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New space telescope has a big appetite for X-rays
Monday, December 06, 1999 By Byron Spice, Science Editor, Post-Gazette
X-ray astronomy, a field made possible with the advent of spacecraft, may be entering its golden age.
The South African government is moving ahead with plans for the Southern Hemisphere's largest telescope, deciding last month to recruit a project team and begin construction of the South African Large Telescope.
Carnegie Mellon University officials last year said they want to raise $6 million for the project, which would ensure observing time on a major telescope for the university's astrophysicists.
The South Africans' Nov. 25 decision to "green light" the project closely followed an agreement with the Polish government to help finance the telescope, which is expected to cost $22 million to build and $8 million to operate for the first 10 years. The Polish contribution, combined with cash from Rutgers University and a German university, means a total of $19 million has been committed.
As of yet, Carnegie Mellon hasn't found the money for its share and isn't ready to make a firm commitment, said astrophysicist Richard Griffiths. But fund raising continues, he added, and the window of opportunity for joining the project still remains open.
The South African Large Telescope would be built at the South African Astronomical Observatory near Sutherland, about 230 miles northeast of Cape Town. Construction could begin this spring and is expected to take five years.
It would be similar in design to the Hobby-Eberly Telescope, which became operational this fall at the McDonald Observatory in Texas. Penn State University is part of a consortium that built and operates that telescope, which is named for Uniontown philanthropist Robert E. Eberly and former Texas Lt. Gov. William P. Hobby.
Both the South African and Texas telescopes feature giant, segmented mirrors roughly 400 inches in diameter. To dramatically slash costs, the large mirrors are mounted at a fixed angle on a rotating base. This design compromise means the scopes can track a star for only an hour or so, while conventional telescopes can track a star all night long if need be.
It began in July, when NASA launched its Chandra X-ray Observatory, and should be further burnished on Friday when the golden-eyed X-ray Multi-Mirror spacecraft is scheduled to lift off aboard an Ariane-5 rocket from French Guiana.
The Chandra telescope has an exceedingly sharp eye, with resolving power equivalent to being able to read a stop sign from 12 miles away. Already, it has sent back a number of startling images, showing structures invisible to optical telescopes and previous X-ray telescopes.
The European Space Agency's XMM telescope, by contrast, won't see nearly as much detail as Chandra, but will be capable of capturing much more X radiation, playing the glutton to Chandra's gourmet.
Richard Griffiths, an astrophysicist at Carnegie Mellon University, said XMM's light-gathering ability will allow it to detect very faint sources of X-rays.
It also will thoroughly analyze the X-rays from brighter sources, providing information about the temperature of the objects emitting X-rays and about their chemical makeup.
"On some objects, we'll be able to combine observations from Chandra and XMM," said Griffiths, a co-investigator on Chandra's High Resolution Camera and one of two U.S. researchers serving on XMM's scientific team. With Chandra showing shape and structure and XMM providing chemical composition and energy levels, scientists should be better able to explain what is happening in X-ray sources such as quasars, supernovae and the regions outside black holes.
XMM and Chandra will be joined early next year by yet another new X-ray telescope, Japan's Astro-E.
X-rays are abundant in space, but they are invisible to the human eye and are absorbed by the Earth's atmosphere. The existence of cosmic X-rays was confirmed only when rockets became available to lift instruments above the atmosphere. The first satellite dedicated to X-ray astronomy, Uhuru, was launched in 1970 and a succession of X-ray observatories, including Einstein, Exosat and Rosat, followed.
Chandra already has been a revelation to astronomers. For instance, it detected a brilliant ring around the heart of the Crab Nebula, the remnants of an exploded star that is the most studied object outside of the solar system.
Jeff Hester of Arizona State University said finding the never-before-seen ring may help explain how a pulsating neutron star, called a pulsar, has managed to keep the nebula glowing brightly 1,000 years after the explosion.
"It's like finding the transmission lines between the power plant and the light bulb," he explained.
Likewise, astrophysicists have puzzled for 20 years over what is powering a jet of hot gas that shoots out of the galaxy Centaurus A and extends for 25,000 light-years -- a length equivalent to the diameter of our Milky Way.
Ethan Schreier of the Space Telescope Science Institute said Chandra's images show X-ray emissions from the jet are produced by high-energy electrons spiraling around a magnetic field. The telescope also shows a bright source of X-rays at the heart of Centaurus A near what is suspected to be a supermassive black hole. Matter not sucked into the black hole can be explosively ejected.
Last week, Griffiths was analyzing Chandra data from M82, a young, "starburst" galaxy where he and a colleague last year found evidence of what may be a new class of mid-sized black holes.
The XMM spacecraft will include three barrel-shaped telescope modules. Each module has 58 concentric mirror shells, which are nested one within another like Russian dolls. Each flimsy shell is made of wafer-thin nickel with a gold reflective surface. The total mirror surface of the three modules is practically the size of a tennis court.
As an XMM mission scientist, Griffiths will get more than 100 hours of observing time on the telescope for his own projects. He hopes to use this opportunity to find the origin of the X-ray background, a faint, mysterious glow of X-rays from all over the sky. He suspects the sources of this radiation are black holes at the center of very distant galaxies, which XMM should be able to see.
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