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.
A significant number of discrete X-ray sources detected in the survey coincide with optically identified supernova remnants. When a massive star explodes in a supernova, it expels a large amount of material (often many times more than the mass of the Sun) at thousands of kilometers per second. This high-speed gas then collides with the interstellar medium, heating it up to millions of degrees, and the heated gas can be detected by X-ray telescopes.
Although point-like sources account for the most of the X-ray luminosity of M31, there is another important source of X-rays within this galaxy. Analyzing a recent, 15-hour-long XMM-Newton observation of the northern disk regions of M31, the team of researchers discovered a faint, extended, diffuse X-ray glow coming from the disk of M31.
The spectral properties of this emission component are quite distinct from the typical emission from the point sources. This indicates that most of the diffuse X-ray emission comes from a very hot, million-degree gas concentrated in the spiral arms of Andromeda Galaxy. The significance of the presence of this hot plasma goes far beyond the case of M31, as this galaxy is often considered as the prototype for the population of early-type galaxies, including our own. And since our field of view across our own is often heavily obscured, this is an important new insight into the substance within the Milky Way’s spiral arms.
The researchers plan to continue this survey, one that will eventually cover a whole galaxy. “This will allow us to get a complete X-ray picture of the Andromeda Galaxy, helping to highlight similarities and differences between this galaxy and our own Milky Way,” said Trudolyubov.
The international research team for this investigation includes Sergey Trudolyubov, William Priedhorsky, Oleg Kotov and Konstantin Borozdin of Los Alamos National Laboratory; Keith Mason of the Mullard Space Science Laboratory, UK and France Cordova of the University of California at Riverside.
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. For further information on the satellite and its mission, see: http://sci.esa.int/xmm/. The results of these XMM-Newton observations appear in recent and upcoming papers in The Astrophysical Journal. A complete set of photos is available at http://www.nis.lanl.gov/~tsp/press_release_AAS_202_pictures.html.
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