Columbia Team Is First To Weigh Neutron Star By Measuring X-Rays From Innermost Orbit

May 29, 1997

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 general relativity.

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.

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