Near-infrared image of the region of the microquasar GRS 1915+105 taken with the Palomar 200-inch telescope. The other bright objects are nearby stars in the constellation Aquila. These are used for comparison purposes to look for changes in the infrared brightness of the black hole.

ATLANTA -- Something really shocking is going on in a microquasar, or black hole, dubbed "Old Faithful," some 40,000 light years from Earth. It seems to be behaving like a giant particle collider, with massive shock waves generating eruptions every 45 to 90 minutes.

This is the second time that Old Faithful, the first known microquasar in our galaxy, the Milky Way, has been observed to be acting strangely. Two years ago astronomers presented evidence, from X-ray and infrared observations, that the microquasar is sending out jets of hot gas at close to regular half-hour intervals.

"The system is erratic enough to be called chaotic," says Cornell University astronomer Stephen Eikenberry.

Eikenberry presented the latest findings from what he calls "the greatest show-off of all black holes" in a poster-paper report at the national meeting of the American Astronomical Society in Atlanta today (Jan. 14). Eikenberry's colleagues on the latest observations, which were made in July 1998 with the Palomar Observatory 200-inch telescope and the Rossi X-ray Timing Explorer satellite, were Keith Matthews of the California Institute of Technology; Michael Muno, Ronald Remillard and Edward Morgan of the Massachusetts Institute of Technology; and Philip Blanco of the University of California, San Diego.

The discovery that a black hole is producing explosive "shock fronts," says Eikenberry, might be a clue to the behavior of much more massive, extra-galactic black holes, called quasars, where eruptions seem to occur on scales of years to decades. By comparison, Old Faithful, a microquasar called GRS 1915+105 in the constellation Aquila (the eagle), has in the past produced flares, or eruptions, as few as 20 seconds apart.

A black hole is a massive collapsed star of such density that its intense gravitational pull prevents even light from escaping. Like other collapsed stars, such as neutron stars, black holes often are observed orbiting a nearby star from which they are sucking in gaseous material. This gas forms the accretion disk, a spiraling band of material around the black hole. In some cases the black

hole appears to eject some of this material in jets moving at nearly the speed of light.

The Old Faithful microquasar first revealed itself as a bright X-ray source in 1992, and in 1994 radio observations found jets streaking away from the black hole at more than 90 percent of the speed of light. Two years later, infrared and X-ray observations revealed smaller, rapid jet ejections occurring nearly every 30 minutes.

Now, says Eikenberry, a third class of flare, made up of synchrotron radiation (the radiation produced by fast-moving charged particles in a curved path), is being emitted from an expanding plasma bubble. The bubble, he says, is a cloud of high-energy electrons swirling around in a magnetic field and possibly created by shock waves. The magnetic field curves the trajectory of the electrons and causes them to emit radiation. One or more such plasma bubbles make up a jet ejection, theorizes Eikenberry, an assistant professor of astronomy at Cornell.

What is new here, says Eikenberry, is a different view of what can occur when material flows into a rotating black hole. "Something happens at the interface between the innermost part of the accretion disk and the black hole that creates a jet. The innermost accretion disk is somehow being affected by the black hole and that causes part of the disk to be ejected in the form of a jet and part of it to fall in," he says.

Previous observations showed a disruption in the inner accretion disk followed by the appearance of a jet ejection. These new observations, however, show the plasma bubble appearing before the inner disk activity.

"Since we always thought that the black hole in the inner disk drives jet activity, this was a real surprise," says Eikenberry. "We have a few possible ideas to explain it though. One idea is that a wave of material, a shock front, is moving through the accretion disk." Another theory, he says, is that the black hole is emitting a continuous outflow of material and the observed flares are shock fronts moving through the jets.

The newly discovered synchrotron radiation jets "are clearly different from the jets previously seen," says Eikenberry. "This is telling us there are different ways to make jets or plasma bubbles in microquasars."