Enigmatic cosmic rays hit the earth

Energetic electrons from an unknown source near the solar system

Cosmic Radiation (Illustration) © Simon Swordy / University of Chicago
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That the earth is constantly bombarded by cosmic rays is not new. But now scientists have measured a puzzling additional influx of high-energy electrons. The source of this cosmic radiation is still unknown, as astronomers report in the current issue of Nature. However, it is suspected near the solar system. It may consist of dark matter.

The galactic background radiation consists of subatomic particles that are accelerated by supernovae or other explosive cosmic events almost to the speed of light. They race through the Milky Way and form a veil of energetic particles that reach the solar system from all directions. Typically, the high-energy particle mix contains mainly protons and heavier atomic nuclei with a small fraction of electrons and photons as rounding.

Strange surplus of high-energy electrons

Now, however, scientists around John Wefel of Louisiana State University have discovered an unusual peak in this "weird noise." For their studies, they used a NASA radiation detector, the Advanced Thin Ionization Calorimeter (ATIC), which was transported to the upper atmosphere with stratospheric balloons over the Antarctic. The researchers also expected the usual mix of particles in these measurements, but the calorimeter found something else: a surplus of high-energy electrons. The device registered 70 electrons in the energy range of 300 to 800 Gigaelektronenvolt within five weeks, which does not sound much, but is extremely unusual in this area.

Electron measurement result of the ATIC instrument. The black curve shows the energy that would be released in a collision of two dark matter particles according to the Kaluza-Klein theory. © Nature / J. Chang et al.

"That's a big discovery. It's the first time we've seen a discrete source of accelerated cosmic rays sticking out of the general galactic background, "explains Wefel. He compares the discovery with a drive on the highway, surrounded by normal small cars and trucks, in which suddenly a group of Lamborghinis breaks through the normal traffic. "You do not expect to see so many sports cars on the road - and you do not expect so many high-energy electrons in the mix of normal cosmic rays."

Source is near the solar system

"The source of these electrons must be relatively close to the solar system - not more than one kilo-second away, " said Jim Adams, co-author of NASA's Marshall Space Flight Center study. One kilo-kilo-sec corresponds to about 3, 260 light-years. Energy-rich electrons lose their energy very fast as they fly through the galaxy. First, they lose energy when they collide with low energy photons, that is the so-called inverse Compton scattering. And secondly, they emit some of their energy as they race through the galaxy's magnetic field. If high-energy electrons are found in the measurement, they must therefore be of local origin, possibly a few hundred light-years away. display

Collisions of Dark Matter as a Cause?

But what is the source of this radiation? That is so far completely unclear. The most obvious explanations include a nearby pulsar, a microquasar, or a black hole. In theory, all of these objects can accelerate electrons to high energies. A far more intriguing possibility, however, is dark matter, according to astrophysicists.

According to a physical theory, the so-called "Kaluza-Klein" theory, the basic building blocks of dark matter could hide in extra dimensions of space and thus normally be invisible to us. Only by their attraction we perceive the Dark Matter at all. According to the theory, there is also a corresponding antiparticle for every particle of dark matter. When both types of particles collide, they cancel each other out and release energy in the form of high-energy photons and electrons. The latter materialize in the three dimensions of the space we can grasp and thus appear as cosmic rays.

Hypothetical particle would have the appropriate energy

"Our results could be explained by a lump or cloud of dark matter located in the neighborhood of the solar system, " explains Wefel. "Concretely speaking, there is a hypothetical small Kaluza particle with a mass near 620 gigaelectronvolts, which, when quenched, should produce electrons in the same energy range as we observed."

However, testing this hypothesis is anything but easy, because dark matter, as its name implies, is extremely difficult to detect. New telescopes, such as NASA's Fermi Space Telescope, may be looking for gamma rays and other products of this event. "Whatever it is, " Adams said, "it's absolutely amazing."

(NASA, 24.11.2008 - NPO)