Galaxy nucleus is a mystery

Gammasatellit Fermi and HESS telescopes explore Blazar

Artistic representation of a supermassive black hole in the center of an active galaxy like PKS 2155-304. In the middle, a jet of high-energy particles shoots out. © NASA
Read out

An international team of astrophysicists has discovered surprising changes in the emitted radiation at an active galaxy nucleus. The image resulting from these observations in the visible, X-ray and gamma-rays is considerably more complex than expected and questions the previous theories for generating radiation in such objects.

For these measurements, for the first time simultaneously the HESS telescopes in Namibia and the gamma satellite Fermi of NASA targeted the active nucleus of the galaxy PKS 2155-304, an object called blazar. The researchers report on their findings in the current issue of the journal "The Astrophysical Journal".

Jets shoot out of blazars

Blazars belong to the family of active galaxies. As with most of these Milky Way galaxies, blazar guns fire oppositely directed jet streams - called jets - at near the speed of light when matter falls into a central supermassive black hole. In Blazars one of the jets is aimed directly at the ground - as in PKS 2155-304.

The object is located 1.5 billion light-years away in the southern constellation Piscus Austrinus and is usually a weak source of gamma radiation. But when, as in 2006, there is a huge burst of radiation, the blazar becomes the brightest source in the sky in high-energy gamma light - 50 trillion times more energetic than visible light. But even from the very highest sources, at most one single high-energy gamma photon per square meter per month reaches us.

Short-lived particle showers

Gamma radiation is absorbed in the earth's atmosphere and produces very short-lived particle showers. When these ultrafast particles fly through the Earth's atmosphere, they produce weak blue light flashes called the Cherenkov radiation. The High Energy Stereoscopic System (HESS) in Namibia with four large telescopes has captured these light flashes from PKS 2155-304. display

Lower energy gamma radiation was measured directly by the Large Area Telescope (LAT) aboard NASA's Fermi satellite. "The observatory gives us for the first time the opportunity to measure this huge galaxy over as many wavelengths as possible, " says Werner Hofmann, spokesman for the HESS team and director at the Max Planck Institute for Nuclear Physics in Heidelberg. In addition to observations throughout the entire gamma-ray range, the galaxy was simultaneously X-rayed by the Swift and Rossi X-ray Timing Explorer (RXTE) satellites. The HESS Automatic Telescope for Optical Monitoring also registered the visible light.

Mysterious Blazars

From 25 August to 6 September 2008, the telescopes jointly observed PKS 2155-304 in a calm state. The results of the 12-day campaign are surprising: In active phases of this and other blazars, the emissions in the X-ray and gamma areas increase and decrease together. But in the quiet state of PKS 2155-304 this is not the case - and nobody knows why.

Even more surprising was the discovery that the emission of visible light rises and falls along with the high-energy gamma radiation. "It's as if you were observing a blast-furnace burner in whose flame the highest and lowest temperatures vary, but not the average temperatures, " says Berrie Giebels, astrophysicist cole Polytechnique in Paris, working for both the HESS and Fermi LAT teams.

"We learn from this that the various components of jets in blazars interact in complex ways, generating the radiation we observe, " says Jim Chiang of the Fermi team at Stanford University in California. These observations may contain first clues that help us to decode the events deep inside a blazar.

(MPG, 20.03.2009 - DLO)