The Crab’s Shimmering Shock Waves

Crab waves
These composite X-ray and visible-light images reveal a ripple of matter (arrows) racing outward at half the speed of light from the Crab Nebula's bright inner X-ray ring. Time-lapse movies made from images spanning several months show the outward expansion more clearly. Also evident is a bright jet shooting out from the pulsar's spin pole.
Courtesy Jeff Hester (Arizona State University) and others, NASA.
When a supernova exploded in the constellation Taurus in 1054, it left behind a wispy, glowing remnant known as the Crab Nebula and, at its heart, a highly magnetized pulsar spinning 2½ million times faster than Earth. Astronomers have suspected for decades that the inner nebula is evolving, and now they've got proof. Jeff J. Hester (Arizona State University, Tempe) and his colleagues have combined images from the Chandra X-Ray Observatory and the Hubble Space Telescope to create an intricately detailed movie highlighting the nebula's dynamics.

Hester and his team concentrated on an X-ray halo around the pulsar first observed by Chandra three years ago, and their movie captures a never-before-seen sequence of events. The ring, it turns out, marks the location of an "unstable, quasi-stationary shock" powered by a wind of electrons and antimatter electrons thrown outward from the pulsar's equator. As these particles crash into the ring, several knots periodically brighten and darken, and from there thin, bright filaments of matter race outward at half the speed of light.

Shock waves form when objects move faster than the wave speed of the medium surrounding them. In the Crab's case, the matter-antimatter electrons break the electromagnetic equivalent of the sound barrier as they move outward. Team member David Burrows (Pennsylvania State University) says the shock wave is analogous to a stream of water from a faucet. When it hits the sink below, the water sprays out in all directions in a smooth, seamless flow. But at some point the character of that flow changes dramatically, becoming turbulent and bubbly. The boundary between those two styles of flow in the Crab Nebula, Burrows explains, is the shock.

Pulsars are highly magnetized neutron stars with dizzyingly fast rotations (the Crab's rotates 30 times per second). But nobody has yet found an answer to "the fundamental physical problem of how to convert this rotational energy into this spectacular emission that you see — these particles of matter and antimatter," says Victoria Kaspi (McGill University). The Chandra-Hubble movie will help solve this mystery, and it has already heightened interest in studying about three dozen other Crab-like remnants in the Milky Way.

Hubble observed the Crab pulsar's surroundings with the Wide-Field Planetary Camera 2 at 11-day intervals between August 2000 and April 2001; Chandra viewed it on eight occasions between November 2000 and April 2001 using its Advanced CCD Imaging Spectrometer. Details of the observations appear in the September 20th issue of Astrophysical Journal Letters and in a NASA press release.