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The CfA Almanac Vol. XIII No. 2, July 2000
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HOT "BUBBLES" FOUND AT THE HEART OF THE MILKY WAY

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 the massive black hole 26,000 light-years distant at the heart of our own Milky Way Galaxy.

Jun-Hui Zhao of the CfA 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.

galactic center
A VLA image of the central region of the Milky Way. The bright white dot in the center is Sgr A*, the object believed to be our galaxy's exact center. (Image by Jun-Hui Zhao, CfA, and Miller Goss, NRAO)
"We think there is a rapidly-rotating disk of material that is being drawn inward toward the black hole," explains 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," says Goss, NRAO's Director of VLA/VLBA Operations. "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 says.

Ramesh Narayan of CfA 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 variation 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 estimated 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 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."

--David Finley, NRAO

solar flare solar flare
On June 6, 2000, TRACE observed two X-class flares (the largest, most intense class of solar flares). The image on the left is a composite image taken at 15:00 UT showing a combination of white-light, ultraviolet, and extreme-ultraviolet images. The image on the right is a 171A EUV image taken at 15:10:15 UT, showing the very bright core of the flaring region and some very bright loops around it. The structure running from the image center to the top is in eruption, forming the start of a large coronal mass ejection.

"PRESSURE-COOKER" MODEL MAY HELP EXPLAIN--AND PREDICT--GIANT MASS ERUPTIONS FROM THE SUN

A three-dimensional numerical model developed by scientists at the Naval Research Laboratory (NRL) and "field tested" by astronomers at SAO may help explain the nature and origin of massive solar eruptions. These eruptions are known to trigger stormy space weather, which, in turn, can damage communications satellites, endanger astronauts in space, and disrupt transmissions along electrical power lines on Earth.

A theoretical reconstruction of the evolving magnetic fields in a flaring region on the Sun, using NRL's "pressure-cooker model" (as it has been informally nicknamed), matched actual observations of the so-called "Bastille Day" Flare on July 14, 1998, made by SAO scientists using NASA's Transition Region and Coronal Explorer (TRACE).

According to the NRL-SAO team, this close agreement between theoretical predictions and observations demonstrates the viability of the model for understanding how energetic solar eruptions occur and perhaps even predicting them in advance.

The key strength of the pressure-cooker model is its novel description of the conditions allowing the release of energy leading to eruptive flares and coronal mass ejections (CMEs). Magnetic energy emerges from the solar interior, appears low in the solar atmosphere, and accumulates under a magnetic "lid." When the lid has a weak spot--in this case, the null point, where the positive and negative poles of the magnetic field cancel each other--the accumulated low-lying energy can blow the lid off the "pressure cooker."

The NRL "pressure cooker" differs from competing CME models in that the magnetic field is assumed to have a more complicated (quadrupolar) geometry, with a null point located high in the solar atmosphere. Other models, which assume a simpler (bipolar) geometry and rely on different mechanisms for triggering eruptions, "have never been compared with actual solar observations as precisely as we did in our study," notes Guillaume Aulanier of NRL.

This is one of the first attempts by the NRL-SAO research collaboration to reconstruct and interpret the complex magnetic field of a real observed flaring region. TRACE's Bastille Day coverage provided some of the finest detailed observations of an eruptive flare ever obtained. In particular, the continuous telemetry of the instrument provided more data on pre-flare activity than has previously been available, which was essential for validating the model and confirming the pressure cooker process. The scientists intend to use additional TRACE data, SOHO/EIT observations, and future data from the SOLAR-B and STEREO missions to further test the pressure-cooker model.

The scientific paper describing this research will be published in the Astrophysical Journal in September 2000.

--Judith Karpen, NRL

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