Shocks jolt jet set galaxy, X rays reveal.

A huge jet of high-energy particles shoots out from a supermassive black hole at the core of nearby galaxy Centaurus A. A new X-ray snapshot catches that activity in unprecedented detail. Besides depicting one of the universe's most spectacular types of pyrotechnics, the view will help clarify theories about the formation of such jets, astrophysicists say.

The image was taken by NASA's orbiting Chandra X-ray Observatory, which began transmitting X-ray portraits of cosmic objects in July 1999 (SN: 10/21/00, p. 266). The instrument was designed to detect the high-energy X rays emitted by some of the most violent events in the universe, such as jet-producing collisions between stars and a galactic core's supermassive black hole, which can weigh in at millions of times the mass of the sun.

The X rays in these jets are produced when charged, high-energy particles spin in magnetic fields. Scientists studying the extreme physics of jet formation have long suspected that these particles get a boost of energy as they speed beyond the core of a galaxy. The new data provide solid evidence for this.

The image reveals 31 regions with enhanced X-ray emissions within the jet of Centaurus A. Previous images created by less discerning telescopes portrayed jets as having a uniform composition, says team leader Ralph P. Kraft of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Kraft says he suspects that the 31 X-ray-rich regions represent "shocks" where particles somehow get a boost in energy. Such shocks may occur when fast-moving particles catch up to and then rear-end slower-moving particles in the jet. The new Chandra image supports this view, says Kraft.

Jonathan Grindlay, also at the Harvard-Smithsonian Center, agrees. "It's a spectacular image," he says. Grindlay isn't a member of Kraft's team, which announced its findings earlier this

The new data revealed, in addition the jet architecture, over 200 X-ray point sources within Centaurus A--10 times as many as previous analyses had shown. Most of the point sources, says Kraft, result from cannibalism. When a stellar-size black hole or a much smaller, superdense neutron star filch material from a neighboring star, huge amounts of energy are released. That energy is visible as a bright spot called an X-ray binary.

In concurrent observations, Chandra has tabulated the number of X-ray point sources in several other nearby galaxies. Analyses of these data are already sufficient to invalidate some ideas about X-ray binary formation. For example, one theory has it that the most massive galaxies, as inferred by measurements of their total starlight, have the most X-ray binaries. Instead, says Kraft, the Chandra data suggest that the X-ray binary population may be determined by factors such as the number of dense groupings of stars, where increased star proximity may encourage cannibalism.

"Something very interesting is going on," concurs Grindlay.

Centaurus A is the nearest galaxy to Earth that has an active supermassive black hole. With the new data in hand, astronomers can now train Chandra's eye on more-distant galaxies of the same type and gather more data for honing their theories about how X-ray binaries and jets form.