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News and Public Affairs
Los Alamos National Laboratory

Contact:
Nancy Ambrosiano, nwa{at}lanl.gov, (505) 667-0471

May 26, 2003

03-069

XNM-Newton satellite uncovers diffuse X-ray emission and the first
accreting X-ray pulsar in Andromeda Galaxy

Part 1 of 2

LOS ALAMOS, N.M. -- In the most sensitive X-ray survey of our
neighboring galaxy, Andromeda (M31), the X-ray Multi-Mirror satellite
observatory (XMM-Newton) has uncovered hundreds of X-ray sources and
provided new insights into the nature of the interstellar medium in
the spiral arms of our own galaxy as well as those of Andromeda.

Examining new satellite data, an international team of scientists led
by researchers at the Los Alamos National Laboratory discovered
information relating to two key types of X-ray sources. These
sources, X-rays both discrete and diffuse, more clearly lay out the
life cycle of our neighbor galaxy from the formation of stars through
their supernovae death throes. 

One of the discoveries is a faint X-ray glow from a very hot gas in
the disk of this giant spiral galaxy. "It is likely that much of this
hot gas is produced and heated in supernova explosions, and massive
star winds," said Sergey Trudolyubov, a Los Alamos scientist who will
be presenting these and other results today at the 202nd Meeting of
the American Astronomical Society in Nashville, Tenn.

"Our first results show that we are probably dealing with millions of
solar masses of a hot plasma in the disk of M31. It can be traced up
to the distance of 8 kiloparsecs from the center of M31, or as far as
our Sun from the center of our own Galaxy."

Another exciting result of the survey is the discovery of an
accreting X-ray pulsar, a strongly magnetized neutron star that is
drawing in material from its neighbor.

"The observed X-ray modulation and energy spectrum of the source make
it the first X-ray pulsar detected in M31," explains Trudolyubov. "It
is likely that we have found a binary system, where a strongly
magnetized, spinning neutron star accretes matter from a young
massive super-giant star." While there are many X-ray sources within
the galaxy, categorizing this one as a pulsar is an important event.

The neutron star, XMMU J004415.8+413057, appears to be spinning such
that its brightness varies with a regular interval of 198 seconds.
The star is the third object showing periodic X-ray modulation
discovered in the XMM-Newton survey of Andromeda galaxy. Scientists
hope to find more such periodic modulations in systems within the
Andromeda Galaxy, and to learn more about their geometry and
dynamics.

The Andromeda Galaxy, the closest giant spiral galaxy to our own,
located only 2.5 million light years away, is a unique object for the
study of X-ray astronomy. M31 is a remarkably similar analogue of our
own galaxy and often called its "twin sister." It offers an advantage
for Earthbound astronomers though, because it sits at an angle to our
own galaxy, so we can view its components more clearly than those of
our home galaxy. M31 hosts hundreds of X-ray sources, observed at a
nearly uniform distance, and due to the favorable orientation of M31,
they are less obscured by interstellar gas and dust than those in the
our galaxy. 

There are two kinds of X-ray sources of interest in this satellite
survey, discrete and diffuse. Discrete X-ray sources in M31 (X-ray
binary stars and supernova remnants) are a fossil record of the
stellar population, denoting stellar endpoints, and may be used as a
probe of star formation history. Diffuse X-ray emission, widely
distributed in the disk of M31, traces the hot gas component and
indicates recent star formation. 

With earlier X-ray surveys scientists were able to study only a
handful of bright X-ray sources in the disk of M31. The new survey of
M31 performed with XMM-Newton revealed another collection of the
sources -- fainter and more uniformly distributed along the giant
disk of the galaxy, and these news sources provide new insights into
the nature of the interstellar medium in the spiral arms. 

XMM-Newton, one of the two most powerful X-ray observatories ever
placed in orbit, performed the survey of M31 during a series of
observations during 2000 to 2002. The international team of
researchers detected several hundred discrete X-ray sources, with
more than 200 of them appearing for the first time. The scientists
found that slightly more than one third of the total number of the
detected X-ray sources belongs to M31. The remaining two thirds are
more distant background objects like active galactic nuclei and
quasars or just ordinary stars within our own Galaxy.

Most of the discrete X-ray sources belonging to the Andromeda Galaxy
are X-ray producing binary star systems. X-ray binaries are made up
of a normal star and a collapsed star (neutron star or a black hole).
These pairs of stars produce X-rays if the stars are close enough
together that material is pulled off the normal star by the gravity
of the dense, collapsed star. The X-rays come from the area around
the collapsed star where the material that is falling toward it is
heated to very high temperatures. 

While these star pairs are often X-ray producing, they do not qualify
as pulsars since they do not show a pulsing emission of radiation.
About 300 pulsars have been identified in our own galaxy, but most
pulsars detected to date release radio waves, as opposed to X rays.

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