MIT researchers announced a handful of new findings at last week's 195th meeting of the American Astronomical Society (AAS) in Atlanta. Institute scientists reported the discovery of Earth's closest black hole; unveiled an animated video that reveals the X-ray sky with unprecedented clarity; and described revelations from the Chandra X-ray Observatory, including an expanding ring-like structure of oxygen and neon that provides details about the creation and dispersal of heavy elements necessary to form planets like Earth and a faint source that may be the long-sought X-ray emission from a known supermassive black hole at the center of our galaxy.
The discovery of the closest black hole yet, a mere 1,600 light years from Earth, was heralded by four of the most dramatic, rapid X-ray intensity changes ever seen from one star, said researchers from MIT and the National Radio Astronomy Observatory (NRAO) operated by the National Science Foundation (NSF).
The black hole in the constellation Sagittarius, along with a normal star dubbed V4641 Sgr, form a violent system that briefly flooded part of our Milky Way galaxy with X-rays and ejected subatomic particles moving at nearly the speed of light one day last September. At the peak of its X-ray output, V4641 Sgr was the brightest X-ray emitter in the sky.
Astronomers call this type of system an X-ray nova because it suddenly becomes a bright source of X-rays, but this object shows characteristics never seen in an X-ray nova.
"V4641 Sgr turns on and off so fast that it seems to represent a new subclass of X-ray novae," said Donald A. Smith, a postdoctoral associate in MIT's Center for Space Research. Dr. Smith worked on data from this object with MIT principal research scientist Ronald Remillard and NRAO astronomer Robert Hjellming. "In X-rays, the intensity rose by a factor of more than 1,000 in seven hours, then dropped by a factor of 100 in two hours," Dr. Remillard said.
V4641 Sgr excites astronomers because it is close and because it acted so differently from other microquasars. In other microquasars, outbursts have dimmed more slowly over weeks or months rather than hours.
"Because this system happens to be so close to us, it is very likely that there are more objects like V4641 Sgr waiting to be discovered," Dr. Smith said. "The rapidly flaring systems in our galaxy may have been too faint and too fast for us to notice them."
What makes it so different? Astronomers aren't sure, but Dr. Remillard speculated that "in V4641 Sgr, either the matter can flow into the black hole without forming a large accretion disk, or the black hole itself is significantly different in its mass, spin or charge."
If future searches for brief X-ray flares reveal that there are more objects like V4641 Sgr, "we will have a whole new source of information that can help us decipher just how jets in X-ray binaries work," Dr. Remillard said.
CHANDRA'S POWERFUL VISION
The results on an expanding ring-like structure of oxygen and neon that was hurled into space by the explosion of a massive star, called E0102-72, were reported by Professor Claude Canizares, director of the Center for Space Research. Research scientists Kathryn Flanagan, David Davis and John Houck of MIT collaborated with Professor Canizares.
E0102-72 is the remnant of a supernova explosion located in our neighbor galaxy, the Small Magellanic Cloud, nearly 200,000 light years away. It was created by the 1,000-year-old explosion of a star that was more than 10 times as massive as our sun. Shock waves are heating gas to temperatures of nearly 10 million degrees, so it glows with X-rays that are detected by Chandra's instruments.
By using the High Energy Transmission Grating Spectrometer (HETG), astronomers were able to pinpoint the distribution of each chemical element individually and measure the velocities of different parts of the expanding ring. They also show the shock wave in a kind of "freeze-frame," revealing the progressive heating of the stellar matter as it plows into the surrounding gas.
This is the first time such detailed X-ray information has ever been obtained for a supernova remnant, and should provide critical clues to the nature of supernovae.
The grating spectrometer, which was built by an MIT team led by Dr. Canizares, spreads the X-rays according to their wavelength, giving distinct images of the object at specific wavelengths characteristic of each chemical element. Small wavelength shifts caused by the Doppler effect are used to measure the expansion velocities of each element independently.
"We've been studying these supernova remnants for decades, but now we're getting the kind of information we need to really test the theories," Dr. Canizares said.
"Understanding supernovae helps us to learn about the processes that formed chemical elements like those which are found on Earth and are necessary for life," Dr. Flanagan said.
Most of the oxygen in the universe, for example, is synthesized in the interiors of relatively few massive stars like the one being studied here. When they explode, they expel the newly manufactured elements, which become part of the raw material for new stars and planets. The amount of oxygen in the E0102-72 ring is enough for a thousand solar systems.
BLACK HOLE AT THE CENTER
Frederick K. Baganoff and colleagues from Pennsylvania State University at University Park, and the University of California at Los Angeles presented their findings on Sagittarius A*, a point-like, variable radio source at the center of our galaxy. It looks like a faint quasar and is believed to be powered by gaseous matter falling into a supermassive black hole with 2.6 million times the mass of our sun.
Dr. Baganoff, lead scientist for Chandra's Advanced CCD Imaging Spectrometer (ACIS) team's "Sagittarius A* and the Galactic Center" project and a postdoctoral research associate at MIT, said the precise positional coincidence between the new X-ray source and the radio position of a long-known source called Sagittarius A* "encourages us to believe that the two are the same."
Chandra's remarkable detection of this X-ray source has placed astronomers within a couple of years of a coveted prize: measuring the spectrum of energy produced by Sagittarius A* to determine in detail how the supermassive black hole that powers it works.
"The race to be the first to detect X-rays from Sagittarius A* is one of the hottest and longest-running in all of X-ray astronomy," Dr. Baganoff said. "Theorists are eager to hear the results of our observation so they can test their ideas."
But now that an X-ray source close to Sagittarius A* has been found, it has taken researchers by surprise by being much fainter than expected. "There must be something unusual about the environment around this black hole that affects how it is fed and how the gravitational energy released from the infalling matter is converted into the X-ray light that we see," Dr. Baganoff said. "This new result provides fresh insight that will no doubt stir heated debates on these issues.
"The luminosity of the X-ray source we have discovered already is a factor of five fainter than previously thought, based on observations from an earlier X-ray satelllite," he added. "This poses a problem for theorists. The galactic center is a crowded place. If we were to find that most or all of the X-ray emission is not from Sagittarius A*, then we will have shown conclusively that all current models from Sagittarius A* need to be rethought from the ground up."
Astronomers believe that most galaxies harbor massive black holes at their centers. Many of these black holes are thought to produce powerful and brilliant point-like sources of light that astronomers call quasars and active galactic nuclei. Why the center of our galaxy is so dim is a long-standing puzzle.
A SUPERNOVA IN ACTION
Shortly before the AAS meeting, MIT scientists used Chandra to capture a rare glimpse of X-ray radiation from the early phases of a supernova, one of the most violent events in nature.
Although more than 1,000 supernovae have been observed by optical astronomers, the early X-ray glow from the explosions has been detected in less than a dozen cases.
The Chandra observations were made under the direction of a team of scientists from MIT led by Walter Lewin, professor of physics, and graduate student Derek Fox. When combined with simultaneous observations by radio and optical telescopes, the X-ray observations give astronomers deeper insight into the last days in the life of a massive star. These events are responsible for the production and dispersal of carbon, oxygen, iron and other heavy elements found on Earth.
Chandra observed an X-ray glow with the total power of 50,000 suns. Ten days later, it observed the glow (dubbed SN1999em) for another nine hours and found that the X-rays had faded to half their previous intensity. The optical luminosity, which had the brightness of 200 million suns, had faded somewhat less. No radio emission was detected at any time.
With this information, the MIT group and their colleagues are already piecing together a picture of the catastrophic explosion.
The Chandra observation also provides an inside look at the hectic, exciting world of the international "quick response" network that scientists have set up to track and investigate supernovae.
On Friday, Oct. 29, Alex Fillipenko of the University of California at Berkeley notified Bob Kirshner at Harvard-Smithsonian that his automated supernova search project had a good candidate in a relatively nearby spiral galaxy, NGC 1637. (Nearby in this case means about 25 million light years from Earth.) Wei Dong Li, who is visiting Dr. Fillipenko's group from the Beijing Astronomical Observatory, called his colleagues in Beijing, who confirmed the supernova when the Earth rotated into a position to make viewing from China possible.
Radio astronomers Kurt Weiler at the Naval Research Laboratory, Schuyler van Dyk at the California Institute of Technology and Richard Sramek at the NRAO's Very Large Array (VLA) in New Mexico were alerted. Dr. Kirshner then got in touch via e-mail with Harvey Tananbaum, director of the Chandra X-ray Center, a little before 11pm on Saturday night.
After checking with the Chandra operations team, which worked furiously over the weekend to develop the list and timing of commands necessary to interrupt Chandra's observing plan without jeopardizing the space observatory, Dr. Tananbaum gave the go-ahead. By Monday morning, Chandra was pointed at the supernova and observed it for about nine hours.
Professor Lewin, who had been awarded the rights to Chandra's first observation of a nearby supernova, was ecstatic. "This is a unique chance that we have been hoping for!" he wrote in an e-mail to Dr. Tananbaum.
ANIMATING THE X-RAY SKY
A video animation of the violent X-ray sky was presented at the AAS meeting by Professor of Physics Hale Bradt, co-recipient of the Rossi Prize of the High Energy Astrophysics Division of the AAS for his role in the Rossi X-ray Timing Explorer (RXTE) project. He was assisted in its preparation by MIT graduate student Michael Muno and principal research scientists Smith, Alan Levine and Remillard, and Robin Corbet of NASA's Goddard Space Flight Center.
The All-Sky Monitor (ASM) experiment on the RXTE satellite, launched in December 1995, provided data on about 150 X-ray sources for the animation. X-ray cameras in space like the Rossi are our window to these extremely energetic and often violent processes in the universe.
The seven-minute video "demonstrates dramatically the extreme brightening, fading, disappearances and sudden appearances of these stars from 1996 to 1999," Professor Bradt said. "By contrast, the visible sky we usually see is relatively unchanging and seemingly quite peaceful."
The animation depicts X-ray stars on a map of the entire sky. The X-ray stars are pinpoints, just like visible stars, but in the animation they are indicated with colored circles. The size of a circle represents the brightness. The color of the circle is the "X-ray color" -- blue generally represents higher-temperature gases and red represents lower-temperature gases.
The animation runs about 350,000 times faster than actual time. Four days of time pass in about one second, and the entire four years passes by in around seven minutes. The rectangular image below the sky image is a magnification of the center of our Milky Way galaxy, which has a high density of X-ray stars.
Objects other than compact X-ray binaries are shown. Supernova remnants such as the Crab nebula are the leftover nebulae from the collapse of a star. They are quite steady emitters of X-rays. Active galactic nuclei such as quasars are most likely massive black holes in the centers of distant galaxies. They undergo violent flaring, which can be seen in a few cases, but because of their great distances, they are faint in the ASM data.
The dramatic variability of the galactic "microquasars" is evident in the animation. These sources exhibit intense jets of material seen by radio observers. The material in the jets is probably the same infalling material that causes X-rays.
RXTE's primary objective is to study variations in brightness of X-ray stars. Of its three instruments, the ASM developed at MIT and operated by MIT and Goddard continuously surveys the entire sky about once every two hours, snapping an X-ray "picture" every two minutes as it rotates. ASM is the Rossi's eyes and ears. The other two instruments on board focus on specific stars for detailed studies.
Scientists at MIT and Goddard use data from the ASM to calculate the intensities of the 150 sources in near real time. The data are immediately made available to the public, so scientists using RXTE or other observatories such as the Italian BeppoSAX or Chandra will know the state of X-ray sources they may wish to study.
"The ASM data reveal the existence of previously unknown sources, alert astronomers to new types of behaviors and help us understand the energetic processes in the sources, such as the creation of jets of material being ejected from black holes," Professor Bradt said.
Other members of the ASM team at MIT are principal research scientist Edward Morgan; graduate student Linqing Wen; programmer Doug Allen; and data aides Alan Wood and Joan Quigley, all at the Center for Space Research.
The VLA is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The RXTE is a NASA explorer mission consisting of X-ray instruments built by teams at Goddard Space Flight Center, MIT and the University of California at San Diego.
A version of this article appeared in MIT Tech Talk on January 26, 2000.
