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Chandra observes star's corona using MIT instrument

The high-energy transmission grating (HETG) consists of 336 gold grating facets mounted on an assembly that can be swung into position behind the Chandra mirrors. The inner two rings are high-energy grating facets, and the outer two rings are medium-energy grating facets.
The high-energy transmission grating (HETG) consists of 336 gold grating facets mounted on an assembly that can be swung into position behind the Chandra mirrors. The inner two rings are high-energy grating facets, and the outer two rings are medium-energy grating facets.
Image courtesy / MIT Center for Space Research

NASA's Chandra X-Ray Observatory opened a new era in astronomy last weekend by making the most precise measurements ever recorded of the energy output from the 10-million-degree corona of a star. The observations came after the successful August 28 activation of an instrument developed at MIT that will allow a thousandfold improvement in the capability to measure X-ray spectra from space.

"This was absolutely exhilarating for me and everyone else [present]," said Professor Claude R. Canizares, principal investigator for the instrument and associate director of the Chandra X-ray Observatory Center (CXC).

The new measurements, made with the High Energy Transmission Grating (HETG) Spectrometer, join spectacular Chandra images released last Thursday of the aftermath of a gigantic stellar explosion. The instrument that made those first images was developed jointly by MIT and Pennsylvania State University (see MIT Tech Talk, August 11). Two other key instruments aboard Chandra will be turned on over the next two weeks.

HETG spreads the X-rays from Chandra's mirrors into a spectrum, much as a prism spreads light into its colors. The spectrum then can be read by Chandra's imaging detectors like a kind of cosmic bar code from which scientists can deduce the chemical composition and temperature of the corona. A corona is a region of hot gas and magnetic loops that extends hundreds of thousands of miles above the star's visible surface and is best studied via X-rays.

"The success of the new spectrometer is definitely a major milestone for modern astronomy," said Professor Canizares, the Bruno Rossi Professor of Physics and director of MIT's Center for Space Research. "Within the first hour, we had obtained the best X-ray spectrum ever recorded for a celestial source. We can already see unexpected features that will teach us new things about stars and about matter at high temperatures."

HETG measured X-rays from the star Capella, which is 40 light years away in the constellation Auriga. Capella is actually two stars orbiting one another and possibly interacting in ways that pump extra heat into the corona, which appears to be more active than that of the sun. How a star manages to heat its corona to temperatures a thousand times higher than its own surface is still a puzzle, which astronomers hope can be solved by observations like this one.

Other prime targets for Chandra's spectrometers over the next few months include black holes, quasars and supernova explosions.

The grating spectrometer consists of hundreds of gold gratings, each about the size of a postage stamp. The surface of each grating resembles a precise picket fence, with microscopic gold pickets 500 times thinner than a human hair. These are spaced every 2000 angstroms, or less than half the wavelength of visible light.

The instrument was developed at the Center for Space Research by adapting techniques usually used to make computer chips. Some of these adaptations have found their way back as improvements in the chip-making industry.

The grating spectrometer is one of two such devices carried by Chandra. The other, a low-energy grating built by a Dutch-German team, will be activated next week. Chandra also contains two detectors. One, built by researchers at Pennsylvania State University and MIT, was turned on two weeks ago and has recorded all the images and spectra seen so far. The second, built by the Smithsonian Astrophysical Observatory, is being activated this week.

Dr. Stephen Murray of the Harvard-Smithsonian Center for Astrophysics summarized the expected impact of Chandra's high resolution X-ray spectroscopy: "A picture is worth a thousand words, a spectrum is worth a million."

Overseeing the HETG are Professor Canizares; Drs. Mark Schattenburg, Daniel Dewey and Kathryn Flanagan (lead scientists); Eugene Galton (project manager) and Dr. Michael McGuirk (deputy project manager). The co-investigator is Professor Henry I. Smith of the Department of Electrical Engineering and Computer Science. The late Dr. Thomas Markert was also a project scientist.

A Center for Space Research web page shows Capella's spectrum and further information about Chandra's high-energy transmission grating spectrometer. The first Chandra images and more information on the Chandra X-ray Observatory Center are available on Harvard and NASA web sites.

The Chandra X-ray Observatory Center was named in honor of the late Nobel laureate Subrahmanyan Chandrasekhar. NASA's Marshall Space Flight Center manages the Chandra program. TRW, Inc. in Redondo Beach, CA is the prime contractor for the spacecraft. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science and flight operations from Cambridge.

A version of this article appeared in the September 1, 1999 issue of MIT Tech Talk (Volume 44, Number 4).

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