Signs In The Sun, The Moon and The
Stars
11 May 2011 Last updated at 10:20 ET
Crab Nebula's massive gamma-ray flare mystifies astronomers
By Jason Palmer Science and technology reporter, BBC News, Rome
A Hubble classic: The Crab Nebula is about 6,500 light-years from
Earth
BBC - The Crab Nebula has shocked astronomers by emitting an
unprecedented blast of gamma rays, the highest-energy light in the
Universe.
The cause of the 12 April gamma-ray flare, described at the Third
Fermi Symposium in Rome, is a total mystery.
It seems to have come from a small area of the famous nebula,
which is the wreckage from an exploded star.
The object has long been considered a steady source of light, but
the Fermi telescope hints at greater activity.
The gamma-ray emission lasted for some six days, hitting levels 30
times higher than normal and varying at times from hour to hour.
While the sky abounds with light across all parts of the spectrum,
Nasa's Fermi space observatory is designed to measure only the
most energetic light: gamma rays.
These emanate from the Universe's most extreme environments and
violent processes.
The Crab Nebula is composed mainly of the remnant of a supernova,
which was seen on Earth to rip itself apart in the year 1054.
At the heart of the brilliantly coloured gas cloud we can see in
visible light, there is a pulsar - a rapidly spinning neutron star
that emits radio waves which sweep past the Earth 30 times per
second. But so far none of the nebula's known components can
explain the signal Fermi sees, said Roger Blandford, director of
the Kavli Institute for Particle Astrophysics and Cosmology, US.
"The origin of these high-energy gamma rays has to be some other
source," he told BBC News.
"It takes about six years for light to cross the nebula, so it
must be a very compact region in comparison to the size of the
nebula that's producing these outbursts on the time scales of
hours."
Since its launch nearly three years ago, Fermi has spotted three
such outbursts, with the first two reported earlier this year at
the American Astronomical Society meeting.
These events are unleashing gamma rays with energies of more than
100 million electron-volts - that is, each packet of light, or
photon, carries tens of millions of times more energy than the
light we can see.
But the Crab's recent outburst is more than five times more
intense than any yet observed.
'Big puzzle'
What has perplexed astronomers is that these variations in gamma
rays are not matched by changes in the emission of other light
"colours". Follow-up studies using the Chandra X-ray telescope,
for example, showed no variations in the X-ray intensity.
Kavli Institute researcher Rolf Buehler outlined the details of
the Crab's flashes to the meeting on Thursday.
"If you look in optical light, the Crab is very steady; in radio
emission, it's very steady; in very, very high-energy gamma rays
it's very steady. Only in this part between do we see it varying,"
he told BBC News.
"That's why people hadn't found this before; there was not an
instrument like Fermi sensitive enough to capture it."
Understanding the flare, however, may take some time, Dr Buehler
said.
"To have something that puts almost all of its energy into gamma
rays is an unusual thing," he said. "We're looking at a big puzzle
and are probably going to need a couple of years to understand
it."
The best guess so far is that in a region near the neutron star,
intense magnetic fields become opposed in direction, suddenly
re-organising themselves and accelerating close-by particles to
near the speed of light.
As they move in curved paths, the particles emit the gamma rays
seen by Fermi.
Fermi project scientist Julie McEnery said that the find was a
testament to the power of the Fermi telescope to elucidate new
physics in the cosmos.
"It's just so extraordinary that so many telescopes over so many
years have been looking at the Crab and it's been constant all
that time, and suddenly we discover that it's not," she told BBC
News.
"With Fermi, we have the opportunity to catch it when it's in this
extraordinarily flaring state - it really brings home the
advantage of having an instrument that looks at the whole sky all
the time, because you catch the unexpected."
The US-space-agency-managed telescope was launched in 2008. It
honours Enrico Fermi, the great Italian-American physicist who
worked on the development of the first nuclear reactor and who was
awarded the Nobel Prize in Physics in 1938 for his work on
radioactivity.