The emission line of helium-like iron at energy about 6.7 keV is a salient feature of a hot plasma (temperatures around 10-50 million Kelvin). Such plasmas are present in different types of astrophysical objects, ranging from chromospheres of normal stars through atmospheres of accreting white dwarfs to the intracluster gas in clusters of galaxies. However, the discovery of a 6.7-keV line in the diffuse X-ray emission distributed along the Galactic plane (the so called Galactic ridge X-ray emission) came as a surprise for astronomers in the 1980-ies. The main problem is that this line indicates that the temperature of the emitting plasma cannot be less than ten million K. Although such hot gas can be kept bound in the deep gravitational potential wells of compact objects, e.g. white dwarfs, or of clusters of galaxies, the potential well of our Galaxy is not sufficiently deep for this puprose. Therefore, should this hot plasma be distributed through the interstellar space, it must be outflowing from the Galactic plane. To sustain a stationary situation one needs to constantly pump an uncomfortably huge amount of energy into the interstellar medium.

An alternative solution is that this hot plasma does not occupy the interstellar space but belongs to a large number of descrete X-ray sources in the Galaxy, which are too weak to be resolved separately by X-ray telescopes. In this case the surface brightness distribution of the "diffuse" X-ray emission is expected to follow the distribution of the parent sources over the Galaxy. This however could not be checked until recently, mainly because of the relatively low surface brightness of the ridge emission and its very large extent (more than 100 degrees along the Galatic plane and a few degrees across the plane). Another major problem was the severe contamination of the diffuse component by bright point X-ray sources (mainly accreting neutron star and black holes).

Recently the team of MPA astronomers showed that the distribution of background-like X-ray emission of the Milky Way very closely follows the distribution of the Galactic near-infrared surface brightness (which is a good tracer of stellar mass distribution) in selected regions of the sky, not contaminated by bright point sources (see current research March 2006). Now it became possible to construct a much more detailed map of the Galaxy in this background light, making use of the flux of the 6.7 keV line. The constructed map confirms that the Galactic background X-ray emission is distributed exactly as stars and that it originates from superposition of X-ray emission from millions of weak Galactic X-ray sources -- accreting white dwarfs and coronally active stars. It was also been shown that the X-ray-near-infrared correlation is the same throughout the Galaxy, demonstrating that the population of weak X-ray sources is virtually uniform.

Revnivtsev M., Molkov S., Sazonov S.

Publication:

Revnivtsev M., Molkov S., Sazonov S., Map of the Galaxy in the 6.7 keV emission line, ArXiv Astrophysics e-prints, arXiv:astro-ph/0605693 (2006)