Spring 2002 -- Vol. 19, No. 1

X-Ray Images of Quasars Confirm Results of Penn State Graduate Student

Astronomers at the Harvard-Smithsonian Center for Astrophysics have published research results that provide confirmation of a common feature among quasars, which are accreting, supermassive black holes that gobble up matter at a rate of more than one solar mass each year and produce enormous amounts of energy and light. The researchers' results, based on observations made with the Chandra X-ray Observatory, confirm results published earlier by a team led by Sarah Gallagher, a graduate student at Penn State.

According to Gallagher's research, "shrouded" quasars, those that are seen through material blowing off the accretion disk in an energetic wind, do produce important X-ray emissions. Her esearch showed definitively for the first time that there was absorption of X-rays by material along the line of site and the researchers from the Harvard-Smithsonian Center for Astrophysics have confirmed this result with a larger sample.

She made her initial findings in 1999 and 2000 with the Advanced Satellite for Cosmology and Astrophysics (ASCA), a joint project between NASA and Japan that has since fallen back to Earth. She has been working on extending those results using Chandra.

"You can think of the quasar as being sort of a messy eater," Gallagher said. "It does not eat everything that comes close to the black hole and maybe it spits out as much as it eats as a wind coming off the accretion disk. If you look through the wind, then you see a 'hrouded' quasar."

Having found a way to see through the shroud to the X-ray source close to the black hole itself has provided an important opportunity for astronomers.

"X-rays are generated closer to the black hole than other type of light, and so they are important because they give you information about the environment that you could not get in any other way," Gallagher said. "The reason this particular class of quasars is interesting is that they're the only ones that allow you to study the wind. If you want to know what the environment is like, you want to look through that material. Our next step would be to make a connection with what we see with an optical telescope to try to predict what we're going to see when we look at a shrouded quasar with an x-ray telescope."

Chandra's Advanced CCD Imaging Spectrometer (ACIS) was conceived and developed for NASA by Penn State and Massachusetts Institute of Technology under the leadership of Penn State professor of astronomy Gordon Garmire. The ACIS detector is a sophisticated version of the CCD detectors commonly used in digital cameras and video cameras. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. TRW, Inc., of Redondo Beach, California, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Massachusetts.

Steve Sampsell

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