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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.
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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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