Universe Today
Illustration of a black hole, surrounded by a torus
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The European Space Agency's High Energy Space Telescopes have discovered a donut-shaped cloud filled with a black hole
Using the Integral and XMM-Newton observatories, an international team of astronomers discovered additional evidence that massive black holes are surrounded by a doughnut-shaped cloud of gas, called a torus. At certain viewing angles, the torus may block the black hole in its center from our eyes. The team looked directly at the torus and discovered salient features never seen before with such clarity.
Black holes are objects so compressed that they create such strong gravity that even light cannot escape from them. Scientists estimate that supermassive black holes reside in the cores of most galaxies, including our own, the Milky Way galaxy. The massive black holes may contain a mass tens of thousands of millions times greater than the mass of our sun in a volume that does not exceed the volume of the solar system. The black holes are apparently surrounded by a thin, hot disk of gas, and at a greater distance from the black hole is the thick torus.
If the torus is in certain planes relative to Earth, it may hide the black hole and the hot disk of gas. Galaxies, where the torus hides the light from the hot gas disc, are called Seyfert 2 galaxies, and they are usually not bright enough to be picked up by optical telescopes. According to another theory, these galaxies appear quite faint, because the central black hole does not absorb gas, so the disk surrounding it is not so bright.
An international team of astronomers led by Dr. Volker Beckmann (Beckmann) from NASA's Goddard Space Flight Center in Greenbelt, United States, studied one of these galaxies, which is relatively close. It is a spiral galaxy, called NGC 4388, located 65 million years away in the constellation Virgo. The relative proximity of NGC 4388 makes it appear bright, making it easier to study.
Astronomers mainly study black holes, where the wide side of the torus is facing us, to avoid obscuring the black hole. Beckman's group, however, chose to take the less common route and explore the black hole through the torus. The integral and the XMM-Newton allowed them to discover some of the X-rays and gamma rays emitted by the gas cloud around the black hole. "Looking into the torus made us see the phenomenon of black holes in a whole new light, or actually the absence of light in this case," said Beckman.
Beckman's team observed how different processes around the black hole emit light at different wavelengths. For example, some of the gamma rays, created near the black hole, were absorbed by iron atoms in the torus and re-emitted at lower energies. Thus, in fact, the scientists knew that they were observing light that had undergone some process. Also, the view of NGC 4388 allowed them to know that the origin of the iron is in the torus, which is in the same plane as the accretion disk, and not in gas clouds, which are "below" or "above" the disk.
Looking through the clouds of matter has provided new insights into the relationship between the black hole, its accretion disk and the donut. In addition, the evidence supports the torus model in several ways.
Gas in the accretion disk reaches high speeds and extreme temperatures (over 100 million degrees, a temperature higher than that of the Sun) as it approaches the black hole. The gas radiates mainly at high energies in the X-ray wavelength range.
Beckman says that light is able to escape the black hole, since it has not yet entered its boundary, but it eventually collides with matter in the torus. Some of the light is absorbed; Some of it is reflected in different wavelengths, similar to sunlight, which penetrates through the clouds, and the most energetic gamma rays pass through the torus and break out. "This torus is not as dense as a donut or a real 'German karpen' (type of pastry), but it is much hotter – its temperature can reach up to 1,000 degrees – and it has a lot more calories," Beckman said.
In addition, the new observations may help identify the sources of the high-energy emission from the galaxy NGC 4388. While the lower-energy X-rays detected by the XMM-Newton appear to originate far from the black hole in the gas clouds, the higher-energy X-rays detected by the Integral are directly related to the activity the black hole
The team deduced the structure of the torus and its distance from the black hole based on the light that was reflected back from it or completely swallowed. The torus itself appears to be several light-years away from the black hole, although observations could not measure its diameter from the inside out.
The new results are the clearest observations of a black hole hidden from view in the "colors" of X-rays and gamma rays, which constitute a region nearly a million times wider in the spectrum of electromagnetic waves than the visible light range, from red to violet. Studies of black holes in many energy ranges are of great importance to understanding these objects, as a study published earlier this year showed. In May 2004, the European Astrophysical Virtual Observatory project, in which the European Space Agency ESA takes an important part, found 30 extremely massive black holes, which were previously hidden behind clouds of dust.
Translation: Dikla Oren
The article in Universe Today
Knowledge of astrophysics - stars and galaxies
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