28 September 2007

Mysterious radio signal from deep space

Cosmos Online
Astronomers hunting pulsars have detected a mysterious radio burst unlike anything observed before, and appears to originate from the deepest reaches of the cosmos.
Mysterious radio signal from deep space

Curious signal: The radio burst came from the same directions as the Magellanic Clouds, but is thought to herald from farther away than any other object known. Credit: Lorimer et. al

SYDNEY: Astronomers hunting pulsars have detected a mysterious radio burst unlike anything observed before, and appears to originate from the deepest reaches of the cosmos.

“We believe we’ve discovered an exotic phenomenon,” said lead author and astrophysicist Duncan Lorimer of West Virginia University in Morgantown, USA. “We were really shocked to find it … [it] could tell us something fundamental about the material in the universe.”

The find is reported today in the U.S. journal, Science.

Pulsating stars

Pulsars, short for ‘pulsating stars’, are rotating neutron stars – the remnants of massive suns that have collapsed into extremely dense objects. Rather like the beam of a lighthouse, they emit electromagnetic radiation in the form of radio waves as they rotate. This makes the stars appear to emit a regular pulse from Earth. The period of time between the peaks of the radio pulses typically occurs on the scale of seconds, but occasionally pulsars are found that rotate much more slowly – on the scale of hours.

To make the unusual discovery Lorimer, and a team of astronomers in the U.S. and Australia, revisited six-year old data from the radio telescope in Parkes, New South Wales, which surveyed the Magellanic Clouds for pulsars over 480 hours. Somewhere in this mountain of data, the scientists discovered an unknown signal lasting less than five milliseconds.

“It’s completely different from radio signals because it doesn’t repeat,” Lorimer said. “When we see radio bursts from neutron stars they repeat every hour or two in a rare case. But in this instance, we saw only one burst in 90 hours.” The apparent non-repetition of the signal makes it unique, Lorimer said.

Hawking radiation?

One potential culprit is a supernova – an exploding star – or the collision of two neutron stars. A third and more speculative possibility is that the pulse is ‘Hawking radiation’ from a so-called evaporating black hole (see Cosmos, issue 16, page 78). These were theorised to exist by British cosmologist Stephen Hawking, who suggested that over time, a black hole eventually loses more matter than it gains, thus shrinking and emitting radiation in the process.

Following the discovery, the astronomers worked backwards from the data to estimate the distance of the source. “There’s a characteristic time delay” between different frequencies of radio waves in the same signal, as pulses arrive at the telescope, said co-author Matthew Bailes, director of the Swinburne Centre for Astrophysics and Supercomputing in Melbourbe.

In a process known as dispersion, different frequencies of the same radio pulse arrive at Earth at fractionally different times, because ionised gases slow the waves down by different extents. The more ionised gas the radio pulse has to pass through on its long journey, the more it becomes dispersed. Using this method, the scientists estimate that the source of the radio burst could be as many as three billion light-years from Earth – farther than almost any other object studied.

“Something from a long way away

“It really couldn’t be anywhere nearby,” Lorimer said. “The amount of dispersion far exceeds that of anything in this galaxy. Effectively [the signal] is much farther away and has had to traverse through many galaxies.”

“They make a very convincing case that they have detected something from a long way away – an extra-galactic burst,” commented Richard Manchester, an astronomer at the Australia Telescope National Facility in Sydney, who was not involved in the discovery. “Nothing like this has ever been seen before. The fact that only one has been found despite a lot of searching means that they’re very rare.”

The find may have wider implications too – possibly even serving as a tool to measure the redshift of the universe.

The experts believe they can turn the correlation around, using dispersion and distance to estimate the amount of ionised gas, and therefore matter, in the universe. “If we can really pinpoint the position of these bursts in the sky, and then associate that with a distant galaxy using Hubble’s law [the expansion of universe], we can find its redshift. Up until now we’ve had no good way of measuring this.”


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