The Crab Nebula, a tangled web of cosmic debris cast off by a supernova nearly 1000 years ago, is starring in a new action-packed film. The Hubble Space Telescope and the Chandra X-ray Observatory teamed up to take more than 30 images of the nebula's heart. The dynamic sequence--which spans about 8 months--has thrilled astrophysicists accustomed to static snapshots or mere points of light.
The spiky nebula is the famed remnant of a giant star that exploded when it ran out of nuclear fuel. At the Crab's center a dense neutron star spins 33 times each second, unleashing pulses of radiation. As it gradually slows down, this pulsar sheds energy along the axis of its intense magnetic field at a fantastic rate. The rotation and magnetism combine to whip particles around the pulsar into a frenzy approaching the speed of light, but how that works is poorly known.
Now, the new images have exposed jets, wisps, knots, and other features that roil the nebula's innermost cauldron, dramatically changing its shape from week to week. Hubble zeroed in on the nebula's core 24 times between August 2000 and April 2001, while Chandra took eight x-ray images during the same interval. The results, released 19 September at NASA headquarters in Washington, D.C., and published in the 20 September Astrophysical Journal Letters, illuminate striking sets of shock waves near the pulsar. "This is relativistic astrophysics in action," says team leader Jeff Hester, an astronomer at Arizona State University in Tempe.
A blazing x-ray ring girdles the plane of the pulsar's equator. At that spot, says Hester, a violent but steady wind streaming from the pulsar careens into a frothy shock front of disordered electrons. Wisps of particles flit outward from the x-ray ring at half the speed of light. The wisps form crisp, narrowly defined arcs confined to the equatorial plane, probably held in place by tight lines of magnetic field whipping out from the pulsar. Meanwhile, at right angles to the plane, diffuse jets of particles blast into the nebula from the pulsar's rotation poles. One jet looks like a puffy plume from an industrial smokestack on a windy day, buffeted to and fro by turbulence around it.
Debates about the mechanisms driving the shocks will take time to settle, says Chandra project scientist Martin Weisskopf of NASA's Marshall Space Flight Center in Huntsville, Alabama. "There is so much detail," he says. "We all want to know how this pulsar converts its rotational energy into electromagnetic radiation with such amazing efficiency. It's a fascinating puzzle."