In its first look at the Andromeda Galaxy, known as M31, the X-ray Multi-Mirror satellite observatory has revealed several unusual X-ray sources. In examining new satellite data, an international team of scientists, including researchers at the United States Department of Energy's 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.

The XMM-Newton X-ray image of the central region of the Andromeda Galaxy taken in June 2000. The newly discovered X-ray Nova XMMU J004234.1+411808 and a transient supersoft X-ray pulsating source are marked by red and yellow circles respectively. Photo: S. Trudolyubov

The report on these and other results was presented by Dr. Sergey P. Trudolyubov of Los Alamos National Laboratory at the 198th American Astronomical Society Meeting in Pasadena, Calif.

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." explains Dr. 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, 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 understanding 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 Los Alamos 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.

XMM-Newton X-ray image of the central region of the Andromeda Galaxy taken in December 2000. Photo: S. Trudolyubov

The unprecedented sensitivity of XMM-Newton allowed a detailed study of spectral and temporal properties of several dozens of 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," says Dr. Trudolyubov.

One class includes relatively bright objects with extremely soft energy spectra, implying the temperature of accreting gas lower than 1 million degrees Kelvin, ten or even a hundred 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 an 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 such systems.

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 of X-ray instruments is needed (for example, to detect short thermonuclear bursts from the neutron star sources).

The European Space Agency's XMM-Newton was launched from Kourou, French Guiana on Dec. 10, 1999. 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.

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 international research team for this investigation includes Drs. Sergey P. Trudolyubov, Konstantin N. Borozdin and William C. Priedhorsky of the Los Alamos National Laboratory; Dr. Robert Shirey of the University of California at Santa Barbara; Dr. Julian P. Osborne of the University of Leicester, UK; and Prof. Keith O. Mason and Dr. Roberto Soria of the Mullard Space Science Laboratory, UK.

This announcement is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the United States through NASA

Los Alamos National Laboratory is operated by the University of California for the US Department of Energy's National Nuclear Security Administration.