A team led by Dr. Jon Miller of the Harvard-Smithsonian Center for Astrophysics (CfA) zeroed in on gas very close to two suspected "intermediate-mass" black holes -- material that would soon take that final plunge. Using the European Space Agency's XMM-Newton satellite, the scientists precisely measured the temperature of this gas and obtained the most accurate mass measurement of the black hole systems to date.

Miller presented these results today at a press conference at the meeting of the High Energy Astrophysics Division of the American Astronomical Society at Mt. Treblant, Quebec. His colleagues include Drs. Giuseppina Fabbiano of CfA, Cole Miller of the University of Maryland, and Andrew Fabian of the University of Cambridge.

"Evidence is mounting that these elusive intermediate-mass black holes may really exist," says Jon Miller. "The mystery, really, is how they can exist."

Black holes are objects so dense and with a gravitational potential so strong that nothing, not even light, can escape the pull if it ventures too close. Black holes are invisible, yet the gas and dust falling into a black hole are heated to high temperatures and glow furiously.

Scientists agree that there are at least two classes of black holes. Stellar black holes, with a mass of up to about ten suns, are the remains of massive stars whose cores have imploded. Supermassive black holes contain the mass of millions to billions of suns confined to a region about the size of our solar system. These monstrous objects likely form from immense gas clouds and are thought to reside in the cores of most galaxies.

Scientists are not in agreement over the existence of intermediate-mass black holes, however, which seem to harbor the mass of hundreds to tens of thousands of suns. Fabbiano first observed objects suspected to be intermediate-mass black holes in 1989 with the Einstein X-ray Observatory. Several more objects were discovered through the 1990s and were labeled ultra-luminous X-ray sources (ULXs), for they are exceedingly bright yet compact.

Over the last three years, several observations provided compelling evidence that ULXs were black holes. Yet scientists could not rule out the possibility that these bright objects were less exotic sources with all of their energy (or light) beamed in our direction, making them appear intrinsically brighter than they really are.

---New Evidence For Mid-sized Black Holes---

Jon Miller and his colleagues have new X-ray data that, when combined with recent optical and radio observations, strongly support the intermediate-mass black hole interpretation for two specific ULXs. The scientists observed these two objects in a spiral galaxy about 10 million light years from Earth called NGC 1313. One source, called NGC 1313 X-1, is approximately 3,000 light years from its galaxy's center. The other source, NGC 1313 X-2, is approximately 25,000 light years from the center. The XMM-Newton observations concentrated on the temperature of the gas orbiting the black holes in a disk, called an accretion disk.

The inner ring of the accretion disk, closest to the black hole, is the hottest part of the disk, glowing primarily in X-ray light. Perhaps counter-intuitive, however, is the black hole theory predicting that the inner ring of an accretion disk is hotter in small, stellar-mass black holes compared to supermassive black holes. This is because spacetime curves more gently near a large black hole than near a small one. Thus, the material falling into a supermassive black hole remains cooler over this larger surface area. The temperature of this inner disk is inversely proportional to the mass of the black hole, growing cooler with increasing black hole mass.

Jon Miller and his colleagues found the temperatures of NGC 1313 X-1 and X-2 to be in line with black holes containing at least 100 solar masses, and likely 200 to 500 solar masses. The scientists needed the superb resolution and collecting area afforded by XMM-Newton to be confident of the interpretation of their data.

While evidence supporting the existence of intermediate-mass black holes continues to flow in, scientists still do not know how such black holes would form. "Three basic scenarios have been suggested," says Cole Miller, "direct collisions and mergers of stars within globular clusters; the collapse of extremely massive stars that may have existed in the early Universe; or the merger of smaller black holes. Each scenario has strengths and limitations."

Jon Miller's research was supported by the National Science Foundation, through its Astronomy and Astrophysics Postdoctoral Fellowship Program.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into six research divisions study the origin, evolution, and ultimate fate of the universe.