June 28, 2007
O ne of the astonishing qualities of a black hole is its ability to power great jets of energy and matter into space.
Now, using NASA's orbiting Chandra X-ray Observatory, a team of astronomers, led by Sebastian Heinz of the University of Wisconsin-Madison, has obtained evidence that neutron stars — the dense, collapsed cores of exploded stars — produce jets that, relatively speaking, rival those produced by black holes.
The new work reveals the first-known neutron star with a powerful large-scale jet, a quality previously attributed only to black holes. Because jets can be diagnostic of extreme objects like black holes and their environments, the new finding may help astrophysicists better understand the nature of both black holes and neutron stars.
"We've known for a long time that black holes are good at making jets," says Heinz, a UW-Madison assistant professor of astronomy. "This discovery shows that neutron stars are just as good at making jets."
The object, known as Circinus X-1, is actually a pair of stars in mutual orbit, a binary system located in the southern hemisphere constellation Circinus about 20,000 light-years from Earth.
The neutron star is locked in a celestial waltz with a companion star several times the mass of the Sun. Gravity draws material from the companion star into a spinning disk that spirals slowly toward the neutron star, accreting onto its surface and somehow powering the plasma jets that beam energy and matter into space at nearly the speed of light.
"Jets are these mysterious beams of matter," Heinz says, explaining astronomers are only beginning to figure out how the beams are made and what they're made of. "These are plasma beams that get launched in the inner regions of accretion flows."
The jet observed by the international team of astronomers is about 3 light-years long, but it emanates from an object that has a mere 10-kilometer radius. Jets from super-massive black holes, on the other hand, can extend for several million light-years.
However, measured on the scale of the object generating it, Circinus X-1 is "certainly very impressive. This is one of the largest jets we know of relative to the size of the object that powers it. In terms of power, it seems to have the same efficiency as a black hole," Heinz says. "The fact that neutron stars are just as efficient in making jets, despite having shallower gravitational potential and none of the gimmicks that spinning black holes have, is an important new insight."
Some black holes have what are called "ergospheres," which are akin to twisting funnels that suck everything toward the center, forcing matter into a faster and faster rotation. "It is this twisting of space-time that occurs around a spinning black hole that people had thought might be responsible for making black hole jets so efficient," Heinz explains.
The Circinus X-1 jets measured by Chandra behave much like jets from black holes: Tightly-bundled energy mixed with a pinch of matter is blasted into space like water from a fire hose. When the jets reach the gas of interstellar space, they seem to inflate radio nebulae, lobes of very hot, radio-emitting gas.
Knowing how jets from black holes and other objects work is essential because they are thought to be engines for powering the universe, says Heinz.
"People have realized black holes and jets from black holes are important for energizing the universe. They carry huge amounts of energy in kinetic form, and the release of this energy is very important in terms of how structures form in the universe at large."