Shocks jolt jet set galaxy, X rays reveal

A huge jet of high-energy particles shoots out from a supermassive black hole at

A jet (upper left) shoots out the supermassive black hole at the center of the galaxy Centaurus A. Around the galaxy, interstellar gas forms a haze that’s punctuated by X-ray point sources. These probably represent black hole or neutron star raids on neighboring stars. The gas at lower right balloons outward, possibly in response to pressure from a second, undetected jet. Kraft et al./NASA/SAO

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 month.

The new data revealed, in addition to 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.

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