LONDON: When compact, city-sized neutron stars collide they emit a robust radio signal that could help astronomers detect gravitational waves, locating their original source, new research predicts.

Neutron stars are incredibly dense stellar remnant composed almost entirely of subatomic particles called neutrons, which result when large supernova stars collapse. Roughly 5% of these stars enter into a binary system.

Mergers of neutron star binaries are very rare, but are prime candidates for the release of gravitational waves, which occur following violent astronomical events and have long remained undetected.

However, a new study, published today in Nature, shows that the interaction of ejected particles from these mergers with the surrounding medium can create detectable electromagnetic signatures - some with peak emissions of 1.4 gigahertz that can persist at observable levels for several weeks.

The detection of accompanying electromagnetic signals - or radio 'flares' - in conjunction with gravitational waves "will enable researchers to point radio telescopes towards the region in the sky from which the candidate signal came from," said astrophysicist and co-author Tsvi Piran from the Hebrew University of Jerusalem.

"A radio flare of the kind we predict will confirm that indeed a neutron star merger took place - providing an independent support to the discovery."

Elusive waves explained

Albert Einstein first predicted gravitational waves in 1916 as part of his theory of general relativity.

The presence of large amounts of mass or energy can distort the space-time fabric causing it to curve. When these masses move suddenly, this curvature ripples outward - like the ripples in a pond after a fish jumps.

Alternatively, it can be likened to a bowling ball placed on a stretched sheet. The steep, indentation closest to the ball describes the curvature of space-time, and how it is affected by mass.

Where space-time is stretched by the presence of a very large mass, it can be punctured by the sheer weight of the mass, creating a black hole.

Binary neutron star mergers often result in black holes, creating the most theoretically significant gravitational waves.

Predicting the rate of mergers

New simulations detailed in the research show that particles could rush out from these mergers anywhere from one-tenth to half the speed of light. The velocity of the outflow determines the frequency and duration of the radio signal's peak.

According to Piran, this research also means that radio searches for such flares will enable scientists to more accurately determine the rate of neutron star mergers.

"The rate is at present highly uncertain and knowledge of this rate will help the design of gravitational radiation detectors and, in particular, will give a clear idea at what stage, and at what sensitivity, the detectors are sensitive enough to detect merger events at a reasonable rate," he said.

Detection experiments shaping up

In 2010, NASA announced plans to commence a program by 2016 in collaboration with the European Space Agency (ESA) to observe gravitational waves using the Laser Interferometer Space Antenna (LISA).

LISA will sense ripples coming simultaneously from tens of thousands of sources in every direction, with the instrument acting like a microphone picking up sound.

The detection of radio flares will considerably narrow the direction from which the gravitational waves originated. It is a complementary project to the United States-based Laser Interferometer Gravitational-Wave Observatory (LIGO).

Bernard F. Schutz, director of the Max Planck Institute for Gravitational Physics in Germany said the projects will "open up hidden chapters in the history of the universe, by listening to the waves made by the very first stars, the earliest black holes, and by some of the oldest stars in existence today".

Some doubts linger

Finding an example of a neutron merger could be quite difficult, as they are rare and the ejected particles are only visible for a short duration.

Still, the researchers point to a previously observed transient object identified by astronomers at the University of California, Berkeley.

Transient objects are astronomical events that appeared once and when observed a few months down the track, have subsequently vanished.

Reviewing the findings of the American astronomers, Piran points to one object, RT 19870422, as a possible signal from a neutron merger. .

"[It] shows all the expected properties of the radio remnant of a compact binary merger," he wrote. "This transient is therefore an excellent candidate to be the first observed radio remnant of a merger."

He noted that it wasn't possible to rule out a radio supernova as the source, but reiterated that this would have required a supernova "brighter by an order of magnitude" than any previously observed.