MAGIC and CTA: Gamma ray telescopes

The MAGIC and CTA telescopes enable scientists to study gamma radiation in the universe. Gamma rays have the highest energy of the whole electromagnetic spectrum. They are produced along with the cosmic radiation that constantly strikes the Earth's atmosphere - and scientists have been trying to elucidate their origin for over 100 years.

Gamma rays can be used to study very high energy objects in the universe. These include the remnants of supernova explosions as well as star-forming regions, pulsars and binary systems and other types of gamma ray sources in our galaxy. The telescopes also observe other galaxies, especially those with active supermassive black holes at their centers.

Scientists from the Max Planck Institute for Physics are involved with both observatories. MAGIC is a twin telescope on the Canary Island of La Palma, while CTA is a major telescope array currently being built on La Palma and in Chile.

Physics background

In the year 1912, the scientist Victor Hess discovered that Earth is continually bombarded by subatomic particles from space. These particles, consisting mainly of protons, are known as cosmic rays. The source of cosmic rays remains obscure because galactic magnetic fields conceal the direction they come from.

Gamma rays are generated at the same locations as cosmic rays, and since they have no electrical charge, they are not deflected by the magnetic fields and so maintain their crucial directional information. In this way, gamma rays lay a trail to the sources of cosmic radiation.

Essentially, gamma radiation is simply light, but with a much higher frequency and far greater energy per photon than visible light. Telescopes cannot observe gamma rays directly, because the rays react with the molecules of Earth's atmosphere. This gives rise to showers of secondary particles, called air showers.

Many of the secondary particles in air showers move faster than light in a transparent dielectric medium. This may sound surprising at first, but since the speed of light in air is a little lower than in a vacuum, it does not violate any law of physics. In 1934, Russian physicist Pavel Cherenkov proved that particles that move faster than light in air generate a kind of optical shock wave - a bluish glow known as Cherenkov light.

These flashes are not visible to the human eye, as they are too short (only a few thousandths of a second) and too weak (compared to the brightness of the night sky). They can, however, be detected by gamma telescopes, as these are equipped with large reflectors that capture each and every particle of light. In addition, the imaging cameras on the telescopes have thousands of extremely sensitive and ultra-fast light sensors that can provide snapshots of the air showers in Cherenkov light, intergrated for only several nanoseconds.

But that is not all. A further trick involves using a number of telescopes so that a great many images of the same gamma ray air shower are captured from different angles. This provides a very high level of accuracy when calculating the arrival direction.