ASTROPHYSICISTS, it could be said, have the universe for a laboratory. And what a laboratory it is, with conditions that cannot be duplicated in an earthly setting--nearly perfect vacuum, extraordinary temperatures and pressures, and enormous distances. But the very vastness that provides these conditions makes study of astrophysical phenomena difficult.|
The observable phenomena in the universe often are the result of complex interplay between several physical processes, each of which operates over a scale that cannot be controlled or modified by the experimenter. Obviously, a researcher cannot perform any type of controlled experiment on objects outside the solar system. Theory and computer simulations must be called upon to fill the void left by the absence of controlled experiments.
To complement their observations, astrophysicists must leave their laboratory of the universe and return to the more modest facilities on Earth. Lawrence Livermore, with its advanced computational resources and laser plasma research capabilities, is a natural place to conduct this research.
Occupying that particular spot on Earth is astrophysicist Duane Liedahl. Along with astrophysicist Christopher Mauche, Liedahl is working to shed some light on the properties of cosmic x-ray sources while also using the data from space-borne experiments to refine and improve the accuracy of computer simulations of these phenomena.
Works in Both Directions
The goal of Liedahl's project is to improve and experimentally benchmark a sophisticated suite of computational tools for modeling the radiative properties of astrophysical plasmas. Liedahl and his colleagues are approaching solutions to problems in astrophysics along four avenues: astronomical observations, laboratory experiments, computer simulations, and theory (see figures for the interplay of these approaches). Historically, laboratory experiments have been performed to identify elements by measuring wavelengths that can be matched to stellar spectra. But the interaction of computer simulations and laboratory experiments works in both directions. Data from experiments are used to refine the computer models, and the computer models help scientists understand the problem and develop theories.
Especially now, in the Department of Energy's nuclear-test-free Stockpile Stewardship and Management Program (SSMP), x-ray astrophysical observations and related modeling will play an essential role in benchmarking our ability to understand the physics of thermonuclear weapons because much of the physics is common to both fields. For example, high-quality x-ray observations from satellites may well be the source of future data supporting the SSMP. Lawrence Livermore's current leadership position in modeling x-ray sources is a result of its work to understand high-energy-density physics, which is required to predict the behavior of weapons.
"In short, theory draws from computer simulation, and computer simulation draws from experiment," Liedahl says. "Livermore's computational modeling for the SSMP will benefit from the improved atomic models that allow us to verify the accuracy of our computational models."
Liedahl also works closely with Peter Beiersdorfer of Lawrence Livermore's Electron-Beam Ion Trap (EBIT) facility. At EBIT, measurements of electron-impact ionization, excitation, and recombination can be made that are crucial to understanding high-temperature plasmas. These experiments yield data that can be used to verify the completeness and accuracy of atomic models of the emission properties of various elements involved in astrophysical processes. Liedahl and his colleagues use these improved atomic models, along with data from space-borne experiments, to calibrate astrophysical models. In turn, these improved models allow scientists to refine theories about the behavior of plasmas and highly charged ions--essentially, our basic understanding of matter in extreme environments.
Science Born by Chance
"The science of x-ray astronomy was born in 1962 during a rocket-based experiment to detect x-ray-induced fluorescence on the lunar surface," Liedahl says. "By chance, the Moon's orbit passed close to the position of the star Scorpius X-1, and a dramatic increase in flux--changes in the radiation emitted--was detected. This discovery indicated that x-ray observations could reveal new and exotic cosmic phenomena that are largely invisible to conventional optical and radiotelescope techniques."
Our solar system is inside a million-degree ball of gas--purportedly carved out by an ancient supernova--that is radiating x rays. The Sun, because of its proximity to Earth, is our brightest source of x rays. However, most objects in the universe--stars, supernova remnants, galaxies, and black holes--also produce x rays. Scorpius X-1 is a much brighter source of cosmic x rays, 100 billion times stronger than our Sun. But it is also 100 million times more distant than our Sun, and the apparent brightness decreases with the distance squared.
"We've found that x-ray spectroscopy is a very useful measuring tool for cosmic plasmas," Liedahl continues. "However, its real usefulness in astrophysics depends on significant improvements in its sensitivity and capabilities. This usefulness will be realized only after we can make significant improvements in our spectroscopic modeling tools. Some of the unique characteristics of cosmic plasmas include ultralow density (down to 10-3 atoms per cubic centimeter, roughly a million times better than the best vacuum achievable on Earth), high radiation-energy density, ultrahigh magnetic fields, relativistic gas flows, and very-high-temperature shock waves."
Traditionally, spectroscopy has been used to identify elements. As data quality improves, the demands placed on spectroscopic models will become much more stringent because astrophysicists will want to know the physical conditions of the plasmas in which the elements exist. Liedahl's approach seeks to identify the detailed behavior of atoms in a wide range of physical environments. His team uses these data to build atomic models to hypothesize about the composition and physical conditions of cosmic plasmas. Then the team uses these atomic models to refine the astrophysical models and improve accuracy.
"Atomic physics operates the same way on Earth as it does in space," Liedahl says. "By improving our atomic models under conditions we can control, we develop the confidence to apply them to more complex astrophysical environments, which we can't control."
Liedahl's work helps further our understanding of both the relevant atomic physics and the astrophysics of the sources themselves. Unfortunately, acquiring high-quality x-ray spectra of cosmic sources poses experimental challenges because the sources are extremely faint, and observations must be conducted from space. Although the interstellar medium is an extremely good vacuum, it is not perfect and thus is not entirely transparent to x rays. However, our ability to collect high-quality data will be dramatically improved in the near future when new satellites are launched. The U.S. project AXAF; the European XMM, for which Lawrence Livermore collaborated with the University of California at Berkeley to construct the grating arrays in the spectrometers; and Japan's Astro-E will provide more than order-of-magnitude improvements in sensitivity and resolution. "We also are expecting to achieve great improvements in the versatility of x-ray spectroscopy analysis tools," Liedahl adds.
The tremendous quantity of data expected from the new satellites launched by the U.S., Europe, and Japan will provide a basis for significant advances in our understanding of a wide range of phenomena. Lawrence Livermore's ability to coordinate large-scale technology, formidable computational power, and an experienced team of researchers can have a major impact on the astrophysics community by helping to maximize the scientific yield from major space missions.