A giant x-ray flare from the heart of our galaxy is helping researchers test ideas about a black hole they believe lurks there.

A giant x-ray flare from the heart of our galaxy recorded by the Chandra X-ray Observatory last year has brought astronomers closer than ever to a black hole that most believe lurks there. "This discovery takes us to the extreme," says Fulvio Melia, an x-ray astronomer at the University of Arizona. The brilliant burst came from only light-minutes away from the black hole's surface, Melia and others claim. Although researchers are still speculating about the cause of the flare, they have already used it to test several theories about the black hole and to put new limits on its size.

Since the late 1970s, astronomers have kept watch on an object at the Milky Way's core known as Sagittarius A-star. When viewed through a radio telescope, Sagittarius A-star appears so bright that many believe only a black hole's gravity could fuel it. Black holes are massive stellar objects whose gravitational fields cause the space and time around them to fold in upon itself creating a so-called singularity from which nothing, not even light, can escape. Researchers believe that Sagittarius A-star is a black hole surrounded by a whirling disc of gas. Gravity accelerates the nuclei and electrons in the gas atoms, causing them to radiate different kinds of light. Most of that light scatters into the interstellar dust that lies between us and the Milky Way's core, but radio waves and x-rays can penetrate the shroud of dust to reach the earth.

Only radio waves can break through our dense atmosphere, so it wasn't until the launch of NASA's state-of-the-art Chandra X-ray Observatory two years ago that scientists were able to get a look at the x-rays coming from Sagittarius A-star. At first, Sagittarius A-star appeared much fainter than expected, and astrophysicists concluded that there must be less material around the black hole than they previously thought. Their revised calculations predicted that large flares would periodically erupt from the black hole as chunks of matter fell into it. But no one saw such a flare until last year, when Chandra witnessed a burst of x-ray light streaming from the galactic center. The flare was 50 times as bright as the sun and lasted for about three hours. When it was over, researchers were convinced that they had witnessed a spectacular event from the Milky Way's black hole.

Researchers have since used information about the flare to put new constraints on the size of the black hole and the material surrounding it. For 10 minutes during the three-hour event, the flare momentarily dimmed. That flicker was all that astronomers needed to determine the flare's size. Because different regions of the flare can react to each other only at the speed of light, if the entire flare dimmed over a period of 10 minutes then the flare could not be more than 10 light-minutes wide.

Ten light-minutes corresponds to a distance of 100 million miles, roughly the distance from the earth to the sun, so researchers determined the flare was no larger than the earth's orbit. That made it about 20 times larger than the predicted size of the black hole, but researchers believe that it could be even smaller. Near the surface of a black hole, time slows to a stop, explains Penn State University's Gordon Garmire, a member of the team that discovered the flare. As a result, the apparent 10-minute interval researchers used to determine the flare's size could actually have been shorter. Regardless of the precise time, the gas creating the flare was closer to the black hole's surface than any other object ever observed.

Prior to this discovery, researchers could not rule out other explanations for the 2.7-million-solar-mass gravitational field at our galactic center. For example, a cluster of dead neutron stars or a massive ball of neutrinos could cause the pull at the galactic core. But the small area of the flare put these and other theories to rest, leaving a black hole as the only possibility. "At this time, we don't have another physical explanation," says Frederick Baganoff of the Massachusetts Institute of Technology, the head of the research team.

Although the flare provided crucial information about the Milky Way's black hole, scientists are still unsure what caused it. One possibility is that the giant magnetic fields thought to exist around the surface of the hole sparked the flare. Like the fields around our sun, these black hole's magnetic fields can periodically "reconnect," a process that could catapult material from near its surface. Another theory is that a comet-size object falling into the black hole sparked the x-ray storm. If that were true, it would be "extremely exciting," according to Baganoff. "It's as if the material there sent us a postcard just before it fell in," he says.

Sometime in the near future, astronomers hope to observe a second flare like this time while both Chandra and radio telescopes are watching. The combined radio wave and x-ray information could help them to more precisely determine the location of the flare and would further corroborate the black hole theory. But ultimately, astronomers will be unable to prove that a black hole lurks at our galactic center until they have actually seen its surface. "That, to me, would be definitive proof," Baganoff says. And it will take at least another 10 years before a new generation of observatories allows that to happen.

— NRAO: The National Radio Astronomy Observatory