Pulsar Find Boosts Hope for Gravity-Wave Hunters

Neutron star pairs may merge and give off a burst of gravity waves
about six times more often than previously thought, scientists
report in today’s issue of the journal Nature [4 December]. If so,
the current generation of gravity-wave detectors might be able to
register such an event every year or two, rather than about once a
decade – the most optimistic prediction until now.

Gravity waves were predicted by Einstein’s general theory of
relativity. Astronomers have indirect evidence of their existence
but have not yet detected them directly.

The revised estimate of the neutron-star merger rate springs from
the discovery of a double neutron-star system, a pulsar called PSR
J0737-3039 and its neutron-star companion, by a team of scientists
from Italy, Australia, the UK and the USA using the 64-m CSIRO
Parkes radio telescope in eastern Australia.

Neutron stars are city-sized balls of a highly dense, unusual form of
matter. A pulsar is a special type – a spinning neutron star that emits
radio waves.

PSR J0737-3039 and its companion are just the sixth known system of two
neutron stars. They lie 1600-2000 light-years (500-600 pc) away in our
Galaxy.

Separated by 800,000 km (500,000 miles) – about twice the distance
between the Earth and Moon – the two stars orbit each other in just
over two hours.

Systems with such extreme speeds have to be modelled with Einstein’s
general theory of relativity.

“That theory predicts that the system is losing energy in the form of
gravity waves,” said lead author Marta Burgay, a PhD student at the
University of Bologna.

“The two stars are in a ‘dance of death’, slowly spiralling
together.”

In 85 million years the doomed stars will fuse, rippling spacetime
with a burst of gravity waves.

“If the burst happened in our time, it could be picked up by one of
the current generation of gravitational wave detectors, such as
LIGO-I, VIRGO or GEO” said team leader Professor Nicolo D’Amico,
Director of the Cagliari Astronomical Observatory in Sardinia.

The previous estimate of the neutron-star merger rate was strongly
influenced by the characteristics of just one system, the pulsar
B1913+16 and its companion. PSR B1913+16 was the first
relativistic binary system discovered and studied, and the first
used to show the existence of gravitational radiation.

PSR J0737-3039 and its companion are an even more extreme system,
and now form the best laboratory for testing Einstein’s prediction
of orbital shrinking.

The new pulsar also boosts the merger rate, for two reasons.

It won’t live as long as PSR B1913+16, the astronomers say. And
pulsars like it are probably more common than ones like PSR
B1913+16.

“These two effects push the merger rate up by a factor of six or
seven,” said team member Dr. Dick Manchester of CSIRO’s Australia
Telescope National Facility, which operates the Parkes telescope.

But the actual numerical value of the merger rate depends on
assumptions about how pulsars are distributed in our Galaxy.

“Under the most favourable distribution model, we can say at the
95% confidence level that this first generation of gravitational
wave detectors could register a neutron star merger every one to
two years,” said Dr. Vicky Kalogera, Assistant Professor of
Physics and Astronomy at Northwestern University in Illinois,
USA.

Dr. Kalogera and colleagues Chunglee Kim and Duncan Lorimer have
modelled binary coalescence rates using a range of assumptions.

The new result is “good news for gravity-wave astronomers,”
according to team member Professor Andrew Lyne, Director of the
Jodrell Bank Observatory of the University of Manchester in the
UK.

“They may get to study one of these cosmic catastrophes every
few years, instead of having to wait half a career,” he said.

PUBLICATION

M. Burgay, N. D’Amico, A. Possenti, R.N. Manchester, A.G. Lyne,
B.C. Joshi, M.A. McLaughlin, M. Kramer, J.M. Sarkissian, F.
Camilo, V. Kalogera, C.Kim & D.R. Lorimer. “An increased
estimate of the merger rate of double neutron stars from
observations of a highly relativistic system”. Nature 426,
531-533 (2003).

CONTACTS

ITALY
Ms Marta Burgay
University of Bologna, Department of Astronomy
+39-051-209-5719
burgay@tucanae.bo.astro.it

Professor Nicolo D’Amico
Director, Cagliari Astronomical Observatory
Tel: +39-070-711-80-208
Cell: +39-329-660-38-28
damico@ca.astro.it

USA
Assistant Professor Vicky Kalogera
Northwestern University
Vicky@northwestern.edu
+1-(847)-491-5669

UK
Professor Andrew Lyne
University of Manchester, Jodrell Bank Observatory,
+44-(0)-1477-572640
agl@jb.man.ac.uk

AUSTRALIA
Dr. Dick Manchester
CSIRO Australia Telescope National Facility
+61-2-9372-4313
Dick.Manchester@csiro.au

IMAGES AND ANIMATIONS

1. Pulsar J0737-3039 and its neutron-star companion. The
system is emitting gravity waves, shown here as ripples
in a spacetime grid. Credit: John Rowe Animation

JPEG IMAGE (109K)
http://www.atnf.csiro.au/news/press/neutron_binary/images_3/anim1_0272.JPG

TIFF IMAGES – BROADCAST RESOLUTION (400K and up)
http://www.atnf.csiro.au/news/press/neutron_binary/images_1/

MPEG2 ANIMATION FOR WEB (320×256, 3MB)
http://www.atnf.csiro.au/news/press/neutron_binary/web/BinaryFirstAnim320x256.mpg

NTSC MPEG2 ANIMATION FOR BROADCAST (16MB)
http://www.atnf.csiro.au/news/press/neutron_binary/ntsc/BinaryFirstAnimNTSC.mpg

PAL MPEG2 ANIMATION FOR BROADCAST (20MB)
http://www.atnf.csiro.au/news/press/neutron_binary/web/BinaryFirstAnim320x256.mpg

2. The pulsar and its companion merging.
The merged object may become a black hole.
Credit: John Rowe Animation

JPEG IMAGE (88K)
http://www.atnf.csiro.au/news/press/neutron_binary/images_3/anim2_0475.JPG

TIFF IMAGES FROM ANIMATION – BROADCAST RESOLUTION (400K and up)
http://www.atnf.csiro.au/news/press/neutron_binary/images_2/

MPEG2 ANIMATION FOR WEB (320 x 256, 5MB)
http://www.atnf.csiro.au/news/press/neutron_binary/web/BinarySecondAnim320x256.mpg

NTSC MPEG2 ANIMATION FOR BROADCAST (24MB)
http://www.atnf.csiro.au/news/press/neutron_binary/ntsc/BinarySecondAnimNTSC.mpg

PAL MPEG2 ANIMATION FOR BROADCAST (29MB)
http://www.atnf.csiro.au/news/press/neutron_binary/pal/BinarySecondAnimPAL.mpg

3. The CSIRO Parkes radio telescope.