ONE of astronomy鈥檚 most elusive goals鈥攆inding the mass of a neutron
star鈥攎ay now be within reach.
Neutron stars consist of a ball of protons and neutrons crushed into a sphere
some 20 kilometres across. A chunk of this material the size of a sugar cube
would weigh as much as the entire population of the Earth.
The new estimates of mass come from looking at of X-ray binary systems, in
which a neutron star and a normal star circle one another. As they spin, the
dense neutron star sucks a stream of matter off its companion, which spirals
down onto the neutron star鈥檚 surface, losing energy as X-rays. Neutron stars
rotate hundreds of times per second, and the spiralling matter orbits even
faster.
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For the past year, scientists have studied these systems with NASA鈥檚 Rossi
X-ray Timing Experiment (RTXE), a satellite launched in December 1995. One
surprise is that several such binaries show 鈥渜uasi-periodic oscillations鈥�,
high-frequency X-ray pulses whose frequency drifts over time, says Tod
Strohmayer of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland. At one
binary, 4U 1728-34, these oscillations begin at around 800 hertz and gradually
increase to 1200 hertz, before disappearing.
Strohmayer believes that the pulses represent the orbits of matter spiralling
towards the neutron star鈥檚 surface. If so, the upper frequency limit reflects
the matter鈥檚 closest possible approach鈥攚hen it is orbiting 1200 times per
second鈥攂efore it plunges into the neutron star.
The height and frequency of this innermost orbit depends directly on the mass
of the star, according to a new theory by Coleman Miller of the University of
Chicago. Applying the 1200-hertz limit, Strohmayer calculates that the neutron
star in 4U 1728-34 must have a mass 1.8 times that of our Sun.