Ultra high-energy cosmic rays come from black holes! Print E-mail
 
In the latest issue of “Science”, the Pierre Auger collaboration announced having linked for the first time ultra high-energy cosmic rays with violent black holes. “The age of cosmic-ray astronomy has arrived”, said Nobel Laureate James Cronin.
 
A seminal article by the Pierre Auger Collaboration in the November 9 issue of the journal Science gave the first indication that Active Galactic Nuclei are the most likely candidates for the source of the highest-energy cosmic rays observed on the Earth. The origin of these ultra high energy (UHE) cosmic rays has been a long standing mystery in Astrophysics.

Active Galactic Nuclei (AGN) consist of black holes with a huge mass, between a billion to ten trillion times that of our Sun, which keeps growing by gravitationally attracting gaseous matter. During the in-fall of matter into the black hole, electromagnetic radiation is emitted, such as radio, infrared, optical, ultraviolet, X-rays, or gamma rays, in large amounts. AGNs are the most luminous objects in the Universe. They also have been investigated as sources of very high-energy cosmic rays, though the exact mechanism of how they could accelerate particles to energies 100 million times higher than the most powerful particle accelerator on Earth is still not well understood.
 

Auger map 

The above stations, shown in color on the Auger map, have recorded, on March 11, an air shower created by a 69.8 EeV UHE cosmic ray.
Credits: © Auger Collaboration .

 

While the observatory has recorded almost a million cosmic-ray showers, only the rare, highest-energy cosmic rays can be linked to their sources with sufficient precision. Auger scientists so far have recorded 81 cosmic rays with energy above 40 EeV (4x1019 electron-Volts), the largest number of cosmic rays with this energy recorded by any observatory. At these ultra-high energies, the uncertainty in the direction from which the cosmic ray arrived is only a few degrees, allowing scientists to determine the location of the particle’s cosmic source.

The Auger collaboration discovered that the 27 highest-energy events, with energy above 57 EeV, do not come equally from all directions. Comparing the clustering of these events with the known locations of 381 Active Galactic Nuclei, the collaboration found that most of these events correlated well with the locations of AGNs in some nearby galaxies, such as Centaurus A.

Cosmic rays are protons and atomic nuclei that travel across the Universe at close to the speed of light. When these particles smash into the upper atmosphere of our planet, they create a cascade of secondary particles, called an air shower, which can spread across 40 or more square kilometers as it reaches the Earth’s surface. Low energy cosmic rays lose their initial direction when traveling through galactic or intergalactic magnetic fields and thus cannot reveal their point of origin when detected on Earth. But, UHE particles, are only slightly deflected so they come almost straight from their sources.

The experimenters of the Auger Observatory can then “see” the sources by reconstructing their direction. They record cosmic-ray showers through an array of 1,600 particle detectors placed 1.5 kilometers apart in a grid spread across 3,000 square kilometers. Twenty-four specially designed telescopes record the emission of fluorescence light from excitation of atmospheric nitrogen by an air shower, and large water tanks are used to detect Cherenkov light produced by the particles of a shower.

Cosmic ray on the sky map

The blue circle shows the direction of the 69.8 EeV UHE cosmic ray on the sky map. It seems to come, within 0.9 degrees, from Centaurus A.
Credits: © Auger Collaboration .

 

“We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays, first revealed by French physicist Pierre Auger in 1938,” said Nobel Laureate James Cronin, of the University of Chicago, who conceived the Pierre Auger Observatory together with Alan Watson of the University of Leeds. “We find that the southern hemisphere sky as observed in ultra-high-energy cosmic rays is non-uniform. This is a fundamental discovery. The age of cosmic-ray astronomy has arrived and in the next few years our data will permit us to identify the exact sources of these cosmic rays and how they accelerate these particles.”

The Auger Observatory was established in 1992 and is located in Argentina. It is a worldwide collaboration with a large involvement of European astroparticle physicists from many countries.

Submitted by Georgios Fanourakis (Demokritos)
with the help of Alan Watson, spokesperson of the Auger collaboration.

 

 
 Podcast

 

Three questions about Auger to Dr Alan Watson 

 

 
 
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