Earth-bound 'star' impersonates black hole, neutron star:Z, which reaches temperatures of the Sun, to help astronomers interpret Chandra data

November 07, 1999

Albuquerque, N.M. -- In an inconspicuous, flat-roofed building on the high desert of New Mexico, a machine that creates temperatures rivaling those of the sun is helping physicists examine up-close what happens to iron in the grip of black holes and neutron stars.

Experiments on Sandia National Laboratories' Z machine -- the most powerful X-ray generator on Earth -- usually focus on the defense of the United States and harnessing nuclear fusion for electrical power.

But data from recent tests undertaken there by researchers from the Department of Energy's Lawrence Livermore and Sandia national laboratories should help astronomers trying to interpret images from the billion-dollar Chandra X-ray observatory now orbiting Earth. (Also benefiting will be two billion-dollar X-ray orbiting observatories expected to be sent aloft from Europe and Japan in the next year.)

The results will further human understanding of black holes, neutron stars, and the evolution and eventual expiration of the universe, predicts Livermore physicist Mark Foord, one of the leaders of the joint Sandia-Livermore project. The methods developed in the work also can be used in weapons physics, says Sandia project collaborator and physicist Jim Bailey.

Apprentice cooks in empty kitchens
Iron is of interest to astronomers because it is among the most complicated of elements widespread in the universe, and therefore among the hardest to understand. Several explanations are possible for the effects it creates in images taken by the recently launched Chandra orbiting telescope.

The problem for astronomers, who now hold bleacher seats to watch titanic energies transforming elements on a scale never before seen, is somewhat similar to that faced by apprentice cooks in empty kitchens who watch master chefs cook on TV: Without being able to cook along, many fine points of the observed process can only be guessed at.

Creating a neutron star or black hole on Earth for scientists to understand distant reactions is something of a problem, since there would be no more Earth. But because "neutron stars and black holes emanate X-rays similar in effect to those emanated by Z, we realized we have a chance to test astrophysical theoretical models that have never been tested experimentally," says Foord.

Says Bailey, "We're looking with spectroscopic eyes at the atomic physics of ionized iron so that these can be compared with theoretical calculations. Astrophysicists will have to consider what implications our figures have for their models."

"We have a collaboration with four or five groups around the world whose main job is to analyze data from Chandra using their own codes," says Foord. "They have made some predictions that we're going to compare to our data." If necessary, the codes will be modified.

The results will be compared with astrophysical calculations embedded in computer codes of how neutron stars affect the widespread element.

Interpreting data from the stars
The experiments proceed by placing square centimeters of iron, a few hundred angstroms thick, close to the Z pinch at the heart of Z. (A Z-pinch achieves its output by generating a powerful magnetic field that collides ions at an appreciable fraction of the speed of light.) This exposes the metal to temperatures of more than one million degrees for a few billionths of a second, ionizing the metal.

"Iron has thousands of spectral lines," says Foord. "We know their positions [on a spectroscopic scale] but they differ in intensity depending on which electrons, or how many, have been stripped away in the ionizing heat of a neutron star. Calculating these relative intensities is uncertain. This makes it difficult to predict how highly ionized iron is, or should be, in other star systems. If we can measure in the lab what the actual figures are, we learn how to interpret our data from the stars."

A Sandia instrument that takes seven images temporally, and a Livermore instrument that takes one image in time and two images spatially, will help determine effects of the intense X-ray pulse from Z on iron samples in terms of spectrum, temperature, density and ionization.

"Our first two shots were in late October and we are now analyzing the data," says Foord. "It appears that we were successful at producing highly ionized iron and were able to obtain an accurate measurement of the radiation produced from the Z-pinch. We hope to be back in a few months to do follow up experiments and then return again in the summer to explore different regimes."

Other collaborators in the experiments are Livermore's Bob Heeter, Bob Thoe, and Jim Emig. Heeter, a recent Princeton graduate and Livermore postdoctoral fellow, is the lead experimentalist for the project. Sandia's Tom Nash, Gordon Chandler, Dan Nielsen, Dan Jobe, and Pat Ryan also contributed. An invited paper will be given at Goddard Space Flight Center at a workshop entitled "Atomic Data Needed for X-Ray Astronomy," Dec. 16-17 in Bethesda, Maryland.

Livermore's Office of Laboratory Directed Research and Development funds the two-year project. Sandia is a multiprogram DOE laboratory, operated by a subsidiary of Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major research and development responsibilities in national security, energy, and environmental technologies.
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DOE/Sandia National Laboratories

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