Astrophysicists have found a pair of pulsars that could prove the existence of gravity waves, reports Dick Ahlstrom
Astronomers are always happy to discover a pulsar. They are now ecstatic, having discovered not one but two, a pair of pulsars orbiting one another. A team of scientists from the UK, Australia, Italy and the US has just announced the discovery in the journal Science Express. The first known example of a double pulsar, it is causing great excitement.
"While experiments on one pulsar in such an extreme system as this are exciting enough, the discovery of two pulsars orbiting one another opens up new precision tests of general relativity and the probing of pulsar magnetospheres," says Andrew Lyne, professor of physics at the University of Manchester and director of Jodrell Bank Observatory.
Even without a twin, pulsars are remarkable objects that almost defy comprehension. A pulsar is formed by the collapse of a massive star that has ended its life in a supernova explosion, one of the most violent occurrences in the universe.
After the explosion the huge content of the star collapses inwards, to form an ultradense ball of matter called a neutron star, with a mass equal to or greater than that of the sun but just 20 kilometres across (the sun, in contrast, is 1.4 million kilometres wide).
Some neutrons also then become pulsars, emitting powerful beams of radio waves that sweep round the sky like a lighthouse, a feature that shows another aspect of these objects, their rapid spin rate. The collapsed stars can spin hundreds of times a second.
The team, which also includes members of the Australia Telescope National Facility, Cagliari Astronomical Observatory, in Italy, and Columbia Astrophysics Laboratory, in the US, were studying a "compact object" orbiting PSR J0737-3039A, known as a 23-millisecond pulsar after the time it takes to rotate.
They originally took the object, which orbits pulsar A every 2.4 hours, to be an ordinary neutron star. Instead they found that it was a second pulsar, now named PSR J0737-3039B, rotating once every 2.8 seconds.
The team reported in Nature last December that pulsar A was rapidly losing energy by gravitational radiation and that the stars would fuse in about 85 million years.
The discovery is remarkable not only because of its novelty but also because what it tells us about how frequently stars might merge, according to the research team. They expect it to be a more common occurrence than originally thought, with each fusion of stars releasing a ripple of gravity waves across the universe.
"The news has been welcomed by gravitational-wave hunters, since it boosts their hopes for detecting these elusive signals," says Prof Nichi D'Amico of the Cagliari observatory.
The double neutron star system was first detected using the 64-metre Parkes radio telescope in New South Wales. Later observations made both at the Parkes Observatory and with the 76-metre Lovell Telescope, at Jodrell Bank, revealed the presence of radio pulses every
2.8 seconds from the companion star.
The pair were used almost immediately to study gravitational theories proposed by Albert Einstein. Already four effects beyond those explained with simple Newtonian gravity have been measured; they are consistent with Einstein's ideas, according to Dr Richard Manchester of the Australia Telescope National Facility.
"The fact that both objects are pulsars enables completely new high-precision tests of gravitational theories," he says. "This system is really extreme."
Happily for astronomers on Earth, we are observing these objects almost on edge, with the sweeping beams of radio waves crossing our path rather than passing us by in space. "This provides us with a wonderful opportunity to probe the physical conditions of a pulsar's outer atmosphere, something we've never been able to do before," says Dr Andrea Possenti of Cagliari Astronomical Observatory.