25-Year Mystery of X-ray Emissions Solved

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25 years ago, astronomers discovered diffuse X-ray emissions coming from the plane of the Milky Way, but were puzzled by the source of those emissions. The mystery has now been solved by an international team of astronomers using the Chandra X-ray Observatory. These diffuse emissions do not originate from one single source but from white dwarf stars and stars with active outer gas layers.

Energetic X-ray emissions usually originate from very hot gases in a temperature range between 10 and 100 million degrees Celsius. And this so called “Galactic Ridge X-ray Emission” (GRXE) can also be found in very hot, optically thin plasma.

However, a gas with these thermal properties would immediately dissipate. Cosmic particles colliding with the interstellar medium could also be ruled out as an explanation for the GRXE.

Recently observations from two different satellites, the RXTE and Integral satellites, have shown that the X-ray emissions of the Milky Way exhibit the same distribution pattern as the stars. Since then, it has been assumed that a large portion of the GRXE originates from individual stars. These findings motivated the international team to carry out more precise measurements with the Chandra X-ray telescope.

The test area chosen was a small celestial region near the center of the Milky Way, and was about one and a half time the size of a full moon. Chandra identified 473 point sources of X-rays in a sector of the search field covering only 2.6 arcminutes. In a further step, the group used measurements from the Spitzer space telescope to prove that the results of the sector observed could be applied to the whole galaxy.

Most of the 473 X-ray sources are likely white dwarfs, which accrete matter from their surroundings. The sources could also be stars that have high activity in their outermost gas layer, the corona. White dwarfs are the remnants of extinct, low-mass suns. These cooling dead stars frequently orbit a partner, and in such a binary star system the white dwarf extracts matter from its larger partner until it becomes a Type Ia supernova.

The resolution of the diffuse X-ray emissions in our galaxy into discrete sources has far-reaching consequences for our understanding of a number of astrophysical phenomena. Astronomers can use the GRXE as a calibration for the spatial distribution of star populations within the Milky Way, for example. The results are also relevant for research into other galaxies, to determine if diffuse X-ray radiation from these objects also originates from white dwarfs and active stars.

The work was done by Mikhail Revnivtsev from the Excellence Cluster Universe at the TU Munich and his colleagues at the Max Planck Institute for Astrophysics in Garching, the Space Research Institute in Moscow and the Harvard-Smithsonian Center for Astrophysics in Cambridge, and was published in the April 30, 2009 edition of Nature.

Source: Max Planck Institute