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Fine-Tuning Astronomy

The advent of precise spectroscopy--allowing a much sharper view of the composition of celestial objects--is revolutionizing modern astronomy
Centaurus A



NASA/SAO/R. KRAFT ET AL.
Astronomy is not usually associated with high-precision measurements. Consider: this is a field in which results given to within a factor of two are still received without scorn. No one in the field seemed particularly fazed, for example, when it was erroneously announced, not long ago, that some objects (called globular clusters, groups of tightly packed stars) were measured to be older than the universe itself.

But all that is changing fast. Across most of the electromagnetic spectrum, extending from the longest radio waves to the shortest cosmic rays, astronomers now have a sharper view of objects of study. This is thanks to newer instruments that provide precise spectroscopy, which offers resolutions and light-collecting capabilities many times more sensitive than those from just a few years ago (see sidebar on spectrographs). Spectroscopy is the science of measuring line emissions from elements and using these measurements to get clues on the state (temperature, density, ionization) of the atoms.

The recent data from the new and better instruments have invalidated some old theories, allowed new ones to flourish and uncovered unexpected results. In short, they have triggered a discovery fever that is changing astronomy. A quick look at an astronomical database called the NASA Astrophysics Data System reveals that the number of papers linked to high-resolution spectral analysis submitted in the first two months of 2002 was greater than that from all the recorded years before 1996 combined.

X-Ray Eyes

Among the recent findings is the discovery of planets in other stellar systems (see sidebar). Perhaps most dramatic, however, has been a series of revelations in x-ray astronomy, which is marking its 40th anniversary in 2002. When two x-ray satellites, the NASA Chandra X-Ray Observatory and the European Space Agency's XMM (X-Ray Multi-Mirror)-Newton, both launched two years ago, x-ray astronomy leaped ahead.

The study of the universe in x-rays, which reveal details of some of the most high-energy events in the cosmos, is a relatively new addition to astronomy. That's because every x-ray detector has to be space- or airborne (see sidebar on historical missions); the atmosphere stops most of the incoming x-rays. The first detection of x-rays from space (excluding those from the sun) was made on June 18, 1962. The rocket flight that lofted the instrument lasted for less than three minutes but found what is now known to be a neutron star with a low-mass companion star, called Sco-X1. Today x-ray astronomy has become the fastest-growing area of the field. From the largest structures to the smallest objects, x-ray observations have contributed key discoveries.

Warm Absorbers, Hot Controversy

It seems almost certain that most galaxies (our Milky Way included) have at their center a supermassive black hole several millions time the mass of our sun. Only in some of these galaxies, though, is this central black hole accreting enough matter to make the galaxy "bright," or active. In this case, the central region is called an active galactic nuclei, or AGN. Richard Mushotzky, an expert on AGN working at the NASA Goddard Space Flight Center, likes to point out that there is not even a basic theory of AGN, just a large numbers of observations that people are trying to understand. X-ray studies are the only available probes of the closest regions to the central black hole--and an area on which precise spectroscopy has recently shed some new light.

One class of AGN, called Seyfert I, has very bright optical emission lines and broad absorption lines in a spectra. Emission and absorption lines are an atom's signature, revealing information about the composition and the state of a gas. Different atoms emit radiation at different energies, so detecting an emission or absorption lines can, in principle, be used to deduce the composition of a gas. In practice, however, most atoms emit many lines, which can blend and be difficult to disentangle. For example, iron at a certain temperature emits radiation that is hard to separate from the contribution of neon.

Because of the low resolution of the instruments used previously, it was impossible to disentangle the contributions from the different parts of the AGN--that is, specific areas closer or farther away from the central black hole. Now, with the recent data, Mushotzky says, it has become possible to ask questions about the fundamental physics underlying what is observed: What causes those absorption lines? Are they evidence of a cool material present near the central black hole? How does this change the general picture for AGN?

Indeed, the new, clearer data from Chandra and XMM-Newton show very strong lines, which in the past year have triggered heated arguments about their correct interpretation. Although astronomers agree that Seyfert I's broad absorption lines are generally explained by the presence of a gas called a "warm absorber," a couple of objects don't seem to match that idea--and therein lies the debate.

Part of the controversy is because Chandra and XMM-Newton don't have exactly the same capabilities. Chandra has greater spatial resolution than XMM, so it can better separate objects and analyze a specific part of a picture. But Chandra collects fewer photons than XMM, so an object must be observed for a far longer period to collect a similarly bright spectrum. As an analogy, imagine using a funnel to collect rainwater that is leaking from a roof. For Chandra, imagine that the funnel is narrow but that it can precisely locate the roof leak. With that narrow funnel, you can collect only a very small amount of water, so it is difficult to analyze the composition of the liquid to tell, for instance, whether it has dirt or other things in it. In comparison, XMM would be like a bigger funnel for that roof leak. You might not know exactly where the leak is coming from, but because there is much more water, its composition is easier to analyze.

With their data, the XMM-Newton team was convinced that they had detected emission lines from elements located in the nearby vicinity of the massive central rotating black hole. This explanation would constitute a major discovery and would imply a complete revision of the current theories of AGN, because astronomers had never previously seen emission lines that were so distorted by a black hole. But after analyzing their own high-resolution data, the Chandra team argued for the more conventional view of the presence of a warm absorber located farther away from the central black hole. To help sort out the disagreement, astronomers will have to closely compare the two data sets, and will also observe other Seyfert I's to see whether this problem occurs elsewhere.

"I recognize that this is still controversial," says Steven Kahn, the U.S. principal investigator of the high-resolution grating instruments onboard XMM-Newton. "But what is clear is that the standard warm absorber models do not provide good fits to the data. Whatever the eventual explanation is, the data clearly show that the 'soft' [relatively lower-energy] x-ray band is an exciting region to observe these sources."

Supernova Remnants: An In-Space Lab for Emissions

The models will get another workout from a different area of study: supernova remnants (SNRs), which are what is left behind after stars explode. And although these findings are not controversial, they did engender disbelief for another reason: the high quality of the new data took astronomers by surprise.

"When I first saw the Chandra data [on E0102-72, a supernova remnant located in the Small Magellanic Cloud]," says astronomer Kathryn A. Flanagan, "I wasn't sure that all the lines were real." There were so many strong lines, Flanagan recalls, a member of one of Chandra's instrument teams at the Center for Space Research at the Massachusetts Institute of Technology, that at first she was afraid they were caused by instrument contamination.

When everything checked out fine, however, she and her collaborators were able to create the first detailed map of the speed of all the parts of the supernova remnant. The team found that some parts are flying away from Earth, and some are coming toward us. (They won't reach us; the remnant is more than a million light-years away, and the parts are traveling far more slowly than the speed of light.)

The results of the spectral analysis of SNRs will help in refining models of emissions of plasma (charged particles) from the remnants. "Mother Nature is not simple," Flanagan notes, "so no simple model will work." Astronomers depend on models that involve both atomic physics (which predict at what energy each atom will emit radiation if it is at a specific temperature and density and in a certain ionization state) and the physics of the objects they want to analyze. The combination of both is crucial to allow for a meaningful interpretation of the data.

Several efforts to develop models that incorporate most of the atomic physics are under way. Some use well-known and studied objects in the sky as their reference; some are trying to re-create and measure line emission element by element in a laboratory setting. Neither method will solve the entire problem. "We're going to need the feedback from both," Flanagan says.

Randall Smith, a scientist at the Harvard-Smithsonian Center for Astrophysics, is involved in a project aimed at providing the astronomical community with ways to analyze and understand the wealth of data generated by the high-resolution spectrometers on Chandra and XMM-Newton. He says that modeling is only part of the problem, that astronomers need to be more aware of the unique characteristics of the objects of study. The underlying physics are different in AGNs and SNRs, for instance.

As astronomers sort out the details, one thing is clear: they now have more focus to tackle the problems at hand.

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