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The energy gamma rays

All amateur astronomers are accustomed to seeing images of the same object in different energy bands from radio observations, optical, infrared, ultraviolet, X-ray It is not so common, however, hear about most of the energy of the electromagnetic spectrum: gamma rays.

This light has an energy about 10 billion times greater than the light to which the human eye is sensitive. Unfortunately it is not possible to directly observe this radiation as the Earth's atmosphere is not transparent at these energies. The problem can be circumvented in part through the use of space telescopes , but what about an important part of gamma rays, high energy ones, this is not possible for purely logistical reasons: these photons are so rare that the dimensions of the telescope are prohibitive for an experiment to be sent into orbit.

Another way to observe these photons uses the same atmosphere of the Earth as a large particle detector. The gamma rays that pass through the atmosphere interact with the atoms thereof and lose energy, producing a cascade of particles. The particles in the cascade have a speed faster than the speed of light in the atmosphere (but still less than the speed of light in vacuum!) And produce the so-called Cherenkov radiation: this is a cone of light that, when it reaches the ground, it can illuminate an area of \u200b\u200babout 100 meters range!

This light is then gathered by telescopes and analyzed. Among the latest experiments in this field include HESS (High Energy Stereoscopic System), located in Namibia, MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov Telescope), located in La Palma, Canary Islands, and VERITAS (Very Energetic Radiation Imaging Telescope Array System), Arizona, United States. These experiments take advantage of the stereoscopic technique, or use multiple telescopes spread over an extensive area, to cover the cone of light produced by the waterfall: this allows us to determine more certainty the direction from which the photons and their energy.


Each telescope is not fundamentally different from a traditional optical telescope. The main difference is the size of the mirror: the two telescopes MAGIC experiment, for example, have a diameter of about 17 meters each, while that of the telescope to be built during the next phase of the experiment HESS is even over 20 meters ! Of course it is not monolithic mirrors, which consist of a single block, but made up of many small pieces.

Many and various are the astronomical objects that can be studied with this technique: an example is jets of particles emitted by the nuclei of so-called active galaxies, which hide within them a supermassive black hole that devours the surrounding matter, or nebulae of energetic particles ejected from the stars when, at the end of their lives, explode in the form of supernovae.

GIOVANNA PEDALETTI

picture, the four telescopes currently comprising the HESS experiment in the Plateau of Khomas, Namibia. The experiment is fully operational since 2004, will be enhanced in the coming years with more telescopes to achieve even better performance. (Credits: HESS Collaboration)