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  Current Research Highlight :: January 2007 all highlights

Resolving the Galactic X-ray background

A team of X-ray astronomers at the Max Planck Institute for Astrophysics and Harvard-Smithsonian Center for Astrophysics used ultra-deep observations of the Galactic Center region to resolve the thirty-years-old puzzle of the origin of the Galactic X-ray background.

Fig. 1: Image of the Galactic Center region taken by the Chandra observatory. Circles denote the positions of detected point sources.

Fig. 2: Luminosity functions of faint X-ray sources measured in the Solar vicinity (red area) and in the Galactic Center region (gray area). It can be seen that they are quite compatible with each other. The sources in the gray area explain at least 40-50 % of the total X-ray flux measured in the Galactic Center region, while still weaker sources expected to be present in this region by analogy with the Solar neighborhood, probably provide the remaining 50-60% of the total flux.

It has been known since the late 1970s that apart from bright X-ray point sources (mostly neutron stars and black holes in binary stellar systems) there is an unresolved X-ray emission distributed throughout the Milky Way. The origin of this "Galactic ridge X-ray emission", or the Galactic X-ray background, remained unclear for a long time. Numerous studies showed this emission to be strongly concentrated towards the Galactic plane. However the measured X-ray spectrum indicated that the emitting plasma is so hot (kT>5-10 keV) that it cannot be bound within the Galactic gravitational well. The plasma must then be constantly outflowing and carrying away an enormous amount of energy, the source of which was not understood.

An alternative explanation of the Galactic X-ray background, proposed soon after its discovery, is that it is the superposition of X-rays from millions of Galactic point sources, too weak to be detected individually. This would eliminate the problem of keeping hot plasma bound to the Galactic plane. However it was difficult to rigorously test this hypothesis because there was a large uncertainty with regard to the expected cumulative luminosities of different classes of faint Galactic X-ray sources.

MPA reseachers have recently made a number of steps that significantly furthered our understanding of the contribution of point sources to the Galactic X-ray background (see linkPfeil.gif Research Highlight March 2006 linkPfeil.gif Research Highlight April 2006 linkPfeil.gif Research Highlight August 2006 ). However the ultimate answer as to whether the Galactic X-ray background is produced by point sources or by truly diffuse interstellar plasma can only be given by direct observations. The most suitable tool for this purpose is the Chandra X-ray observatory. This is because the angular number density of X-ray sources becomes so high at low fluxes that one needs arcsecond angular resolution (as provided by Chandra) to overcome the problem of source confusion.

Mikhail Revnivtsev and Sergey Sazonov from MPA and Alexey Vikhlinin from CfA-Harvard (USA) used approximately 1 Msec worth of Chandra observations of the Galactic Center region to demonstrate that point sources with X-ray luminosities higher than 1031 erg/s contribute at least 40% to the total X-ray background in this region. Furthermore, a comparison of the reconstructed luminosity function of Galactic Center point sources with the previously measured luminosity function of X-ray sources in the Solar neighborhood showed very good agreement between the two and suggested that the still unresolved 60% of the Galactic X-ray background is probably produced by coronally active stars and white-dwarf binaries with luminosities below 1031 erg/s, the current effective threshold of Chandra.

Mikhail Revnivtsev, Sergey Sazonov

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