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.
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