Mysterious, powerful X-ray sources found in nearby
galaxies may represent a new class of objects, according to
data from NASA’s Chandra X-ray Observatory. These sources,
which are not as hot as typical neutron-star or black-hole X-
ray sources, could be a large new population of black holes
with masses several hundred times that of the sun.
“The challenge raised by the discovery of these sources is to
understand how they produce so much X-ray power at temperatures
of a few million degrees,” said Rosanne Di Stefano from the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Mass., and Tufts University in Medford, Mass. Di Stefano is
lead author of a series of papers published in or submitted to
The Astrophysical Journal and The Astrophysical Journal
Letters.
Until a few years ago, astronomers only knew of two sizes of
black holes: stellar black holes, with masses about 10 times
the sun, and supermassive black holes located at the centers of
galaxies, with masses ranging from millions to billions times
the sun. Recent evidence suggests a class of “intermediate-
mass” black holes may also exist.
Searching for quasisoft sources may be a new way to identify
those X-ray sources most likely to be intermediate-mass black
holes,” said Albert Kong of the Center for Astrophysics (CfA)
and a member of the team.
The enigmatic objects found by the Chandra team are called
“quasisoft” sources, because they have a temperature in the
range of 1 million to 4 million degrees Celsius. On the one
hand this temperature range is below the 10 million to 100
million-degree gas associated with “hard” X-ray sources, such
as neutron stars or stellar-mass black holes. On the other hand
the quasisoft-source temperatures are hotter than the several
hundred-thousand-degree gas associated with “supersoft” X-ray
sources due to white dwarfs.
Di Stefano and her colleagues determined the temperatures of
individual X-ray emitting objects in four galaxies by measuring
their X-ray spectra, or distribution of X-rays with energy.
They found that between 15 percent and 20 percent of all
detected sources fell in the quasisoft temperature range.
The power output of quasisoft sources is comparable to or
greater than that of neutron stars or stellar-mass black holes
fueled by the infall of matter from companion stars. This
implies the region that produces the X-rays in a quasisoft
source is dozens of times larger.
One possibility is the quasisoft sources represent standard
neutron stars or stellar black holes where the associated hot
gas cloud is, for some as yet unknown reason, much larger than
usual. Or the quasisoft X-rays could be coming from the
vicinity of intermediate-mass black holes having masses a
hundred or more times greater than the mass of the sun. This
would increase the diameter of the event horizon and could
explain the larger sizes and lower temperatures associated with
quasisoft sources.
As more quasisoft sources are discovered, the types of galaxies
in which they reside and where they are located in a galaxy
should give astronomers additional clues as to their nature.
The present study indicates that they occur in various
locations in elliptical as well as
spiral galaxies.
Di Stefano and her CfA team observed quasisoft sources in
several galaxies with Chandra including M101, M83, M51 and NGC
4697. NASA’s Marshall Space Flight Center, Huntsville, Ala.,
manages the Chandra program for NASA’s Office of Space Science,
Washington. Northrop Grumman of Redondo Beach, Calif., formerly
TRW, Inc., was the prime development contractor for the
observatory. The Smithsonian Astrophysical Observatory controls
science and flight operations from the Chandra X-ray Center in
Cambridge, Mass.
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