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email@example.com Thursday, May 29, 1997
Columbia Team Is First to Weigh
By Measuring X-Rays From Innermost Orbit
Corroborates Relativity's Prediction of Unstable Orbits Near Compact Stars
A team of astrophysicists at Columbia University, with the help of
Einstein's theory of general relativity, is the first to gauge the mass of a neutron
star by measuring its flickering X-ray radiation. The results are the first
evidence for the existence of unstable orbits close to a compact star as predicted by
Others have weighed neutron stars before, but the Columbia team did so by
measuring X-rays emitted by materials falling from an orbiting companion star
into the neutron star's innermost orbit and then into the neutron star itself. The
scientists are the first to use the presence of the innermost stable orbit, a unique
prediction of general relativity, to weigh a neutron star.
Objects orbiting a compact star such as a neutron star or a black hole must
keep a certain distance or they will spiral into it. That distance is related to the
neutron star's mass by general relativity and is called the innermost stable orbit.
The star, 4U 1636-536, one of about 800 known neutron stars, was found to
have a mass of about 9 X 1030 pounds - a nine followed by 30 zeroes - about twice
that of the sun. The results appear in the May 30 issue of Science magazine.
Neutron stars have masses similar to that of the sun but diameters of only a
few kilometers; they are thought to consist entirely of densely packed neutrons
and other elementary particles. Found in the Norma spiral arm of our own Milky
Way galaxy and visible from the southern hemisphere, 4U 1636-536 pulls clumps
of matter away from a companion star, and that matter emits flickering X-rays.
"From the rate of the X-ray flickering, we can determine the precise position of the
innermost stable orbit and therefore the mass of the neutron star," said Philip
Kaaret, associate professor of physics, who led the group. In the case of 4U 1636-
536, the falling matter emits X-rays that flicker at up to 1171 times per second.
Evidence for the existence of unstable orbits comes from the way the rate of X-ray
flickering varies with the star's brightness, Professor Kaaret said.
The neutron stars weighed in previous studies, which used radio waves
rather than X-rays, were all found to weigh 1.4 times as much as the sun, a result
thought to indicate the birth weight of neutron stars. But the neutron star
measured by the Columbia researchers is significantly heavier. "This neutron
star has probably gained weight by feeding off its companion star for the past few
billion years," Professor Kaaret said.
The Columbia astrophysicists used Einstein's theory to calculate the mass
of the neutron star from the frequency of the X-ray pulses, known among
astronomers as quasiperiodic oscillations. The other team members are Kaiyou
Chen, postdoctoral research scientist, and Eric Ford, graduate research
assistant, both of Columbia.
Matter falling into the neutron star is composed of hydrogen, helium and
other elements, essentially the components of the sun and other stars. The
neutron star's tremendous gravity pulls the material in, heating it into an
accretion disk surrounding the star. It is then slowly drawn into the star itself.
The astronomers who measured neutron star masses using radio waves
were Princeton physicists Russell Hulse and Joseph Taylor, who received the 1993
Nobel Prize in Physics for their studies of so-called relativistic binary pulsars.
The X-ray pulses from 4U 1636-536 were observed by U.S. and Dutch
astronomers Will Zhang and Jean Swank at the NASA Goddard Space Flight
Center and M. van der Klis at the Anton Pannekoek Astronomical Institute of the
University of Amsterdam. The observations used the Rossi X-Ray Timing
Explorer satellite launched by NASA in late 1995. Funding for Professor Kaaret's
research was provided by NASA.
This document is available at http://www.columbia.edu/cu/pr/.