The strongest evidence yet that the universe is home to a new type of black hole was reported by several groups of scientists today.

Using NASA's Chandra X-ray Observatory, scientists have zeroed in on an off-center, mid-mass black hole in the galaxy M82. This black hole -- located 600 light years away from the center of a galaxy may represent the missing link between smaller black holes and the supermassive variety found at the centers of galaxies.

X-ray image of the central region of the starburst galaxy M82. Of particular interest is the bright source near the center of the image, offset from the dynamical center (small green +) of the galaxy by about 600 light years. This source was seen to increase dramatically in intensity over a period of three months, as indicated by comparing the left image to the more-recent observation at right. Short-term flickering in 10-minute intervals also was observed. This fast flickering and the peak intensity of the source are strong evidence that the X-rays are produced by matter accreting onto a black hole with the mass of more than 500 suns. Photo: NASA/CXC/SAO

"This opens a whole new field of research," said Martin Ward of the University of Leicester, UK, a lead author involved with the observations. "No one was sure that such black holes existed, especially outside the centers of galaxies."

The black hole in M82 packs the mass of at least 500 suns into a region about the size of the Moon. Such a black hole would require extreme conditions for its creation, such as the collapse of a "hyperstar" or the merger of scores of black holes.

"This black hole might eventually sink to the center of the galaxy," said

Dr. Hironori Matsumoto of the Massachusetts Institute of Technology (MIT), Cambridge, 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."

Although previous X-ray data from the German-U.S. Roentgen Satellite and the Japan-U.S. ASCA Satellite suggested that a mid-mass black hole might exist in M82, the crucial breakthrough came when astronomers compared the new high resolution Chandra data with optical, radio, and infrared maps of the region. They determined that most of the X-rays were coming from a single bright source.

Repeated observations of M82 over a period of eight months showed the bright X-ray source gradually peaking in X-ray brightness before dimming. Another critical discovery was that the intensity of the X-rays was rising and falling every 600 seconds.

"This flickering of the X-ray intensity 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 10 minute variations. "The brightness of the source requires that the black hole have a mass greater than 500 suns."

An optical image of M82. The bluish band seen running from the upper-left to the lower-right of the image is due to light from stars in the M82 galaxy, which is about 12 million light years from Earth. The red filamentary features extending perpendicular to the galaxy are due to ionized hydrogen gas emitting its characteristic red light at a wavelength of 6563 Angstroms. Photo: Subaru Telescope, NAO Japan

Possible explanations for the object include the merger of stars to form a hyperstar that collapsed, or growth of a black hole through mergers with other nearby black holes and neutron stars. Observations with the Japan Nobeyama Millimeter Array by Dr. Satoki Matsushita of Harvard-Smithsonian and colleagues have revealed a large expanding superbubble of gas centered on the mid-mass black hole in M82. The energy of several thousand supernovae would be required to produce the expanding superbubble.

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 unseen 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.

Scientists from Kyoto University, Japan; Ehime University, Japan; RIKEN (The Institute of Chemical & Physical Research) and Nobeyama Radio Observatory, all in Japan were also involved with the Chandra observations.

The observations were made with the High Resolution Camera (HRC) and the Advanced CCD Imaging Spectrometer (ACIS). The HRC was built for NASA by the

Smithsonian Astrophysical Observatory, Cambridge, Mass. The ACIS instrument was built for NASA by MIT, and Pennsylvania State University, University Park.

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