Simulation of a supermassive black hole as seen from above. Hot, compressed gas swirls around the event horizon. Blue indicates the radio waves oscillating sideways; red indicates waves oscillating vertically. The bright, thin ring is due to light that has circled the black hole once before leaving its environment.
Scientists announced today some of the most compelling evidence to date for the existence of a colossal black hole at the center of the Milky Way Galaxy, also determining a new and much smaller upper limit to the diameter of the mysterious object.
Experts were already fairly sure that the black hole resided at the center of our galaxy, packing the mass of 2.6 million Suns into an area smaller than our solar system. Their confidence came from reading the motions of millions of fast-moving stars that swarm around the dense, central object.
Yet they've never seen it -- black holes by definition are invisible. And they don't really know how much space it takes up.
In fact, no one is even sure it is a black hole. It might instead be a dense concentration of exotic "dark" stars, for example.
But in the new study researchers claim to have seen the edge of darkness by spotting rapidly pulsing X-ray emissions thought to be created when superheated gas or other matter zooms beyond the point of no return, through an "event horizon" that marks the time-bending and space-warping outer limits of a black hole. "This is extremely exciting because it's the first time we have seen our own neighborhood supermassive black hole devour a chunk of material," said Frederick Baganoff of the Massachusetts Institute of Technology, who led the study. "It's as if the material there sent us a postcard before it fell in."
He said the material could be a hot gas or solid matter roughly the size of a comet. Or the pulse of emissions could possibly be the result of some intense magnetic flare, similar to solar flares seen on our Sun.
The X-ray emissions were found near a previously known strong source of radio emissions, known as Sagittarius A* (pronounced "Sagittarius A Star") at the center of the Milky Way. Astrophysicists have long suspected that the compact source of intense radio emissions is related to what's known as a supermassive black hole, the largest type of black hole and a variety suspected of anchoring many large galaxies.
The new observations, detailed here at a Chandra X-ray Observatory symposium, represent what may be the most direct evidence for the black hole's existence, several scientists said. And though they stopped short of calling it proof, researchers are hopeful that a final answer will come within a decade.
Galactic puppeteer
Most of our galaxy is relatively uncrowded. The nearest star to our Sun, for example, is 4.2 light-years away.
But roughly 10 million stars are known to orbit within a light year of the galaxy's center, dashing along at phenomenal speeds of up to 3.1 million mph (5 million kph). This concentration of stars, and the speed with which they orbit, is the main clue that something hugely massive is at the center of the Milky Way, acting like a galactic puppeteer by providing the tremendous gravity needed to tug at stars with invisible strings.
If it is in fact a black hole, then it must meet the technical definition laid out by Einstein's general theory of relativity: All its mass must be locked and hidden inside a sphere known as an event horizon, beyond which nothing -- not even light -- can escape.
Just before matter swirls into a black hole, scientists predict it will approach the speed of light and become superheated, giving off X-rays prior to entering the event horizon.
Generally, the suspected supermassive black hole at the center of the Milky Way is quiet as black holes go, rarely coughing up X-rays or other forms of light radiation. This puzzling feature has made it difficult to study and casts some doubt on whether Sagittarius A*, the radio source, really represents a black hole.
So the X-ray observations announced Wednesday eliminated some of that doubt and filled a data gap.
Intense flares
The new observations, made in October of 2000 by the Chandra space telescope orbiting Earth, span about two hours. Flares of X-rays were 45 times more powerful than typical emissions that had previously been spotted. And wild variations occurred on shorter time frames: The number of X-rays per second dropped by a factor of five over one 10-minute period and then jump again to high levels 10 minutes later.
"Such breathless variability is rarely seen in emissions from ponderous multi-million solar-mass objects," said Fulvio Melia, a researcher at the University of Arizona's Steward Observatory. This kind of activity is typically reserved for smaller objects, like neutron stars.
Melia was not involved in the study but has written an analysis of it for the Aug. 6 issue of the journal Nature, where the results will be published.
Because Chandra only sees the area near the black hole as a point in space, the X-ray peaks represent the entire output around the sphere of the event horizon. So if strong peaks of activity can build in just 10 minutes, then the entire event horizon that contributes to the total emissions must be no larger than the distance light can travel in 10 minutes -- roughly the distance from Earth to the Sun, Melia said.
This implies a size of the event horizon that is some 1,500 times smaller than previous research could determine -- much closer to the size predicted by the theory of general relativity. Though it may be as small as relativity predicts -- 5.6 million miles (9 million km) in diameter -- the new study can only say that it is no larger than 112 million miles (180 million km).
Could it be a cosmic trick?
In 1999, the same research group used Chandra to detect weak but steady X-rays near Sagittarius A*, data that has not been widely reported.
Combined, the two sets of observations strongly indicate that the radiation most likely comes directly from Sagittarius A* and not from some other object, such as a star that orbits close Sagittarius A* in some sort of "binary" configuration, Melia said.
But the observations are not entirely conclusive.
"Nature has been cruel to astronomers in other circumstances, and this probability, although small, is not zero," Melia said. He added that much more convincing evidence would come if astronomers could spot X-ray emissions and radio emissions that occur simultaneously from the same location.
Studies to look for such a convergence are underway, and an answer could come within a year, several researchers agreed.
Baganoff hopes to lead the team that makes that discovery. Will scientists then declare, once and for all, that there is a black hole at the center of the Milky Way Galaxy?
Not quite. Baganoff said researchers would have to actually image the event horizon, not just the emissions from it. If he and his colleagues spot a correlation between radio and X-ray flares in Sagittarius A* within the next year, then he said he would bet his career that the event horizon will be imaged within a decade.
But, he said, he wouldn't be quite ready to bet his life on it.
Contributing to the study were researchers from Penn State University, MIT, UCLA, Caltech, and Insititute of Space and Astronautical Science, Japan.
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