Galactic "engine room" explored

Variable emission of high-energy radiation of a giant radio galaxy detected

The radio galaxy M87 in the optical: The bright central area in which the black hole is located and from which the high-energy gamma radiation was detected, is located at the top left, the relativistic plasma current extends down to the right. © MPG
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The M87 radio galaxy emits gamma rays that are one million times million times more energetic than the visible light. It is particularly surprising that the intensity of this radiation can change drastically within just a few days. This can only be explained by the fact that the source region of the high-energy gamma radiation is unusually compact. As the only eligible region in the immediate vicinity of the supermassive black hole in the center of M87, as researchers now report in the online edition of Science.

At the center of many galaxies scientists today suspect a massive black hole with a mass that can reach millions to billions of solar masses. When this black hole sucks in the surrounding matter, matter flows of relativistic particles can be formed, which move at almost the speed of light. One speaks then of an "active galaxy". If such a stream of matter hits the earth, the corresponding galaxy is called a blazar. Blazars are the only active galaxies that have so far detected high-energy gamma radiation - with the only exception to date being the much closer M87 radio galaxy.

Frequent changes in gamma-ray intensity

The HESS team, an international composite research group of astrophysicists and particle physicists, now reports on the discovery of high-energy gamma radiation from the M87 radio galaxy. The research group operates in Namibia a system of four so-called Cherenkov telescopes, with which they have measured the gamma radiation of the nearby radio galaxy M87 in the last four years. The surprising result is that the intensity of this radiation changes drastically within only a few days.

The radio galaxy M87 is located in the Virgo galaxy cluster, 50 million light-years from Earth. The center of M87 is home to a supermassive black hole with a mass of three billion solar masses. From the central area of ​​M87 emerges a relativistic plasma current, a so-called jet, which is visible in optical, radio and X-ray images. In contrast to previously proven extragalactic sources of very high-energy gamma radiation (blazars), the plasma current of M87 does not point directly to the earth, but points past it at an angle of 30 degrees. The blazar detected gamma radiation is believed to be generated in the plasma streams, with the intensity and energy of the radiation being focused and amplified in the direction of the current due to the high velocity of the plasma stream. But such bundled radiation from the jet in M87 would not hit the earth at all. M87 therefore presumably represents a completely new type of extragalactic sources of high-energy radiation.

Source region no bigger than our solar system

The M87 high-energy radiation variability time scale measured by HESS is very short in a few days - shorter than in any other wavelength range. The source region of the high-energy radiation can therefore only be about as large as our solar system (only about 0.000001 percent of the size of the entire radio galaxy M87). "This is not much larger than the event horizon of the supermassive black hole at the center of M87, " notes Dr. Matthias Beilicke, one of the participating scientists from the University of Hamburg. Relativistic effects, which play a role in other previously proven extragalactic sources (blazars) and which modify the relationship between time variation and source size, should be of minor importance in the case of M87, since the plasma current of M87 is not aimed at the earth. Display

The high-energy gamma radiation will most likely be in the immediate vicinity of the supermassive black hole in the center of M87; other structures in M87, such as the plasma stream, tend to have larger dimensions. However, the physics of emission processes is not really understood yet. Because of the proximity to the black hole, the researchers also discuss novel mechanisms: For example, hydrogen nuclei in the field of a rotating black hole could be accelerated to extreme energies and then emit gamma quanta. In the vicinity of the black hole, part of the matter sucked in by it is also diverted into the relativistic plasma stream; a process that is also not fully understood. The fact that high-energy gamma radiation can escape unhindered from this "active" region may seem surprising at first glance. However, this is possible because in the black hole in M87 obviously comparatively little matter fills and it still represents a rather harmlosen environment compared to many other black holes.

With this new as well as the previous discoveries of extragalactic sources HESS provides an important contribution to the deciphering of the processes that lead to the generation of extremely high-energy gamma quanta. The M87 radio galaxy represents a unique laboratory for studying the nucleus of such an active galaxy, at the center of which a black hole as a powerful "motor" accelerates the charged particles to extremely high energies. M87 can be compared to the more numerous but more distant blazars, but in contrast to M87, the plasma stream hides our view of the central region. With the help of HESS, in the case of M87, a clear insight into the "machine room" of a galaxy was achieved. This will lead to a better understanding of extragalactic sources of high-energy gamma radiation.

(Max Planck Society for the Advancement of Science eV, 27.10.2006 - AHE)