The jets -- which were some of the fastest fountains of material ever detected -- were revealed by recent radio observations conducted by Robert Hjellming, an astronomer at the National Radio Astronomy Observatory in Soccoro, New Mexico.
Hjellming used the Very Large Array (VLA) of radio antennas to develop a rough image (shown above) of the X-ray source. The array is a collection of 27 dish antennas set on a plateau in the New Mexico desert. They can be spread out to cover an area 22 miles (36 kilometers) across to obtain high-resolution images of radio sources.
Hjellming is using the image of V 4641 Sagittarius to calculate the velocity of the jets, and has found them to be moving at about 90 percent the speed of light.
"At the moment there is still some uncertainty of whether it's 85 percent or 90 or 92 percent [the speed of light]," he said. The uncertainty arises from the fact that that nobody is sure exactly how far away the object is. It is seen in the constellation Sagittarius and is somewhere between 1,500 and 3,200 light-years from Earth between our planet and the center of the Milky Way. (Earth's sun is about 24,000 light-years from the galactic center. A light-year is 5.88 trillion miles.)
The discovery of jets is creating excitement among astronomers and astrophysicists because it is one of only a handful of objects that exhibit jets moving at such extremely high speeds. It may provide a case for scientists to learn how blacks holes work, and may allow them to test Einstein's theory of general relativity.
The object's behavior is unique in several respects. It produced one of the brightest X-ray bursts ever seen and, within a matter of days, its X-ray emissions had virtually disappeared.
A similar quick decay of the radio emissions occurred, Hjellming said.
After a few days, the energetic radiation that produced the above image of the jets had faded, and only a few dim points remained: one spot at the position of the center of the image and one at the very south end of the lower jet.
Most astronomers agree that the object is a binary system in which a star and the compact object are orbiting each other, but one question that remains is where in the system the visible star is located.
"The bet now is that the location is actually at the southern extension of the southern jet," Hjellming said.
Further observational work will need to be done on the object to determine the exact distance and the mass of the dense companion object. If the mass turns out to be greater than three times that of the sun, then it is certainly a black hole, Hjellming said.
The great mystery that scientists are working to explain is what produces the intense jets of plasma that create the radio emissions.
Black holes are objects so tremendously dense that nothing, not even light, can escape their gravitation. A common black hole is thought of as being an object the size of a small metropolitan city with the mass several times that of the sun. Material falling into a black hole swirls around it, something like water swirling down a drain, creating a rotating mass called an accretion disk.
It is rather simple to explain X-ray bursts as flashes of energy released by material falling from the accretion disk into a black hole, Hjellming said.
But no one knows how to make the connection between in-falling matter and a high-speed fountain that produces the radio jets, Hjellming said.
"Everybody knows that happens. In NASA's cartoons of black holes the artists always draw jets," he said, "But how those jets produce the type of plasma that we're seeing radio emission from is still a missing link that we haven't figured out."
The physics of what happens around a black hole has confounded everybody who has worked on it.
Steven Eikenberry is an astronomer at Cornell University who has spent the past two years studying another black-hole candidate that has produced jets similar to those seen in Hjellming's observations. Eikenberry and several colleagues have been struggling to develop a model that will explain the way black holes produce jets. He calls current ideas about black holes "toy models" of very complex systems.
The basic picture that Eikenberry and his co-workers have developed is that matter in a black hole's accretion disk may be warping its magnetic fields.
"Basically, as you're dumping material on, it's spiraling in, and that tends to tangle up the magnetic fields," Eikenberry said.
"What may happen is, once you get the magnetic field too tangled, it will just reconnect suddenly," he offered. "It'll just sort of untangle itself and, in doing so, release a whole lot of energy."
This sudden snap back into a stable, undistorted state shoots charged-particle radiation into space. At the same time, it creates ordered magnetic field lines that become an axis along which the charged plasma will project out of the system. The two magnetic pole lines become the conduits along which material flows out in straight jets.
This is only one of the many guesses people are making, though, Eikenberry explained.
Whatever theory scientists come up with to explain the behavior of objects like V 4641 Sagittarii, observing these objects may give scientists a better understanding of not only individual black holes, but also of the engines that power entire galaxies.
Small black holes in Earth's galactic neighborhood could be models that allow scientists to understand the gigantic ones that likely live at the center of galaxies, said Ronald Taam, an astrophysicist at Northwestern University who studies them.
"We know that active galactic nuclei also show evidence of jets," Taam said. "The model for them is a massive black hole in the center of the galaxy."
Such black holes could be as massive as hundreds of millions, or billions of suns, which would make them so massive that they would change so gradually, on time scales so long, that humans would not be able see the variation.
"Here we have objects in our own galaxy which can be thought of analogs of what are happening in galactic nuclei," Taam said. "Here is a system where we might be able to learn about the physics that are taking place on a time scale we can actually study reasonably."
Another great opportunity that black holes afford is that they will allow physicists to test Einstein's theory of general relativity. Einstein predicted that time and space are modified when influenced by extremely strong gravitational fields, like those around black holes.
Taam and others look forward to studying how matter behaves around objects like V 4641 Sagittarii in order to examine whether or not Einstein's ideas about the behavior of space and time are accurate.
"Here you're actually able to probe relativity in a strong-field regime," Taam said. "This is tremendously important because we think that Einstein's theory is the most significant and relevant [one] of the 20th century. We are always looking for ways to test it."
advertisement
