29.04.2009 —

A mysterious X-ray glow in our galaxy, the Milky Way, has kept astronomers busy for more than 25 years. Now scientists have solved the riddle about the nature of this radiation: Mikhail Revnivtsev, a research fellow with the Excellence Cluster Universe and his colleagues in an international research team now have found direct evidence that the majority of the emission does not stem from one single diffuse X-ray-source. Obviously it is caused by discrete sources in our galaxy – very likely by accreting white dwarfs and active stars. The original research work will be published in Nature journal on 30 April 2009 (Vol. 458, Nr. 7242).

In the universe, highly energetic electromagnetic X-rays usually are emitted by hot gas at a temperature of 10 to 100 million degrees. 25 years ago, astrophysicists found this kind of radiation along the disk of our galaxy - with no plausible explanation available: The so called “Galactic Ridge X-ray emission” (GRXE) might be characteristic for a very hot, optically thin plasma. However, an interstellar medium with these thermal properties would “fly away” from our galaxy immediately, causing a major energy loss that can not be replenished by the energy sources in the Milky Way.

Also, attempts to explain the GRXE as result of interaction of cosmic rays and the interstellar medium (ISM) could not be verified. In the past years observations from X-ray satellites RXTE (Rossi X-ray Timing Explorer) and Integral showed that the distribution of the GRXE follows the same pattern as the stars in the Milky Way. This led to the assumption that the bulk of the X-ray emission originates from stars, besides a small fraction being produced by hot, truly diffuse interstellar plasma, heated by stellar explosions (supernovae).

These findings motivated the group around Mikhail Revnivtsev to perform more precise tests with the Chandra X-ray Observatory. The galactic test area they chose was a small field about half the moon’s size near the center of the Milky Way. Explains Revnivtsev: “We chose a region of the Galactic plane that provided good conditions to perform our measurements: In this area we have a high GRXE intensity, which is essential to minimize the effects of X-radiation from extragalactic sources. On the other hand, there is a weak interstellar absorption of X-rays, crucial to maximize Chandra’s sensitivity to discrete sources.”

Left: Image of the central part of our Milky Way. The square marks the region of about 2,6 arcminutes M. Revnivtsev and his team observed to discover the sources for the galactic X-ray emission. Right: Picture of the GRXE (sources: (1) SPITZER Space Telescope/M. Revnivtsev; (2) M. Revnivtsev)

Indeed Revnivtsev and his team succeeded in finding discrete point sources for the GRXE focussing on a yet smaller “high resolution” circle in the test area where Chandra’s angular resolution is best. The scientists analysed the telescope’s images, finding 473 radiation peaks in a small circle of only ~2.6 arcminutes radius. In the next step the research team proved that the results of this limited area can be applied to the whole galaxy. They did so by scanning the same galactic area with another telescope. The Spitzer telescope operates at the near infrared (NIR) spectral band and is specialised to detect stellar mass distribution. Then they put this value into relation to the measured X-ray surface brightness and compared the result to the known GRXE/NIR intensity ratio of the whole Milky Way.

They found that the two values were in perfect agreement. “According to these results”, says Revnivtsev, “we can regard our present study of just a tiny region of our galaxy as representative for the whole Milky Way.” Most of the 473 X-ray sources detected are by all likelihood accreting white dwarfs and binary stars with a high activity in their coronae. White dwarfs are remnants of low-mass stars that accumulate matter from their partner-stars in a binary system.

The final resolution of the diffuse GRXE-“cloud” into discrete source has far reaching consequences for our understanding of a variety of astrophysical phenomena. “Knowing the sources of GRXE we have solved a major energy problem in our Galaxy”, resumes Revnivtsev. “Furthermore, we can use GRXE as a direct measure for variations in stellar populations in our Milky Way. And it also has become clear that, when studying external galaxies, astrophyisicists have to be aware that diffuse X-ray emission from these objects will contain or even be dominated by radiation stemming from stellar type sources like accreting white dwarfs and coronally active stars.”  

Contact:

Dr. Mikhail Revnivtsev, Excellence Cluster Universe, Tel: +49.89.35831-7133

Barbara Wankerl, Excellence Cluster Universe, Tel: +49.89.35831-7105

High resolution images