Binary Neutron Stars - Scientific American Digital

were outside the solar system.
Still, the bursts were
kept secret for several
years, until in 1973 Ray W.
Klebesadel, Ian B. Strong
and Roy A. Olson of Los
Alamos National Laboratory
described them in a
seminal paper. Theorists
proposed more than 100
models in the next 20
years; in the late 1980s a
consensus formed that
the bursts originated on
neutron stars in our own
galaxy.
A minority led by Paczynski
« argued that the
bursts originated at cosmological
distances. In
the spring of 1991 the
Compton Gamma Ray Observatory,
which was more
sensitive than any previous
gamma-ray satellite,
was launched by the National
Aeronautics and
Space Administration. It
revealed two unexpected
facts. First, the distribution
of burst intensities is
not homogeneous in the
way that it would be if the
bursts were nearby. Second,
the bursts came from
all across the sky rather
than being concentrated
in the plane of the Milky
Way, as they would be if
they originated in the galactic disk. Together
these facts demonstrate that the
bursts do not originate from the disk
of our galaxy. A lively debate still prevails
over the possibility that the bursts
might originate from the distant parts
of the invisible halo of our galaxy, but
as the Compton Observatory collects
more data, this hypothesis seems less
and less likely. It seems that the minority
was right.
In the fall of 1991, I analyzed the distribution
of burst intensities, as did Paczynski
« and his colleague Shude Mao.
We concluded that the most distant
JOHN LONG
LIGO INTERFEROMETERS, one of which is shown here under construction,
should one day be able to detect the gravitational radiation
of colliding neutron stars from a distance of billions of lightyears.
If these signals arrive at the same time as gamma-ray bursts,
a decades-old mystery may be solved.
bursts seen by the Compton Observatory
came from several billion light-years
away. Signals from such distances are
redshifted (their wavelength is increased
and energy decreased) by the
expansion of the universe. As a result,
we predicted that the cosmological redshift
should lead to a correlation between
the intensity of the bursts, their
duration and their spectra. Fainter
bursts, which tend to come from farther
away, should last longer and contain
a lower-energy distribution of gamma
rays.
Recently a NASA team headed by Jay
P. Norris of the Goddard
Space Flight Center has
found precisely such a
correlation. The number
of bursts that the Compton
Observatory records
also tallies quite well with
our earlier estimates of the
binary neutron star population.
Roughly 30,000
mergers should occur every
year throughout the
observable universe, and
the satelliteÕs detectors can
scan a sphere containing
about 3 percent of that
volume. Our rough estimates
suggest 900 mergers
a year in such a space;
the Compton Observatory
notes 1,000 bursts.
Although the details of
how colliding neutron
stars give rise to gamma
rays are still being worked
out, the tantalizing agreement
between these data
from disparate sources
implies that astronomers
have been detecting neutron
star mergers without
knowing it for the past 25
years. Researchers have
proposed a few other
sources that might be capable
of emitting the enormous
amounts of energy
needed for cosmological
gamma-ray bursts. The
merger model, however, is the only one
based on an independently observed
phenomenon, the spiraling in of a neutron
star binary as a result of the emission
of gravity waves.
It is the only model that makes a
clear prediction that can be either con-
Þrmed or refuted. If, as I expect, LIGO
and VIRGO detect the unique gravitational-wave
signal of spiraling neutron
stars in coincidence with a gamma-ray
burst, astrophysicists will have opened
a new window on the Þnal stages of
stellar evolution, one that no visiblelight
instruments can hope to match.
About the Author
TSVI PIRAN has studied general relativity
and astrophysics for 20 years. He is a
professor at the Hebrew University of
Jerusalem, where he received his Ph.D. in
1976. He has also worked at the University
of Oxford, Harvard University, Kyoto
University and Fermilab. Piran, with
Steven Weinberg of the University of
Texas, established the Jerusalem Winter
School for Theoretical Physics.
Further Reading
BLACK HOLES, WHITE DWARFS AND NEUTRON STARS: THE PHYSICS OF COMPACT OBJECTS. Stuart
L. Shapiro and Saul A. Teukolsky. Wiley Interscience, 1983.
WAS EINSTEIN RIGHT? PUTTING GENERAL RELATIVITY TO THE TEST. CliÝord M. Will. Basic
Books, 1988.
LIGO: THE LASER INTERFEROMETER GRAVITATIONAL-WAVE OBSERVATORY. Alex Abramovici et
al. in Science, Vol. 256, pages 325Ð333; April 17, 1992.
PROBING THE GAMMA-RAY SKY. Kevin Hurley in Sky and Telescope, Vol. 84, No. 6, pages
631Ð636; December 1992.
X-RAY BINARIES. Edward P. J. van den Heuvel and Jan van Paradijs in ScientiÞc American, Vol.
269, No. 5, pages 64Ð71; November 1993.
Copyright 1995 Scientific American, Inc.
SCIENTIFIC AMERICAN May 1995 61