William.A.Steigerwald.1@gsfc.nasa.gov
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RELEASE NO: 97-141
ASTRONOMERS DISCOVER INDICATOR THAT REVEALS RATE OF MATTER CONSUMPTION BY ENORMOUS BLACK HOLES
Astronomers working with the Japanese/NASA Advanced Satellite for Cosmology and Astrophysics (ASCA) have found an indicator of the rate at which giant black holes at the centers of distant galaxies are swallowing matter from their surroundings. The indicator consists of X-ray emissions from very energetic iron atoms swirling in toward the edge of the black hole.
"Now, we can observe quasars and other Active Galactic Nuclei and explore what's happening in the immediate environs of their black holes," said Dr. Paul Nandra of NASA's Goddard Space Flight Center in Greenbelt, Md. Nandra leads a team of astrophysicists at Goddard's Laboratory for High Energy Astrophysics that published its results recently in the rapid release web pages of the Astrophysical Journal Letters. It will appear in the Oct. 20, 1997 printed edition of that journal.
ASCA was used to observe a so called X-ray "emission line" produced by iron atoms close to giant black holes believed to be lurking in the centers of these remote galaxies. "This black hole indicator can be thought of as the final cry of doomed matter as it slides down the throat of a black hole," said Nandra. "We found that as black holes swallow material at a greater rate, the X-ray emission lines become dimmer and less distorted." As a result, what he calls "this vanishing emission line" was adopted by the research team as an indicator of black hole material consumption.
"The new findings, like all discoveries, need follow - up with further observations and study, but this is a really exciting and useful result," added Dr. Richard Mushotzky, a senior member of Goddard's research team.
Galaxies are immense collections of billions of stars and interstellar matter held together loosely by their mutual gravity. A typical black hole is the collapsed core of a massive star that has exploded. The gravitational field near it is so intense that nothing, not even light, can escape. Much larger black holes, whose origins are uncertain, are believed to be the powerhouses in the hearts of certain distant galaxies with very bright centers. These bright centers, called Active Galactic Nuclei (AGN) are what the research team studied with the ASCA satellite. The researchers observed almost 40 AGN to establish the X-ray emission line as a reliable indicator of the prodigious rates at which the black holes are consuming matter from their surroundings.
"By studying how this indicator disappears, we can understand the physics of material near black holes and determine why some of these AGN are so highly energetic," said Nandra. According to his description of conditions in an AGN, "The gravity of the giant black hole pulls surrounding gas from the densely packed galactic center towards it. As this gas falls into the black hole, it forms a flattened, rotating structure called an accretion disk, much like soap suds swirling around a bathtub drain. As the gas swirls around it is compressed and heated. The gas in the accretion disk is also being bombarded by X-ray radiation, which further energizes the gas and causes it to emit other X-rays as well. These processes cause the swirling matter to produce the iron emission line that our team studied, which was already regarded as a signature of the presence of a black hole when the line is distorted by the black hole's gravity. The new work shows that this signature can also tell how rapidly the black hole is gathering in matter from the accretion disk."
"We studied these indicators from various types of AGN, including the most powerful galaxies known, some of which are at the edge of the observable universe. We found that as the brightness of these objects increased, this indicator became progressively less distorted and weaker until it disappeared altogether."
"We do not believe that this is due to there being no black hole in the most powerful AGN. It is more likely that the conditions of matter being swallowed by the black hole are different."
The researchers believe the difference is due to the AGN's energy output. As the iron atoms become more energetic, electrically charged particles called electrons get torn from them. In the most powerful AGN, conditions are so extreme that all the electrons get stripped from the iron atoms. When this happens, the indicator disappears, because the electrons surrounding the iron atoms produce the light for the emission line in the first place.
The most energetic of the AGN, extremely distant objects called quasars, shine with the light of up to 10,000 galaxies, each consisting of one hundred billion stars. The researchers found that in quasars, the black hole indicator is gone, while in lesser energetic AGN, the indicator is present.
"This, in turn, suggests an increased rate of matter consumption by the black holes in quasars and implies why some AGN are so much brighter than others," said Nandra. "Two things can change the amount of light energy output, or luminosity, from an accretion disk surrounding a black hole. The first is the mass of the black hole, and the second is the rate at which stuff falls into the black hole, called the accretion rate. As the mass of a black hole increases, the temperature of the material in the accretion disk should go down. Conversely, as the accretion rate increases, the temperature should increase as well."
"As we see the iron atoms becoming effectively 'hotter' as the energy output increases, this indicates that the incredible luminosity of quasars is due to a higher accretion rate than that of other AGN, rather than from the existence of a more massive black hole," said Nandra.
This file was last modified on Monday, 13-Sep-1999 15:37:28 EDT
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