The relatively young research field of astroparticle physics has been developing dynamically over the last years. It connects
  • particle physics (describing the interactions of elementary particles) with
  • astrophysics (describing up to the biggest structures in the universe) and with
  • cosmology (studying the history of the universe).


One of the very interesting topics in astroparticle physics is trying to understand the acceleration mechanisms of cosmic ray particles to very high energies, much higher than accelerators on Earth can reach. Only the most violent objects in the cosmos can offer the necessary conditions for that: supernovae with their explosion shock waves, magnetic fields of rapidly spinning neutron stars, colliding galaxies or energetic black hole cores of active galaxies.

An artist's drawing of a particle jet emanating from a black hole at the center of a blazar. Credit: DESY, Science Communication Lab
Some active galaxies, known as blazars, develop narrow twin jets of light and elementary particles, one of which is pointing to Earth, emitted from the poles along the axis of the black hole’s rotation. The picture of ICD 2019 shows one of these two jets. They are powerful cosmic engines that accelerate high-energy cosmic rays. But not only. When a blazar accelerates protons, pions are created as well which then produce neutrinos and gamma rays. Neutrinos are uncharged particles, unaffected by even the most powerful magnetic field. Because they rarely interact with matter and have almost no mass, neutrinos travel nearly undisturbed from their accelerators, giving scientists an almost direct pointer to their source. By using neutrinos and gamma rays we can point to the cosmic ray accelerators.

On the 22nd of September 2017, one of these neutrinos was detected at the South Pole by the IceCube Neutrino Observatory. It was sent 4 billion years ago from the blazar TXS 0506+056 in the constellation of Orion. This neutrino did not travel alone: gamma rays were detected as well by other telescopes on Earth and in space. This is the first observational evidence that cosmic rays, neutrinos and gamma rays are created in the same source. It also marks the dawn of multimessenger astrophysics: by combining information from different cosmic messenger — cosmic rays, neutrinos, gamma rays and gravitational waves — we can learn about the distant and extreme universe. To learn more about this visit this web page:

Credit: DESY, Science Communication Lab

When two stars exist close enough to each other, they bind themselves to each other by gravity. The less massive star, called companion, will orbit around the more massive star, called primary. One of the biggest binaries that we know of in our galaxy is the binary star Eta Carinae. The artistic illustration of this is the cover image of the ICD2020. The picture show the two stars in the middle and the surrounding gas cloud with its snail shell pattern formed by the binary's orbital motion. The primary star of Eta Carinae is about 100 times larger than our sun. In the image this is the very bright area that outshines everything. The companion of Eta Carinae is about 20 times larger than our sun. This is the shining point to the right of the bright area.

Once every 5.5 years, the companion completes its orbit around the primary. These two large and massive stars emit cosmic particles in the form of fast solar winds. The two winds collide with each other and create a shock which accelerates particles, like protons and electrons. The accelerated particles then lose energy by producing photons which are measurable on Earth and in space. Because the stars are orbiting each other, the distance between the two stars changes and so does the particle acceleration. A binary star in which particle acceleration is strong enough to be seen with instruments is called a Colliding-Winds Binary. In the case of Eta Carinae, this process has been known from satellite measurements for a few years. Many of the details are however still missing. On July 1st 2020, the H.E.S.S. telescopes reported on their observation of the most energetic gamma-rays ever seen from Eta Carinae. The H.E.S.S. measurement showed that the physics of Eta Carinae are more complex than what people first believed and that these complexities can teach us a lot about acceleration in general. Many variable gamma-ray sources which accelerate electrons as well as protons are outside of our own galaxy. The farther gamma-rays have to travel, the more likely they will be absorbed before reaching Earth. Eta Carinae is one of the closest cosmic laboratories that we have for studying variable particle acceleration in shocks up to gamma-ray energies.

There are many scientific experiments with huge detectors that aim to unlock the secrets of cosmic rays. If you are interested, have a look at the websites of the following experiments: ANTARESAugerBAIKALFermiHAWC, H.E.S.S.IceCube,  KM3NeTMAGICTelescope ArrayVERITASCherenkov Telescope Array .