Existing neutrino and gravitational-wave detectors can be used in concert to observe gravitational waves given off during a nearby supernova — say physicists in Italy.

Gravitational waves are vibrations of space–time predicted by the general theory of relativity. A number of experiments are trying to detect gravitational waves by measuring tiny changes in the separation of two masses that are expected to occur when the waves traverse a detector. However, none have been successful so far and the most convincing evidence yet for gravitational waves is that the orbital period of the Hulse–Taylor binary star system is shrinking at the precise rate associated with the emission of gravitational waves.

With a little bit of luck, however, the first direct detection of the waves could happen if a supernova occurs in our own galaxy. Such a massive stellar explosion produces a vast amount of light and other radiation, which could help physicists narrow down their search to the precise moment that the gravitational waves reach Earth. This would be a great help in boosting the sensitivity of gravitational wave detectors.

Neutrino pulse

When radiation from supernova SN 1987A was detected here on Earth (in 1987), physicists realized that it included a pulse of neutrinos — or more precisely, electron antineutrinos. Now, Francesco Vissani and colleagues at the Gran Sasso Laboratory and INFN Torino have done calculations suggesting that the detection of neutrinos from a nearby supernova could allow existing gravitational wave detectors to make their first discovery.

At the heart of their theory is the idea that gravitational waves large enough to be detected are created when the supernova reaches the “bounce” phase. This occurs when the star’s iron core collapses rapidly to the density of a nucleus. Incoming material rebounds from the nuclear core, which is incompressible, the result being a sharp outward acceleration of vast amounts of matter.

For a typical supernova, Vissani and colleagues calculate that this bounce should last about 30 ms — creating a burst of gravitational waves of about the same duration. Most astrophysicists agree that the neutrino pulse from a supernova precedes the bounce by about 3 ms. Using a model developed from analysis of neutrino and other data from SN 1987A, the team calculated the time delay between the detection of the neutrinos in detectors on Earth and the arrival of gravitational waves.

The physicists conclude that neutrino detection could be used to narrow down the arrival time of gravitation waves to an uncertainty of about 10 ms.

Welcome reduction

According to Edward Daw at the University of Sheffield, existing gravitational wave detectors operate on the basis that the arrival time of waves can be pinpointed to within 100 ms. He cautions that while the new results do not necessarily mean this can be reduced by a full factor of ten, any significant reduction is welcome.

“This is a useful improvement, especially if you are requiring coincident signals between multiple gravitational wave interferometers within this time window”, he told physicsworld.com. If three detectors are used, a factor of 10 reduction means that the probability of a false detection due to noise in all detectors is reduced by a factor of 1000. This means that the noise threshold of the detectors can be reduced, making them more sensitive to gravitational waves.

Daw, who works on the LIGO gravitational-wave detectors, points out that Vissani’s calculations suggest that LIGO would not be able to detect gravitational waves from a supernova like SN 1987A — even using the neutrino trigger. This is because the supernova occurred outside the Milky Way. Instead, scientists will have to wait until the next supernova is spotted in our galaxy — something that is expected to occur only a few times in a century.

Let’s hope it happens when all the neutrino and gravitational–wave detectors are switched on.

The calculations are reported in Physical Review Letters.