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Cosmic Bar Codes

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see captionFebruary 22, 2000 -- Using NASA's Chandra X-ray Observatory, an international team of astronomers has made a high-energy bar code for hot gas in the vicinity of a giant black hole. These measurements, the most precise of their kind ever made with an X-ray telescope, demonstrate the existence of a blanket of warm gas that is expanding rapidly away from the black hole.

Jelle Kaastra of the Space Research Organization Netherlands in Utrecht and colleagues used Chandra's the Low Energy Transmission Grating in conjunction with the High Resolution Camera to measure the number of X rays present over a range of energies. With this information they constructed an X-ray spectrum of the source.

Right: This figure is the X-ray spectrum of the central region of the galaxy, NGC 5548. The spectrum shows the number of X rays present at each energy or wavelength, and amounts to a cosmic bar code. It allows scientists to take an inventory of the gas around the giant black hole in the center of the galaxy. The deep valleys in the spectrum are produced when a blanket of warm (few million degree) gas absorbs X rays of specific energies from hotter gas close to the central black hole.

Their target was the central region, or nucleus of the galaxy NGC 5548, which they observed for 24 hours. This galaxy is one of a class of galaxies known to have unusually bright nuclei that are associated with gas flowing around and into giant black holes. This inflow produces an enormous outpouring of energy that blows some of the matter away from the black hole.

Astronomers have used optical, ultraviolet, and X-ray telescopes in an effort to disentangle the complex nature of inflowing and outflowing gas at different distances from the black hole in NGC 5548. X-ray observations provide a ringside seat to the action around the black hole. By using the Low Energy Transmission Grating, the Dutch-US-German team concentrated on gas that forms a warm blanket that partially covers the innermost region where the highest energy X-rays are produced.

Below: This might resemble a fried egg you've had for breakfast, but it's actually much larger. In fact, ringed by blue-tinted star forming regions and faintly visible spiral arms, the yolk-yellow center of this face-on spiral galaxy, NGC 7742, is about 3,000 light-years across. Like NGC 5548, this galaxy is a Seyfert galaxy - a type of active spiral galaxy with a center or nucleus that is very bright at visible wavelengths. Galaxies like NGC 7742 and 5548 are thought to harbor massive black holes at their cores.

The NGC7742 Galaxy As the high energy X rays stream away from the vicinity of the black hole, they heat the blanketing gas to temperatures of a few million degrees, and the blanket absorbs some of the X rays from the central source. This produces dark stripes, or absorption lines in the X-ray spectrum. Bright stripes or emission lines due to emission from the blanketing gas are also present. Since each element has its own unique structure, these lines can be read like a cosmic bar code to take inventory of the gas. The team was able to determine what atoms the gas contains and how many, the number of electrons each atom has retained in the hostile environment of the black hole, and how the gas is moving there. They found lines from eight different elements including carbon, nitrogen, oxygen, and iron. The amount of this gas was found to be about 100 times greater than that found with optical and ultraviolet observations.

The team of scientists who conducted this research consists of Jelle Kaastra, Rolf Mewe and Albert Brinkman of Space Research Organization Netherlands in Utrecht, Duane Liedahl of Lawrence Livermore National Laboratory in Livermore, Calif., and Stefanie Komossa of Max Planck Institute in Garching, Germany. A report of their findings will be published in the March issue of the European journal Astronomy & Astrophysics.

The Low Energy Transmission Grating was built by the Space Research Institute of Netherlands and the Max Planck Institute under the direction of Albert Brinkman. The High Resolution Camera was built by the Smithsonian Astrophysical Observatory in Cambridge, Mass., under the direction of Stephen Murray.
Web Links

Chandra home page -from Harvard

Chandra News -from NASA

Black Holes -a tutorial about black holes and accretion disks

X-Rays - Another Form of Light - the basics of X-rays from the Chandra home page at Harvard