RELEASE NO: 98-198

Astronomers have found similarities in both short-term and long-term X-ray fluctuations between apparent supermassive black holes in distant galaxies and smaller, well-studied stellar black holes in our galaxy. Such a link could help determine the mass of these more distant, elusive objects and eventually explain the exotic physical processes that produce the brilliant luminosity observed in the area around a black hole.

Black holes are celestial objects with gravitational fields so powerful that near them, nothing, not even light, can escape. A stellar black hole, the remains of a massive star, is an object with several times to a few dozen times the mass of the Sun. A supermassive black hole, found at the center of a whole galaxy of hundreds of billions of stars, has a mass of hundreds of thousands to several billion times the mass of the Sun.

"We have accurate ways of measuring the mass of stellar black holes from the orbits of their companion stars, which are affected by the black hole's mass and gravitational pull," said Dr. Rick Edelson, an astronomer working at University of California, Los Angeles, and Leicester University in the U.K. "But there's no way you can do that for a supermassive black hole. These variability characteristics may provide that link."

Using NASA's Rossi X-ray Timing Explorer satellite (RXTE), Edelson and Nandra observed galaxy NGC 3516, over 100 million light years from earth. Astronomers believe that this galaxy's exceptionally bright core, a phenomenon known as Active Galactic Nucleus, is caused by a supermassive black hole, or "central engine," pulling matter from nearby stars with such fury that the energy produced in this relatively small region (the size of our solar system) outshines the entire galaxy.

Dr. Kirpal Nandra of the NASA's Goddard Space Flight Center and Edelson observations of NGC 3516 showed surprising similarity in the flickering properties (the change of brightness over time) with Cygnus X-1, a nearby, well-studied stellar black hole in our galaxy. The flickering of the two objects differed by a factor of about a million, which is the same difference in magnitude as their luminosities -- that is, NGC 3516 is about a million times brighter than Cygnus X-1.

If luminosity is related to mass, then the black hole masses should also differ by such a magnitude. And this is just what the astronomers found: Cygnus X-1 at 10 solar masses and NGC 3516 at about 10 million solar masses matches the estimates from earlier spectral studies that attempted to measure the mass of NGC 3516.

"These very strong similarities in the X-ray variability characteristics of supermassive and stellar black holes have never been measured before in such detail," said Nandra. "We now may be able to study by analogy many things about these more distant, supermassive black holes using our experience from stellar black holes, which are closer and better studied."

Nandra also said the flickering similarities suggest that the same physical mechanism may be working on both supermassive black holes in the cores of other galaxies and stellar black holes in our own galaxy.

This observation was possible because the astronomers took advantage of a quirk in RXTE's orbit. With low-orbit satellites like RXTE, which circles the earth every 96 minutes, most sources are blocked by the earth and unobservable for about half of each orbit. This makes it impossible to study the short time scale flickering. Edelson and Nandra noticed, however, that NGC 3516 is near the "pole" of the RXTE orbit, so it could be observed without interruption from earth occultation.

Edelson said that future missions, such as NASA's Advanced X-ray Astronomy Facility (scheduled for launch in mid-1999) will have high-earth orbits, so occultation won't be an issue and short time scale observations will be possible with many other galaxies. Edelson and Nandra will continue sampling X-rays from NGC 3516 about every four days for the next two years.

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