In its first look at the Andromeda Galaxy, known as M31, XMM revealed an unusually bright X-ray nova outburst and a luminous white dwarf in a binary system with the shortest X-ray pulsation period seen to date. (White dwarfs are small, dense Sun-sized stars).
The objects were found when XMM-Newton observed the central region of M31 in June and December 2000 during a mission test that pointed up more than 100 discrete X-ray sources, some of them binaries and some of them previously unknown. An international team of scientists has studied the energy variations in the light emitted by these objects and started to classify the sources by type.
"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 Sergey P. Trudolyubov of Los Alamos National Laboratory, who is among those working with the new data.
The results were made public last week at the annual meetings of the American Astronomical Society Meeting in Pasadena, California.
A white dwarf with a very slow pulse
The white dwarf system identified during the test phase is one of a class of cooler-temperature objects found during this survey that includes relatively bright objects with matter falling onto their surface, burning at temperatures 10 or even 100 times lower than other sources in M31.
Most of the similar sources XMM found are stars orbiting with either a white dwarf, neutron star (a highly dense, collapsed star) or black hole. Matter from the stars falls onto the companion objects.
The white dwarf that most excited astronomers pulses out X-rays with a period of nearly 900 seconds, the shortest ever observed in a binary system.
Another class of objects found during the test phase are thought to contain neutron stars or black holes emitting transient or persistent X-ray bursts. Some of the brighter objects are associated with globular clusters, compact spherical concentrations of tens or even hundreds of thousands of stars. These objects resemble globular cluster sources found in our own galaxy and shown to include neutron stars.
Novelty novae
The X-ray nova XMMU J004234.1+411808 also surprised XMM astronomers as it was extremely bright in June, but was not detected at all half a year later. Novas are stars that suddenly shoot out tremendous amounts of light and then fade away.
In the past 30 years, only two dozen similar outbursts have been detected in our galaxy. Scientists still argue on the physical origin of these events, but some think they are caused by the sudden release of a huge amount of matter spiraling into a black hole.
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 help astronomers understand the structure of these objects.
For that reason, the XMM-Newton group at Los Alamos and the Chandra X-Ray Observatory team at the Harvard-Smithsonian Center for Astrophysics have agreed to inform each other immediately of new sources within M31 to coordinate observations.
The Hubble Space Telescope also will follow up by studying chosen objects in visible light and UV-band.
Sister Andromeda
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, and due to the favorable orientation of the M31, they are less obscured by interstellar gas and dust than those in the 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.'' said Konstantin Borozdin of the Los Alamos National Laboratory. "But we are in a good position to study the Andromeda Galaxy, which is very much like our own Milky Way."
XMM-Newton was launched from Kourou, French Guiana on Dec. 10, 1999. It carries three advanced X-ray telescopes, each containing 58 high-precision concentric mirrors to offer the largest collecting area possible to catch the passing X-rays. This design allows XMM-Newton to detect millions of sources, seeing more of a greater range of high-energy objects than other X-ray telescopes.
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