XMM-Newton makes unusual discoveries in Andromeda Galaxy
In examining new satellite data, an international team of scientists, including researchers at Los Alamos National Laboratory, discovered
an unusually bright spot created by an enormous X-ray
nova outburst. Another mysterious object has been found as
well: One of the "coolest" sources of the central region
appears to be a luminous white dwarf with an extremely
soft energy spectrum and the shortest X-ray pulsation
period seen to date.
XMM-Newton carries three very advanced X-ray telescopes, each
containing 58 high-precision concentric mirrors, nested to offer the
largest collecting area
possible to catch the passing
X-rays. These mirror
modules allow XMM-
Newton to detect millions of
sources, far greater than any
previous X-ray mission.
The report on these and other
results was presented recently
by Sergey Trudolyubov of
Space and Remote Sensing
Sciences at the 198th
The Andromeda Galaxy, the
closest spiral galaxy to our
own – 2.6 million light years
away – is a unique object for
the study of X-ray astronomy.
M31 is in many respects similar to the Milky Way and even called its "twin sister.” The Andromeda Galaxy hosts
hundreds of X-ray sources, which are observed at a nearly
uniform distance, because of the favorable orientation of the
M31, they are less obscured by interstellar gas and dust than
those in the our galaxy.
"Because the solar system is situated in the galactic disk, we have
a somewhat distorted view of our own galaxy, like that of a fly
sitting on the elephant's ear,” explained Konstantin Borozdin, also
of NIS-2." But we are in a good position to study the Andromeda
Galaxy, which is very much like our own Milky Way.”
XMM-Newton, the most powerful X-ray observatory ever
placed in orbit, observed the central region of M31 in June and
December 2000 during the performance verification phase of
the mission. The international team of researchers detected
more than a hundred discrete-point X-ray sources, some of
them previously unknown. Most of the detected sources were
identified with X-ray binaries, accreting systems containing
either a white dwarf, a neutron star or a black hole fed by gas
flow coming from a companion star.
One of the new sources, X-ray nova XMMU J004234.1+411808,
was extremely bright in June but was not detected at all half a
year later. In a previous 30 years, only two dozen similar
outbursts have been detected in our galaxy. Scientists still argue
on the physical origin of these events. However, it is recognized
that they are usually caused by the sudden release of a huge
amount of emitting matter spiraling into a black hole.
The observations of X-ray novae provide unique information on
the processes in the immediate vicinity of the compact objects,
which is why each outburst attracts a great deal of interest.
Novae are bright not only in X-rays, but also in other parts of
the spectrum. Thus,the simultaneous observations in the X-ray,
optical and UV bands are of special importance to under-
standing of the structure of these objects.
Realizing the importance of the coordinated observations of
the X-ray novae with different instruments, the XMM-
Newton group at the Laboratory and the Chandra team at the Harvard-Smithsonian Center for Astrophysics have agreed to
inform each other immediately of new sources within M31. In
addition, another valuable space observatory resource, the Hubble
Space Telescope, will follow up the nova imagery in a visible light
and UV band.
The unprecedented sensitivity of XMM-Newton allowed a detailed
study of spectral and temporal properties of several dozen X-ray
objects. As a result, several main classes of sources were found.
"This is the first time we are really able to study the individual
properties of the binary systems in M31 millions of light years
away and compare them with that of our own galaxy,” said Trudolyubov.
One class includes relatively bright objects with extremely soft
energy spectra, implying the temperature of accreting gas lower
than 1 million degrees Kelvin, 10 or even a 100 times lower than
in the other sources detected in M31. It is likely that most of the
emission of such "cool" sources is created by steady thermonuclear burning of enormous amounts of matter falling onto the
surface of a white dwarf. Most exciting is that one of these sources, first detected by XMM-Newton, demonstrates X-ray pulsations
with a period of nearly 900 seconds, the shortest ever observed in
The remaining classes of objects may be associated with either
transient or persistent X-ray sources containing a neutron star or a
black hole. Several bright objects are associated with globular
clusters, compact spherical concentrations of tens or even
hundreds of thousands of stars. The spectral properties of these
objects are strikingly similar to the globular cluster sources
observed in our own galaxy, proven to be the systems with
neutron star primaries.
The identification of the discrete X-ray sources in M31 with
various types of compact objects, which is based mainly on their
spectral properties, needs further work. To finally resolve a debate
on the mysterious nature of these systems, a greater number of X-
ray instruments is needed (for example, to detect short
thermonuclear bursts from the neutron star sources).
Several more XMM-Newton observations of the Andromeda
Galaxy are scheduled as well. It is expected that they will bring
important insights into the nature of the X-ray sources in M31.
The European Space Agency's XMM-Newton was launched
from Kourou, French Guiana, on Dec.10,1999.
For more information on the satellite and its mission, see
http://sci.esa.int/xmm/ on the Internet. High-resolution digital
versions of the X-ray images and other information associated
with this release are available at http://nis-www.lanl.gov/
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