April 2002
Patenting Human Genes

Putting it All Together

Measuring Cosmic X-ray Fireworks

Understanding Gene Functions Through Mutation

New Fellowships Integrate Teaching and Research

Pursuing Answers to Big Questions

Commentary: The Mentoring Mindset

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Bryn Mawr College
A quarterly newsletter on research, teaching, management, policy making and leadership in Science and Technology

Measuring Cosmic X-ray Fireworks
By Barbara Spector

Jean Hebb Swank ’61, an astrophysicist in the Laboratory for High Energy Astrophysics at NASA’s Goddard Space Flight Center in Greenbelt, Md., studies X-ray "fireworks" from neutron stars and black holes — the brightest X-ray sources in the galaxy, though they are invisible to the eye. She is the principal investigator on the Proportional Counter Array on NASA’s Rossi X-ray Timing Explorer (RXTE) satellite.

Jean Hebb Swank ’61

Isolated black holes do not emit much light, and neutron stars are very small and far away, so they are difficult to observe. RXTE detects X-rays generated when the gravitational fields of black holes and neutron stars pull gas from stars orbiting them in a binary star system. The satellite records variations in the timing of the X-ray flashes. The main instruments produce energy spectra and "light" curves, rather than images, for X-ray sources in our galaxy and for certain very bright sources outside our galaxy. It is not certain how, but the centers of some galaxies, including quasars, are enormous black holes. Stars close to them are pulled in and generate X-rays. Though they are very distant, they are as bright as some in our galaxy.

Swank has studied neutron stars and black holes since the 1970s. She is a senior scientist at Goddard. In 1999, she shared the Bruno Rossi Prize of the American Astronomical Society for her role in developing the RXTE and subsequent discoveries. Both the RXTE spacecraft and the prize are named for the scientist whose experiments launched the field of X-ray astronomy.

Undergraduate Study

Swank relished Bryn Mawr College’s emphasis on scholarship. "I found the peaceful environment and the atmosphere of enjoyment of so many subjects — literature, philosophy, science, mathematics, music — very encouraging," she says. "I also learned that working on technical problems is fun." She majored in physics and recalls that she was "excited at the idea that there were several kinds of forces between different kinds of subatomic particles."

Swank and Melinda Groom ’61 were the only two physics majors in their class. "Melinda and I were in the same physics classes for four years, and the only ones in a lot of them, and that is very much a part of my experience at Bryn Mawr," Swank says. "Both the physics and mathematics departments had only a handful of faculty; and students interacted closely with them all."

Today there are 12 faculty members in the physics and mathematics departments. Sixteen College juniors and seniors have declared majors in physics, and 63 in mathematics.

Graduate Work

Although Swank entered Bryn Mawr College in 1957 — the year of the first Sputnik launch — she didn’t become interested in astrophysics until her days as a graduate student at the California Institute of Technology in the late 1960s.

Walter Michels, then professor and chair of Bryn Mawr’s physics department, was a Caltech alumnus. Another faculty member, Charles Miller, defended his thesis at Caltech while teaching at Bryn Mawr. Their advice was instrumental in Swank’s decision to attend graduate school at Caltech, where she earned her Ph.D. in physics in 1967.

At Caltech, she ended up learning about stars because of elementary particles (neutrinos and anti-neutrinos) that are produced in them. "Calculations about neutrino-electron interactions evolved into my thesis," she says, "and they had an application to the collapse of massive stars and the ejection of the envelope in a supernova explosion." She never aspired to be an astronaut. "It was not the best way to pursue the science of interest to me," she explains.

After graduating from Caltech, Swank taught at California State University at Los Angeles and participated in a summer research program at the University of Maryland. There she met physicist Lowell James Swank. When he accepted a postdoctoral appointment at the National Accelerator Laboratory in Illinois, they married and she took a position at Chicago State University. The next opportunity they found to continue their careers together was at the Middle East Technical University in Turkey. The physics department there was chaired by Hakki Ogelman (now at the University of Wisconsin), a researcher in high-energy astrophysics.

Joining Goddard

Through Ogelman, who had worked with the gamma-ray astronomy group at Goddard, Swank learned that X-ray astronomers were developing an experiment for the eighth Orbiting Solar Observatory (OSO-8), launched in 1975. When she and her husband returned to the United States, Swank applied for a postdoctoral fellowship at Goddard and has been there ever since.

Although OSO-8’s main experiments looked at the sun, it also investigated other phenomena, including spectroscopy and timing studies of galactic and extragalactic X-ray sources. Later, Swank used data taken by instruments on the High Energy Astronomy Observatories (HEAO-1, launched in 1977; and HEAO-2, renamed the Einstein Observatory after its 1978 launch). With the European Space Agency’s X-ray Observatory (EXOSAT), launched in 1983, and the Broad Band X-ray Telescope, flown on the space shuttle Columbia in December 1990, she continued to study the emissions from X-ray stars while being deeply immersed in preparing RXTE, which was launched in December 1995.

Swank became the lead scientist for the Proportional Counter Array (PCA), one of three instruments carried on RXTE. The PCA has five xenon-gas proportional-counter detectors that measure X-rays in the 2,000- to 60,000-electron volt range. RXTE makes accurate measurements of photons in a thousandth of a second.

"That time — one millisecond — is significant because it is the so-called dynamical time scale of neutron stars and also of black holes that have the mass of large stars, 10 to 100 times the mass of our sun," Swank says. "To study oscillations, vibrations and rotations of either the objects or the matter approaching them, one has to resolve small times, and the signal has to be large for that to be worthwhile. The detectors have to be designed to record signals at rates of 10,000 to 100,000 events per second."

Another unique feature of RXTE is its ability to observe "a large class of transient events," Swank explains. Black holes often reveal their most interesting phenomena over just a few consecutive days in a 50-year observational span. There may be thousands of black holes in our galaxy.

"The objects RXTE observes have gravitational and magnetic fields exceeding those that either naturally occur in our solar system or can be produced in our laboratories. RXTE measures how photons, electrons, protons and neutrons interact in these strong gravitational fields and in strong magnetic fields," Swank explains. "We are trying to use such data to check the extrapolations of fundamental physics to extreme conditions."

About the Author

Barbara Spector writes on science and technology as well as business topics. She is the executive editor of Family Business magazine and former editor of The Scientist.

 
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