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May 30, 2000


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VLA Reveals Hot "Bubbles" in Swirling Disk at Milky Way's Heart

Sophisticated computer analysis of 20 years of data from the National Science Foundation's Very Large Array (VLA) radio telescope has revealed evidence of hot "bubbles" in the dense, rapidly- spinning disk of material being sucked into a massive black hole 26,000 light-years distant at the heart of our own Milky Way Galaxy, astronomers announced.

Jun-Hui Zhao of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA; and Miller Goss and Geoff Bower of the National Radio Astronomy Observatory (NRAO) in Socorro, NM, discovered that an object at the Milky Way's center shows pulses in its radio emission every 106 days. The object, known as Sagittarius A* (pronounced "A-star"), was discovered in 1974, and is believed to harbor at its core a black hole 2.6 million times more massive than the Sun.

"We think there is a rapidly-rotating disk of material that is being drawn inward toward the black hole," explained Zhao. "Because the material rotates faster in the inner parts of this disk, friction heats up the inner parts of the disk more than the outer parts. We believe that some of this hot, inner material forms "bubbles" that move into the outer, cooler parts of the disk by convection -- like boiling," Zhao said. The appearance of these bubbles causes the object to emit more radio waves.

"Explaining why such bubbles are formed every 106 days is going to be a challenge for theoretical astrophysicists," said Goss, NRAO's Director of VLA/VLBA Operations, and a veteran researcher and recognized expert on the Milky Way's central region. "The amount of radio emission from this object had been seen to vary before, but this is the first time any regular, quasi-periodic variation has been found," Goss said.

Ramesh Narayan, a Harvard University astronomy professor and noted theorist, called the discovery of the periodic variation "very exciting" because it can provide "powerful clues about the nature of the object."

"What is happening is anybody's guess. It is now up to astronomers to decode the signal and figure out exactly what is going on around this black hole," Narayan said.

Disks, called accretion disks, like that at the Milky Way's center are thought to exist around black holes in double-star systems as well as around the supermassive black holes at the hearts of galaxies and quasars. The physics of these disks, which emit vast amounts of energy and often propel "jets" of subatomic particles at tremendous speeds, is not well understood.

When astronomers find a periodic variation in the brightness of an object, whether in visible light, radio waves or other wavelengths of electromagnetic radiation, they often suspect they are seeing the effect of one distinct object orbiting another. In this case, however, according to Kepler's Laws of orbital motion, an object orbiting the black hole every 106 days would be significantly farther from the black hole than Pluto is from the Sun. "With NRAO's Very Long Baseline Array (VLBA), we could see an object that far away from the black hole, and, since nothing like that appears in VLBA images of the region, we conclude that the variation we observe must be occurring in the accretion disk itself," Zhao said.

"Finding the periodic varation in the radio-wave output of this accretion disk will provide critical information needed to help improve our understanding of how these disks work," said Zhao. The accretion disk is believed to have a diameter approximately equal to that of Jupiter's orbit around the Sun, because that is the size of Sagittarius A*'s radio-emitting region.

The researchers used data taken from the VLA's massive observational archive that includes information produced by more than 20 years of continuous observations. All the observational data from the VLA has been preserved by the NRAO and is available for scientific use. For the study of Sagittarius A*, Zhao, Goss and Bower analyzed more than 500 individual VLA observations made over the past two decades.

"The variation we found could only have been detected by using observational data over many years from the same radio telescope, so we could make exact comparisons," said Goss. "In addition, this project would have taken a prohibitive amount of time with the computing power we had available only a few years ago."

"This is an excellent example of why we keep an archive of all the data produced by the VLA," Goss said.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.