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Friday, October 26, 2012

Pulsars as detectors for gravitational waves

So far, they escape despite kilometer-scale detector systems of discovery: gravitational waves, vibrations of space-time whose survival Einstein forecast in his theory of general relativity. However, the detection of gravitational waves could soon be possible. And not with special detectors, but with ordinary radio telescopes that measure the arrival time of the radio signals from pulsars. The vibrations are there but not - as has been mostly assumed - as a sort of background noise, but it dominate individual signals of individual sources.

Pulsars are neutron stars, ultra-dense stellar corpses, at a diameter of about 20 kilometers, containing the mass of a sun. They emit highly focused radio beams swivel by the rapid rotation of the neutron star as the beams of a lighthouse through space. Is the earth affected by these rays, astronomers register with their antennae, regular radio pulses? Of particular interest to the researchers are rapidly spinning pulsars with periods in the millisecond range. Because the pulses of these objects have stability is greater than the atomic clocks.

Three international projects - so-called pulsar timing arrays - "now deliver high quality time measurements of around 40 of the most stable millisecond pulsars," said George Hobbs of the Australia Telescope National Facility. As the number of known millisecond pulsars grows steadily and a number of new, large radio telescopes under construction, Hobbs see good prospects that the precise time measurements could soon provide an indication of gravitational waves. Of course, the researcher, the signals from pulsars are disturbed on their way to Earth by other effects, for example by density fluctuations in the interstellar gas. But while such fluctuations for different pulsars lead to completely different delays, the influence of gravitational waves depends on the angle between the pulsar and the source of gravitational waves.

As a source come mainly supermassive black holes are suitable that are merging. Almost every galaxy contains a black hole at its center with the million or even a billion times the mass of the sun. Since galaxies in the cosmic history repeatedly collide and merge together, it comes again and again to the mergers of black holes. Vikram Ravi from the University of Melbourne and his colleagues on the basis of established models of galaxy formation studies the impact of these events on the pulsar signals. "We have found that the statistics of the variations is not an isotropic, statistical background of gravitational waves in line," the scientists write in the journal "Astrophysical Journal."

The supermassive black holes produce so no uniform noise of gravitational waves. "The reason is that a few sources of gravitational waves dominate the variations," said Ravi and his colleagues. They noted that there was a good chance to prove that generated from a single pair of merging black holes differences in arrival times of pulsar signals within five years with a Pulsar Timing Array. It was important in the analysis of the data does not find a uniform gravitational wave background, but after such individual events.

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