Origin of the Galactic Ridge X-Ray Emission(Chapter 5. The Galactic Center and Ridge Emissions, The Extreme Universe in the Suzaku Era)

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The origin of the Galactic ridge X-ray emission (GRXE) remained a puzzle for a long time, as attempts to explain it as truly diffuse, hot thermal emission or a result of interaction of cosmic rays with the ISM faced serious problems. Here we briefly review significant advances made recently in our understanding of the GRXE. First, observations from the RXTE and INTEGRAL satellites showed that the global morphology of the GRXE reproduces the distribution of stars in the Milky Way, suggesting that X-ray volume emissivity is proportional to stellar mass density thoughout the Galaxy. Second, the inferred X-ray emissivity of the Galactic ridge per unit stellar mass proved to be consistent with the cumulative luminosity density of faint X-ray sources in the Solar neighborhood. This pointed to a picture in which accreting white dwarfs are the main contributors to the ridge hard X-ray emission. This idea received strong support from the detection by INTEGRAL of an exponential cutoff in the GRXE spectrum at energies above〜20-30keV, which corresponds to the virial temperature of protons at the surface of a white dwarf. Finally, counts of point sources in Chandra deep surveys of the Galactic Center region and in the Galactic plane showed excellent agreement with the predictions based on the luminosity function of nearby X-ray sources. At the present sensitivity limit,〜40% of the GRXE at energies below〜7keV is already resolved into discrete point sources, most of which are likely coronally active binaries and cataclysmic variables. All these facts together make a strong case that the GRXE is the combined emission of numerous faint point sources of well-known classes.
2007-12-19

Origin of the Galactic Ridge X-Ray Emission(Chapter 5. The Galactic Center and Ridge Emissions, The Extreme Universe in the Suzaku Era)

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