Nearly a thousand years ago, people saw an
exploding star.
This is the Crab Nebula, the wreckage of that
event. For most X-ray astronomers, it's the
brightest and steadiest beacon in the sky.
But now they realize it's not as steady as
they thought. Several orbiting X-ray observatories
have seen unexpected variations.
Most X-ray telescopes don't have sharp enough
vision to make images. Instead, they detect
the Crab as a broad source. From 1999 to 2008,
it brightened and faded by as much as 3.5
percent a year. And since 2008, it's faded
by 7 percent.
The Gamma-ray Burst Monitor on NASA's Fermi
satellite detected the decline and Fermi also
spotted two gamma-ray flares at even higher
energies.
What's going on? Scientists think the X-rays
reveal processes deep within the nebula, in
a region powered by a rapidly spinning neutron
star, the core of the star that blew up. This
image from NASA's Chandra X-ray Observatory
shows how complex this inner region is.
But figuring out where the Crab's long-term
X-ray changes are taking place will require
a new generation of hard X-ray telescopes.
Once regarded as an unchanging standard, the
Crab Nebula flickers from energy ultimately
provided by a long-dead star.
