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Hidden X-ray population revealed in satellite survey, new dipper in Andromeda Galaxy

DOE/Los Alamos National Laboratory : 04 June, 2004  (Technical Article)
In the most-sensitive study ever of our neighbor galaxy, the X-ray Multi-Mirror satellite observatory has uncovered a new population of X-ray sources. Examining new satellite data, an international team of scientists led by researchers at the United States Department of Energy's Los Alamos National Laboratory discovered more than 100 X-ray sources in the Andromeda galaxy. The faint sources had gone undetected in previous high-energy missions launched to observe the galaxy.
In contrast with earlier work, the newly discovered sources, within the disk of a giant space spiral, do not concentrate in the center but are uniformly distributed. Another exciting result of the survey is the discovery of the most distant known 'dipper', an X-ray source in a closed binary system, which fades out and then appears again with a regular interval of 2.8 hours.

The report on these and other results will be presented today by Sergey P. Trudolyubov of Los Alamos at the 200th American Astronomical Society Meeting in Albuquerque, NM.

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. The galaxy, known as M31, hosts hundreds of X-ray sources, observed at a nearly uniform distance. Most of the previously known sources are bright and located near the center of the galaxy, in its bulge. The new survey performed with XMM-Newton revealed another population of sources, fainter and uniformly distributed along the giant disk of the galaxy.

'With earlier surveys we were able to study the X-ray capital of the galaxy, its center,' explains fellow researcher Konstantin Borozdin of Los Alamos. 'Now, with data from the most sensitive survey ever performed using X-rays, we can see dimmer lights from the galactic countryside.'

XMM-Newton, the most powerful X-ray observatory ever placed in orbit, performed the survey of M31 during a series of observations performed in 2002. The international team of researchers detected several hundred discrete point X-ray sources, most 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.

The new sources are not quite like the previously known ones. The majority of known bright sources are neutron star binaries accreting the matter from low-mass, late-type stars many times smaller and dimmer than our sun. These systems are bright in X-rays, but dim in the optical range. They were formed when the universe was much younger and smaller than now. In contrast, many of the newly discovered faint sources are black holes and neutron stars paired with hot, gigantic stars in their early stellar childhood. Their typical age is just a few million years. Most of them are still in the galactic kindergarten, bright spiral arms and star-forming gas clouds.

This discovery by XMM-Newton allows researchers for the first time to study whole populations of these enigmatic systems in X-rays. The unprecedented sensitivity of XMM-Newton allowed a detailed study of spectral and temporal properties of several dozen X-ray objects. Even though they are now counted by the dozen, each of the objects is unique and keeps its own secrets for a curious inquirer. Analyzing recent observations of the central part of M31, the researchers discovered that one of the bright X-ray sources fades down and then appears again with a regular interval of 2.8 hours. This discovery allowed extraction of unparalleled information on the nature of the source.

'We found a prominent modulation in the X-ray brightness of the source, which we believe comes from absorption and scattering of the X-ray emission in the ionized gas spiraling towards the compact object in a binary star system,' said Trudolyubov. 'This allows us to determine the orbital period of the system and provides unique information on its geometry.'

The short, 2.8-hour, orbital period implies a very small size of the binary system. The distance between the compact object and its companion star is smaller than 1,000,000 kilometers. 'This binary system is so compact that it can be easily hidden inside a sun-like star,' says Trudolyubov. 'According to our measurements, the compact object is pulling material from the companion star at an enormous rate: it needs only 100 million years to consume a star like our sun.'

This source was also found to lie within one of the globular clusters (Bo 158), a compact, spherical concentration of tens or even hundreds of thousands of stars. This implies a low mass of the companion star and possible neutron star nature of the compact object. The general properties of the source are reminiscent of the Galactic low-mass binaries showing 'dips,', regular drops in the intensity of X-ray emission, making it the most distant 'dipping' X-ray source known to date. The researchers hope to find such periodic modulations in more systems within the Andromeda galaxy, and to learn more about their geometry and dynamics.

The international research team for this investigation include Sergey P. Trudolyubov, Konstantin N. Borozdin and William C. Priedhorsky of Los Alamos National Laboratory; Julian P. Osborne and Michael G. Watson of the University of Leicester, UK; Keith O. Mason of the Mullard Space Science Laboratory, UK and France A. Cordova of the University of California at Riverside.

This announcement is based on observations obtained with XMM-Newton, a European Space Agency science mission with instruments and contributions directly funded by ESA member states and NASA. For further information on the satellite and its mission, see

Los Alamos enhances global security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health and national security concerns.
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