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Public Affairs
Harvard-Smithsonian Center for Astrophysics

For more information, contact: 

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468 
daguilar{at}cfa.harvard.edu 

Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
clafon{at}cfa.harvard.edu

For Release: 10:00 AM EST, Monday, March 24, 2003

Release No.: 03-09

Cool X-Ray Disk Points To New Type Of Black Hole 

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. An
artist's concept of an intermediate-mass black hole
is available at
     http://cfa-www.harvard.edu/press/pr0309image.html

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.

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