the University of Maryland, Baltimore County. His coauthors on the Astrophysical Journal article are: Dr. Neil Gehrels of NASA Goddard; Pascal Favre, Dr. Roland Walter and Prof. Thierry Courvoisier of the INTEGRAL Science Data Centre in Switzerland; Dr. Pierre-Olivier Petrucci of the Laboratoire d'Astrophysique de Grenoble in France; and Dr. Julien Malzac of the Centre d'Etude Spatiale des Rayonnements in France and the Institute of Astronomy, University of Cambridge, U.K.

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