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

29 May 2001 -- 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 research 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, and 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 "shrouded" 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 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.

A complete release regarding the results made by the team from the Harvard-Smithsonian Center for Astrophysics follows.

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CONTACTS:

Sarah Gallagher gallsc@astro.psu.edu
Niel Brandt (814) 865-3509 / niel@astro.psu.edu
Steve Sampsell, PIO (814) 865-1390 / sws102@psu.edu

(more images)

This illustration demonstrates the possible different points-of-view from which astronomers observe quasars with X-ray satellites. If a quasar is oriented so that an observer’s vantage point looks straight down the top of a quasar, then their view will not be obscured by the “donut” of gas and dust surrounding the core. This is the situation that astronomers believe occurs in “normal” quasars. However, 10 percent of quasars appear to absorb a great deal of their own radiation, including low-energy X-rays. Recent data from Chandra indicate that “shrouded” quasars appear this way because they are oriented so that astronomers are looking through the side of the obscuring ring of hot gas and dust. However, Chandra reveals that the underlying supermassive black holes behave like other quasars, and suggests that all quasars are the same types of object but just viewed from different angles. Image: Chandra X-ray Observatory

A QUASAR'S IDENTITY MAY SIMPLY BE IN THE EYE OF THE BEHOLDER

Using NASA's Chandra X-ray Observatory, astronomers have made the first detailed study of a peculiar type of quasar that is shrouded in clouds of gas and dust flowing outward at millions of miles per hour. The results support the idea that this outflow is a common feature of all quasars, highly active supermassive black holes that give the illusion of being different when viewed from various angles.

Quasars are some of the most energetic and distant known objects in the universe. Most quasars are extremely bright in optical light, but about 10 percent appear "shrouded," or hidden, by absorbing clouds of gas and dust. In addition to these obscuring clouds, the same subset of quasars shows evidence of extremely energetic winds blasting outward from the central regions.

Astronomers have debated whether these shrouded quasars represent an early evolutionary stage of black holes when they vigorously consume matter, or whether these energetic outflows are present in all quasars, but detectable only when viewed in certain orientations.

"Because high-energy X-rays can pierce through these clouds, we can use Chandra to observe close to the underlying black hole," said Paul Green of the Harvard-Smithsonian Center for Astrophysics and lead author of paper to appear in The Astrophysical Journal. "Looking through these veils, we find that the extremely hot gas around these supermassive black holes shines just the same way as in non-shrouded quasars."

Green and his colleagues used Chandra to survey ten shrouded quasars through a process known as spectroscopy, the study of how atoms absorb and emit light in the electromagnetic radiation. X-ray spectroscopy provides astronomers with a unique ability to "fingerprint" very high-energy objects at great distances.

"Chandra is beginning to show us that these quasars are all the same underneath, regardless of what they are wearing on the outside," said Tom Aldcroft, another member of the research team.

"Our work lends weight to the theory that all quasars possess obscuring donuts of thick gas and dust," said Smita Mathur of The Ohio State University. "However, the difference is that some 'normal' quasars are being observed through the top into the donut hole, while the shrouded ones are being seen through the side."

The work by Green and collaborators confirms an earlier finding by a Penn State team led by Sarah Gallagher. Her team used the ASCA satellite to examine one relatively nearby quasar with an enshrouding wind. Gallagher is currently working to extend these results with Chandra, which is also revealing the familiar, but previously hidden, X-ray emission from several other shrouded quasars. Their results, along with other observations of more distant and hence younger quasars of this type, indicate that the "donuts" may be more common, or perhaps thicker in younger quasars.

The research team led by Green used the Advanced CCD Imaging Spectrometer (ACIS) instrument to survey 10 different quasars. The exposure times of these observations ranged from 1,300 to 5,400 seconds.

ACIS was conceived and developed for NASA by Penn State and Massachusetts Institute of Technology under the leadership of Penn State professor 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.

Images associated with this release are available on the World Wide Web at: http://chandra.harvard.edu AND http://chandra.nasa.gov

CONTACTS:

Steve Roy
Marshall Space Flight Center, Huntsville, AL
Phone: 256-544-6535

Megan Watzke
Chandra X-ray Observatory Center, CFA, Cambridge, MA
Phone: 617-496-7998
cxcpress@cfa.harvard.edu

These two Chandra images demonstrate the different appearances of "normal" and "shrouded" quasars. In these images, blue represents high-energy X-rays while red corresponds to lower-energy X-rays. The image on the left, created by combining the light from 8 of these shrouded quasars, appears distinctly blue. This is because the low-energy X-rays from the quasars have been absorbed by an obscuring "donut" of gas and dust surrounding their cores. In comparison, the Chandra image of the non-shrouded quasar PG 1634+706 shows that low-energy X-rays escape without the masking clouds of gas and dust. Recent data from Chandra reveal that -- despite their differences in appearance -- these two types of quasars are the same types of object, but viewed from a different angle. (Credit: NASA/CfA/P.Green et al.)