2000年9月12日にNASAから行われる予定のプレスリリース

(注意: 準備する時間の関係でこれは最終版であることは確認していません。よって実際には若干変更されているかも知れませんが、内容には全く変更はありません)

Dolores Beasley
Headquarters, Washington, DC
Phone: 202/358-1753

Steve Roy
Marshall Space Flight Center, Huntsville, AL
Phone: 256-544-6535

Dr. Wallace Tucker
Chandra X-ray Observatory Center, CFA, Cambridge, MA
Phone: 617-496-7998

CHANDRA CLINCHES CASE FOR NEW TYPE OF BLACK HOLE

Several groups of scientists are reporting the strongest evidence yet that the Universe is home to a third type of black hole -- a mid-mass black hole. Located 60 light years away from the center of a galaxy, this black hole may represent the missing link between smaller stellar black holes and the supermassive variety found at the centers of galaxies.

Using the Chandra X-ray Observatory, scientists have zeroed in on an off-center black hole in the galaxy M82 that packs the mass of at least 700 suns into a region slightly larger than the Moon. Such a black hole would require extreme conditions for its creation, such as the collapse of a hypothetical colossal star called a "hyperstar" or the merger of smaller black holes.

The observations are the subject of NASA Space Science Update on September 12. The result comes as Chandra starts its second year of operation and is testimony to how Chandra's power and precision is changing the field of astronomy.

"This black hole might eventually sink to the center of the galaxy," said Dr. Hironori Matsumoto of the Massachusetts Institute of Technology, the lead author on one of three Chandra papers scheduled to be published on the mid-mass black hole, "where it could grow to become a supermassive black hole, or enlarge a supermassive black hole that may already exist there."

"A whole new field of research has been opened up," said Dr. Martin Ward of the University of Leicester, UK, another lead author involved with the observations. "No one was sure that such black holes existed, especially outside the centers of galaxies."

Previous X-ray data from the German-U.S. Roentgen Satellite and the Japan-U.S. Advanced Satellite for Cosmology and Astrophysics suggested that a mid-mass black hole might exist in M82. The crucial breakthrough came when Chandra observed a complex region of the galaxy that has been wracked by the formation and explosion of massive stars over the last 10 million years. By comparing the new high resolution data with optical, radio, and infrared maps of the region, astronomers were able to determine that most of the X-rays were coming from a single bright source that does not line up with the center of the galaxy or with any known object, such as the remains of an exploded star.

Scientists observed the region with Chandra six times over a period of eight months. During these observations, the bright X-ray source was seen to gradually peak in X-ray brightness before dimming. The true nature of the mystery source came with the discovery that the intensity of the X rays was rising and falling every 600 seconds.

"This X-ray flickering is similar to the well-studied characteristics of black holes swallowing gas from a nearby star or cloud. Explanations other than a massive black hole for this object are implausible," said Dr. Philip Kaaret of the Harvard-Smithsonian Center for Astrophysics, lead author on the paper reporting the variations. "The brightness of the source requires that the black hole have a mass greater that 700 suns."

One possible explanation for the existence of this mid-mass black hole is that massive stars merged to form a super-sized hyperstar more than 700 times as massive as the sun. Theory suggests that hyperstars may exist for relatively short periods of time before undergoing catastrophic collapse to form a black hole. Another possible explanation is that the black hole was originally much smaller and grew through mergers with other nearby black holes and neutron stars.

The formation of mid-mass black holes could also be connected with the mysterious gamma ray bursts, Matsumoto said. Observations with a millimeter wave telescope by Dr. Satoki Matsushita of Harvard-Smithsonian and colleagues have revealed a large expanding superbubble of gas centered on the giant black hole in M82. The energy of several thousand supernovae was required to produce the expanding superbubble. A hypernova might have supplied this energy and produced a burst of gamma rays in the process of forming a black hole.

In the past, our Milky Way galaxy could have produced mid-mass black holes during periods of vigorous star formation. Hundreds of these massive black holes may exist in our galaxy, in addition to the dozen or so known stellar black holes and the supermassive black hole that is safely confined to the galaxy's nucleus. The mid-mass black holes may be difficult to detect because there is little gas available for them to swallow.

Other scientists involved with the Chandra observations include: Drs. A. H. Prestwich, A. Zezas, and S.S. Murray of Harvard-Smithsonian; C. Canizares of MIT; T. G. Tsuru and K. Koyama of Kyoto University, Japan; H. Awaki of Ehime University, Japan; N. Kawai of The Institute of Chemical & Physical Research (RIKEN) Japan; R. Kawabe of the Nobeyama Radio Observatory, Japan.

M82 was observed by Chandra 6 times for approximately 30 hours total. The observations were made with the High Resolution Camera (HRC) and the Advanced CCD X-ray Spectrometer (ACIS) X-ray camera. The HRC was built for NASA by the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA. The ACIS instrument was built for NASA by the Massachusetts Institute of Technology, Cambridge, and Pennsylvania State University, University Park.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.

Images associated with this release are available on the World Wide Web at:

http://chandra.harvard.edu

AND

http://chandra.nasa.gov

High resolution digital versions of the X-ray image (JPG, 300 dpi TIFF)
are available at the Internet sites listed above. This image will be available on NASA Video File which airs at noon, 3:00 p.m., 6:00 p.m., 9:00 p.m. and midnight Eastern Time. NASA Television available on GE-2, transponder 9C at 85 degrees West longitude, with vertical polarization. Frequency is on 3880.0 megahertz, with audio on 6.8 megahertz.