Gamma-ray burst blasted Earth - study
2013-01-21 09:10
Paris - A mystery wave of cosmic radiation that smashed into Earth in the
eighth century may have come from two black holes that collided, a study
published on Monday says.
Clues for the strange event were unearthed last year by Japanese
astrophysicist Fusa Miyake, who discovered a surge in carbon-14 - an isotope
that derives from high-energy radiation - in the rings of ancient cedar trees.
Dating of the trees showed that the burst struck the Earth in either 774 or
775 AD.
But what was the nature of the radiation, and what caused it?
Space scientists lined up the usual suspects only to let them go. There was
no evidence that an exploding star, also called a supernova, occurred at that
time, they found.
The Anglo-Saxon Chronicle, a record in Old English, makes a dramatic
reference to the appearance of a "red crucifix" seen in the skies
after sunset. But that happened in 776 AD, which was too late to tally with the
event marked by the tree rings.
Also ruled out was a tantrum by the Sun, which can throw out sizzling cosmic
rays or gouts of energy called solar flares.
Black holes
Writing in Monthly Notices, a journal of Britain's Royal Astronomical
Society, German-based scientists Valeri Hambaryan and Ralph Neuhaeuser have
come up with a new explanation.
The pair suggest that two black holes collided and then merged, releasing an
intense but extremely brief burst of gamma rays.
A collision of neutron stars or "white dwarf" stars (tiny, compact
stars near the end of their lives) may also have been the cause, say Hambaryan
and Neuhaeuser of the University of Jena's Astrophysics Institute.
Mergers of this kind are often spotted in galaxies other than our own Milky
Way, and do not generate visible light.
The event in 774 or 775 AD could only have taken place at least 3 000 light
years from here, otherwise the planet would have fried, says the paper.
If their theory is right, this would explain why there is no record of some
ultra-brilliant event in the sky, or evidence of any extinction event in
Earth's biodiversity at that time.
Future risk
Astronomers should scour the skies because invisible remnants of the event
could well exist today, the paper suggests.
And estimating the risk from a future collision of this kind could be vital.
"If the gamma ray burst had been much closer to the Earth it would have
caused significant harm to the biosphere," explains Neuhaeuser.
"But even thousands of light years away, a similar event today could
cause havoc with the sensitive electronic systems that advanced societies have
come to depend on. The challenge now is to establish how rare such carbon-14
spikes are, or rather how often such radiation bursts hit the Earth.
"In the last 3 000 years, the maximum age of trees alive today, only
one such event appears to have taken place."
