Is it possible to consume gamma rays

Particles on tour

Black holes, pulsars, explosion clouds from former stars - these celestial bodies accelerate particles to enormous energies and emit high-energy gamma radiation. With the two observatories H.E.S.S. and MAGIC, which were created under the direction of the Max Planck Institutes for Nuclear Physics in Heidelberg and for Physics in Munich, this extreme spectral range becomes accessible.

Text: Thomas Bührke

If you ask Werner Hofmann about the most recent discoveries of the H.E.S.S. Observatory, he quickly comes up with a recently completed sky survey. "After a total of 3000 hours of observation, spread over ten years, we have found 77 new celestial bodies that we had never seen before in this energy range," says the director at the Max Planck Institute for Nuclear Physics in Heidelberg. The observatory was built under his leadership, for which he has received numerous awards, most recently with the prestigious Stern-Gerlach Medal of the German Physical Society.

This one sentence outlines the difficulties that astrophysics has to contend with with high-energy gamma rays: It takes a lot of time to observe the weak rays - and the largest telescopes in the world. In addition, a trick is necessary for this, which H.E.S.S. works that High E.nergy S.tereoscopic S.ystem.

The gamma rays cannot pass through the earth's atmosphere, but they are noticeable on the ground. If they penetrate into the air, there is a brilliant physical exchange of blows with the electric fields of atoms. This creates new particles that fly on like an avalanche towards the ground. Individual charged particles race faster than light.

That sounds amazing, but the speed of light in air is a little bit slower than in a vacuum. This also does not violate Einstein's law, according to which no body can move faster than the vacuum speed of light. In the air, however, these particles produce a flash that lasts only a few billionths of a second, a kind of “super-light boom”.

This very weak Cherenkov radiation can be observed with large telescopes on the ground. So high-energy gamma astronomy uses the atmosphere like a giant fluorescent screen. The Cherenkov light spot has a diameter of 250 to 500 meters on the ground. If there is a telescope in it, the direction of origin and the energy of the gamma radiation can be determined from the orientation and the intensity of the flash.

The H.E.S.S. Observatory, located in the highlands of Namibia, consists of four telescopes, each with twelve-meter collecting mirrors and a 28-meter-diameter reflector. The counterpart MAGIC - M.ajor A.tmospheric Gamma-ray I.maging C.herenkov Telescopes - stands on the 2400 meter high mountain Roque de los Muchachos on the Canary Island of La Palma. It has two telescopes, each with 17-meter collecting mirrors.

Two observatories discover 139 springs

"MAGIC and H.E.S.S. together have the entire northern and southern hemisphere in view, ”says Masahiro Teshima, Director at the Max Planck Institute for Physics in Munich. But the two observatories also complement each other a little in terms of their capabilities: “Because of the stereoscopic vision and the large mirrors, the MAGIC telescopes can still receive radiation with less energy than H.E.S.S. That's what H.E.S.S. more sensitive at very high energies and has greater sharpness of detail, ”says Teshima's colleague David Paneque, who is responsible for coordinating the scientific work of MAGIC.

Of all 178 celestial bodies known to date that emit high-energy gamma radiation, 105 alone were identified with H.E.S.S. and 34 discovered with MAGIC - a success for which H.E.S.S. Ranked among the top 10 observatories in the world in 2009, along with the Hubble Space Telescope.

The gamma sources testify to the most violent events in the cosmos, such as star explosions and their consequences. If a star has used up its fuel at the end of its life, energy production stops. The central area collapses in a fraction of a second under the effect of gravity. The outer areas, however, explode, shoot out into space and light up brightly. A supernova shines.

If the collapsing core has no more than three solar masses, a neutron star forms - an extremely compact, rapidly rotating sphere with a diameter of 20 kilometers. In such an object, matter is so compressed that a teaspoon of it on earth would weigh as much as a million long-distance trains.

During this collapse, the magnetic field of the former star is also compressed. This dipole field is similar in shape to that of our earth with north and south poles, but is billions of times stronger and rotates billions of times faster. According to today's ideas, electrically charged particles tear themselves away from the star and are accelerated into space along the magnetic field axis up to almost the speed of light.

This process produces radiation in a complex way, primarily in the direction of movement - like a car headlight. As a result, the swarm of particles creates two cones of light that protrude into space from the north and south poles of the neutron star.

In many cases, the axis of the magnetic field is inclined with respect to the axis of rotation. As a result, the two cones of light sweep through space like the headlights of a lighthouse. When they hit earth, the telescopes register radiation pulses with the body's rotational frequency. In this case, astrophysicists speak of a pulsar. These objects are considered to be cosmic laboratories in which physical processes and theories can be tested under extreme conditions.

In 1989 astrophysicists discovered a celestial body in the high-energy gamma range for the first time. It was the Crab Nebula - the explosion cloud of a supernova that a Flemish monk was the first to discover in April 1054. In the center of the structure, also known as the crab nebula, sits a pulsar that rotates around its own axis 30 times per second. The Crab Nebula can be observed today in all spectral ranges from radio waves to visible light to high-energy gamma rays. It is considered to be the best-studied supernova remnant. Nevertheless, he always poses new puzzles.

The magnetic field plays the central role

The researchers recently received pulsed gamma rays from the object with MAGIC with a record energy of 1.5 billion electron volts or 1.5 teraelectron volts (TeV). This is the most energetic pulsed radiation that has ever been measured on a star. For comparison: visible light has an energy of two to three electron volts.

For the results published in early 2016, the MAGIC team had to evaluate 320 observation hours from October 2007 to April 2014. "It is only clear that the very strong magnetic field of the cancer pulsar plays the central role," says Razmik Mirzoyan, spokesman for the MAGIC collaboration and project manager at the Max Planck Institute for Physics in Munich.

In order to discover how this cosmic accelerator works, the collaboration of astro and particle physicists is required. They come to the conclusion that in the magnetic field electrons and their antiparticles - called positrons - are accelerated to almost the speed of light and eventually are annihilated. But this process can only explain gamma rays with energies up to a few billion electron volts (GeV). Another mechanism must be responsible for the recently observed gamma pulses.

The researchers currently suspect that about 1500 kilometers above the surface of the pulsar, high-energy charged particles form a “reactive mixture” with photons from the UV and X-rays, in which the particles transfer their energy to the photons and transform it up to high-energy gamma quanta. This process is called the inverse Compton effect.

The ones with H.E.S.S. and MAGIC received gamma radiation is therefore a secondary effect. The real cause is particles that are accelerated under extreme cosmic conditions. Since no conversion process is perfect, the scientists assume that the primary particles have more energy than the gamma radiation they generate.

In the surrounding explosion cloud, which expands at 1,500 kilometers per second, it is turbulent. It is also an efficient accelerator. This hot gas cloud is permeated by magnetic fields that move with it from the star. This leads to a nuclear table tennis game: Electrically charged particles, especially hydrogen nuclei (protons), are thrown back and forth between magnetic field fronts and constantly gain energy - until they are so fast that they can escape this ping-pong. This process only works when the magnetic fields are moving.

Captivity lasts for several centuries

“Until recently, we had expected the particles to remain trapped in the fog for thousands of years before they could escape,” explains Jim Hinton, who heads the “Non-Thermal Astrophysics” department at the Max Planck Institute for Nuclear Physics in Heidelberg. "According to our measurements, however, they escape after centuries."