Astronomers are scrambling to understand what caused a fleeting but extremely powerful burst of radio waves that originated beyond the Milky Way. Initial suspects include the merger of two dense stellar corpses called neutron stars and the complete evaporation of a black hole.
Finding more such events in new or archived data could help astronomers make the first ever detection of ripples in spacetime – which should occur when neutron stars merge, according to Einstein’s theory of general relativity.
The burst was discovered by David Narkevic, an undergraduate student at West Virginia University in Morgantown, US, while he was searching through archived data taken in 2001 by the Parkes radio dish in Australia. He was looking for periodic signals from pulsars – rotating neutron stars – within our galaxy.
But he came upon a short burst of radio emission that appeared to come from outside the Milky Way. Because electrons in space cause different frequencies of radio waves to arrive at Earth at different times, the researchers used the delay to estimate that the source probably lay about 1.6 billion light years away. “Our first reaction was, what in the world is this?!” says Narkevic’s advisor, Duncan Lorimer. “I didn’t expect to see anything out there at all.”
The burst released the energy that the Sun puts out in one month, but it lasted just 5 milliseconds. Because light can only travel about 1500 kilometres in that time, the researchers believe the object that caused the burst must be no wider than that. “There aren’t too many things in the universe that can do that,” Lorimer told New Scientist.
Pulsars are the right size, but they emit pulses of radiation at periodic intervals – and this burst was seen only once in 90 hours of observations. That suggests it was a single, cataclysmic event, such as a black hole that evaporates completely after gradually losing its energy in a quantum process called Hawking radiation.
“This is getting speculative, but it could be the last gasp of a black hole, [since] there is supposed to be a little burst of energy associated with that event,” says Lorimer.
Alternatively, it could be the merger of two neutron stars, which theorists calculate should last only a few milliseconds. Neutron stars also have powerful magnetic fields that would cause charged particles to zoom around them at near-light speeds. “That’s what radio emission is – coherent electrons spiralling in magnetic fields,” explains Dale Frail of the National Radio Astronomy Observatory in Socorro, New Mexico, US.
Such mergers are also thought to cause volleys of high-energy radiation, called short gamma-ray bursts (GRBs). But when the team searched archived data from gamma-ray observatories operating at the time, they found no corresponding signals.
Lorimer says the gamma-ray photons may have travelled out from the merger in a narrower beam than the radio photons, so no GRBs were detected. “We might have just missed the gamma rays completely because they weren’t pointing at us,” he says.
The team says hundreds of similar bursts may explode across the sky every day, but they have gone undetected until now because today’s radio telescopes focus on only small patches of the sky at once. The team is now scouring other archived data to see if any bursts were accidentally caught during observations of other objects, and they have already found a few candidate bursts, though none as bright as this event.
“We’re trying to design new detectors that will specifically target these events and we’re thinking of using existing telescopes at Green Bank to stare at the sky and look for more of these systems,” says Lorimer.
Needle in a haystack
Future radio observatories, such as the Allen Telescope Array (pictured at right) being built in California, US, will be able to survey larger fractions of the sky at once and could turn up more of the bursts, says Frail.
Frail is particularly excited about the possibility of using the bursts to pinpoint possible mergers between neutron stars, which general relativity predicts should emit gravitational waves – a signal that has so far eluded detection.
“This is the exciting part from my point of view,” he told New Scientist. “Radio astronomy is linked in a vital way to the detection of gravitational waves. You can scan the sky looking for the signal of these collapses.”
Lorimer agrees. Gravitational wave observatories have a bewildering array of possible signals to sift through in their search over the whole sky. “They have a huge computational problem on their own,” says Lorimer. “But if they can get a pointer – why don’t you look for gravitational waves from there – it greatly narrows down the haystack they have to go through to find the needle.”
Journal reference: Sciencexpress (DOI: 10.1126/science.1147532)