The Crab Nebula is one of the brightest sources
of high-energy radiation in the sky. Little
wonder, it's the expanding remains of an exploded
star, a supernova seen in the year 1054. Recently,
astronomers using satellites sensitive to
the highest energy form of light -- gamma
rays -- have observed incredible flares in
the nebula that theorists are hard-pressed
to explain. The supernova left behind a magnetized
neutron star -- a pulsar. It's about the size
of Washington D.C., but it spins 30 times
a second. Each rotation sweeps a lighthouse-like
beam past us, creating a pulse of electromagnetic
energy detectable across the spectrum. Here's
what the sky looks like in high-energy gamma
rays. The pulsar in the Crab Nebula is among
the brightest sources. As the pulsar spins,
its powerful magnetic field causes particles
to flow. These currents ultimately light up
the nebula. But as bright as the pulsar is
in gamma rays, it isn't the source of the
flares. NASA's Fermi Gamma-ray Space Telescope
can look in between the pulsar's brilliant
pulses to reveal the faint gamma rays from
the underlying nebula. Yet several times since
2009 enormous flares have erupted somewhere
within the nebula. The most powerful one to
date lasted six days and made the nebula 30
times brighter than normal and five times
brighter than previous flares. During the
huge flare, astronomers also studied the Crab
with NASA's Chandra X-ray Observatory. Chandra's
keen X-ray eye saw lots of activity, but none
of it seems correlated to the superflare.
This hints that whatever's causing the flare
is happening within about a third of a light-year
from the pulsar. And rapid changes in the
rise and fall of gamma rays imply that the
emission region is very small, comparable
in size to our solar system. Scientists say
the gamma rays most likely arise from electrons
moving near the speed of light, which emit
gamma rays as they interact with magnetic
fields. But to account for these flares, the
electrons must have the highest energies ever
seen in cosmic sources-- 100 times higher
than can be achieved in the most powerful
particle accelerators on Earth. Even after
a thousand years, the heart of this shattered
star still offers scientists glimpses of staggering
energies and cutting-edge science.
