Summer 2000 -- Vol. 17, No. 1

Chandra Uses X-Rays to Gauge the Wind Near a Supermassive Black Hole

NASA's Chandra X-ray Observatory has revealed in unprecedented detail an X-ray-rainbow coming from a
galaxy with a supermassive central black hole about 10 million times the mass of our Sun, report researchers
from Penn State and Tel-Aviv universities in a paper to be published in June 2000 in the Astrophysical Journal Letters.  The rainbow, which astronomers call a grating-resolution X-ray spectrum, is 50 times more detailed than previous X-ray observatories could obtain before Chandra was launched last year.

Comparable to the rainbow of colors that make up visible light, the X-ray spectrum of this galaxy, named NGC 3783, reveals physical conditions in its violent core.

This information has allowed researchers to gauge the dynamics and chemistry of a powerful wind near the black hole, which itself is impossible to detect directly because nothing within its tremendous gravitational grasp can escape, not even energy like light and X-rays.

The researchers discovered the wind, which is composed of hot gas, is being blown away from the black hole at about a million miles an hour.

The spectrum of the gas, is "fantastically complex and rich," said Shai Kaspi, a postdoctoral fellow at Penn State and the research team's expert at interpreting highly detailed spectra. It reveals that the wind contains ions of oxygen, neon, magnesium, silicon, sulfur, argon, and iron.

Unlike our Milky Way, NGC 3783 is among the small percentage of spiral galaxies classified as a "Seyfert 1," with spiral arms surrounding a very violent center, known as an active galactic nucleus (AGN), powered by a supermassive black hole.

Astronomers knew many of the characteristics of Seyfert galaxies from studies with optical telescopes.

They also knew from previous X-ray observations that hot gas in a wind, a so-called "warm absorber," was blocking Earth's view of much of the abundant X-rays generated by the AGN, but they knew very little about the wind's structure and motions.

Now, using the Chandra X-ray Observatory, the researchers have been able to measure these key quantities.  "This is the most detailed X-ray spectrum ever taken of a Seyfert galaxy," Kaspi said.

The astronomers also found that the warm absorber is a major component of the nucleus of this Seyfert galaxy, containing over 10 times as much gas as the material that produces the most prominent optical emissions.

"The central black hole in NGC 3783 is about the diameter of our inner solar system, but it produces more radiation than a billion stars as it feeds on the matter nearby," explained Niel Brandt, assistant professor of  astronomy and astrophysics at Penn State and one of the leaders of the research team.

This powerful radiation creates heat as it ionizes atoms in the surrounding gas, ripping away their electrons and creating a warm-absorber wind at temperatures of about a hundred thousand degrees Celsius.  The researchers used the X-ray spectra like a traffic officer uses a radar gun, to measure velocities in the extreme environment of the galaxy's core.

"We targeted this galaxy with the Chandra X-ray Observatory because, among all the Seyfert galaxies people have studied, this one had consistently the strongest blocking of its X-ray spectrum by a warm absorber," Brandt explained. The researchers used the powerful X-rays created close to the black hole to penetrate through the warm absorber and reveal its contents--similar to the way medical X-rays on Earth penetrate through skin to reveal bones--by using two of Chandra's instruments to produce the first grating-resolution X-ray spectra of NGC 3783.

First, the team used Chandra's prism-like High-Energy Transmission Grating Spectrometer to separate the X-rays into discrete energy bands.  Then, they used the camera-like Advanced CCD Imaging Spectrometer (ACIS) to detect and record the X-ray-spectra data, which computers translated into a jagged-line plot that looks somewhat like an electrocardiogram.  Specific elements, such as carbon or oxygen, absorb specific X-ray wavelengths and reveal their presence by sharp dips in the jagged-line plot.

The detection of the elements in the AGN supported existing theoretical models of Seyfert galaxies that predicted the X-ray signatures of these elements would be there.

Such models have been developed by researchers around the world including Hagai Netzer, the Jack Adler
Professor of Extragalactic Astronomy at Tel-Aviv University in Israel and one of the team's leaders.  Netzer's
analyses of the new Chandra data indicate that the warm absorber is almost completely covering the central black hole in NGC 3783.

"Before Chandra, we would have had only about 15 data points for this entire range of X-ray energies, but now we have on the order of 500," Kaspi said.

"The best spectra obtained by previous X-ray observatories were considered crude by astronomers like Kaspi, who were used to working with the excellent spectra produced by optical observatories like the Hobby-Eberly Telescope," said Brandt.  "Now that we have X-ray spectra as superb as those from optical instruments, we can finally probe the chemical composition and atomic physics of this gas millions of light years away--the same way chemists and physicists on Earth probe substances in a lab."

The X-rays are allowing astronomers to study precisely these extremely powerful gas flows that are thought to be present in most Seyfert galaxies but have been impossible to see clearly before.

The research team includes Kaspi, Brandt, Rita Sambruna, George Chartas, Gordon Garmire, and John
Nousek at Penn State and Hagai Netzer at Tel-Aviv University in Israel.  The High-Energy Transmission
Grating Spectrometer was built by the Massachusetts Institute of Technology.

Back to Science Journal Summer 2000 Index

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