Cambridge, MA — Scientists have more evidence of an exotic, new type
of black hole that is hundreds of times larger than the stellar
variety that dot our Galaxy yet thousands to millions of times
smaller than the supermassive black holes thought to power quasars.

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.