Image: NASA/MIT/F. BAGANOFF et al.
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.
Image: NRAO/NRL/N.E. KASSIM et al.
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.