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Steve Roy
Media Relations Dept.
Marshall Space Flight Center, Huntsville, AL
steve.roy{at}msfc.nasa.gov
(256) 544-0034

For release: 05/27/03

Release No.: 03-080

Major survey suggests finding intermediate-mass black holes will be a
challenge

Using the NASA Chandra X-ray Observatory for the most comprehensive
survey of X-ray sources in nearby galaxies, NASA scientists have
shown that the brightest of these objects are otherwise
indistinguishable from stellar-mass black holes and neutron stars.
This result complicates the ongoing hunt for an emerging but elusive
class of intermediate-mass black holes -- objects whose existence
would defy conventional black-hole formation theory. 

The scientists surveyed over 3500 X-ray sources in 90 nearby galaxies
(other than the Milky Way). Among these sources, the NASA team
identified 120 unusually bright ones, categorized as Ultra- Luminous
X-ray sources (ULXs) -- candidates for intermediate-mass black holes.
The team found that, apart from brightness, the ULX class shared many
of the more fundamental X-ray properties of the dimmer neutron stars
and stellar-mass black holes. 

"Statistically, it looks like many ULXs may be just extreme cases of
rather typical X-ray objects," said Dr. Douglas Swartz of the
National Space Science & Technology Center at NASA's Marshall Space
Flight Center in Huntsville, Ala., who led the analysis. Among
proposed explanations for a ULX is an intermediate-mass black hole.
If ULXs are intermediate-mass black holes, they must have properties
similar to those of neutron stars and stellar-mass black holes. 

Another proposed explanation for an ULX is a micro-blazar -- a
stellar-mass black hole whose radiation happens to be beamed in the
direction of the Earth. "Beaming" occurs when an object emits
electromagnetic radiation (such as X-rays) into a preferred direction
rather than equally in all directions, much like a lighthouse beacon.
If the Earth happens to lie in this direction, then an X-ray source
will appear very bright or luminous. Because of beaming, Swartz said,
a stellar-mass black hole could appear ultra-luminous.

Swartz and colleagues, Dr. Kajal Ghosh and Dr. Allyn Tennant, both of
the NSSTC at Marshall, compared the X-ray characteristics of the ULXs
with those of the other X-ray sources. These characteristics included
the change in brightness over time, the X-ray frequency (X-ray
"colors"), and the spatial distribution of the sources within their
host galaxies. Swartz and his colleagues found that the X-ray
properties of the 120 ULXs looked rather similar to those of the
larger population. It was this that led to their conclusion that the
ULXs differed from their sub-ULX counterparts only in apparent
luminosity, or degree of brightness. 

"Some of the sources we've analyzed have characteristics of
intermediate-mass black holes, others have characteristics of beamed
sources (such as micro-blazars) or of supernovas," Swartz said. "But
the reality is that most of the ULX sources just look like brighter
versions of the other detected sources." 

Nevertheless, he cautioned that these findings don't contradict other
recent studies linking individual ULXs to intermediate-mass black
holes or micro-blazars. "We are analyzing the ULX class as a whole.
Our approach complements research by other groups that scrutinize
individual ULXs both in X rays and at other wavelengths. Both
approaches are important and together we should soon reach a fuller
understanding of these exotic objects," Swartz said. The team hopes
to carry out future surveys with longer observation times and a
larger sample of galaxies to understand the objects more fully. 

NASA's Marshall Space Flight Center manages the Chandra program, and
Northrop Grumman Space Technology (NGST), Redondo Beach, Calif., is
the prime contractor for the spacecraft. The Smithsonian's Chandra
X-ray Center controls science and flight operations from Cambridge,
Mass., for the Office of Space Science at NASA Headquarters,
Washington.

For more information, refer to
      http://wwwastro.msfc.nasa.gov/research/ulx/
after 9:20am CDT, 2003 May 26.

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