Cosmic Accelerators – Astronomy at the Highest Energies
High-energy astrophysics at MPIK is characterized by a very close cooperation between experimentalists and more theoretically oriented astrophysicists. They study the non-thermal phenomena in the Universe using the High Energy Stereoscopic System H.E.S.S. and the High Altitude Water Cherenkov Detector HAWC in Mexico to detect very-high-energy (VHE) gamma rays from the cosmos, and investigate the acceleration mechanisms in cosmic sources of high-energy particles.
Particles in the VHE range cannot be produced as thermal radiation, like the electromagnetic radiation in most other wavelength regimes; only in the Big Bang high enough temperatures were reached for a short time. VHE Gamma radiation is produced when strongly accelerated charged particles react with the ambient medium – either the interstellar gas or the ambient photon fields. In contrast to the charged particles, the gamma rays travel on a straight path from the source to the observer and allow imaging of sources and the study of the processes in the acceleration region.
Charged particles continue to gain energy by diffusively returning many times into the shock front of the giant shock waves generated in supernova explosions or in the plasma jets emerging from the immediate vicinity of the massive black holes at the centres of active galaxies. Considerable effort at the Institute is going into the modelling and theoretical description of processes within the different types of cosmic accelerators.
Since the first telescope became operational in 2002, H.E.S.S. has detected more than 80 VHE gamma-ray sources along the Milky Way that are mostly visible also at other wavelengths. Supernova remnants or pulsar wind nebulae are the most numerous among them. The supermassive black hole in the Galactic Centre turned out to be a cosmic “pevatron” which extremely accelerates charged particles and is probably responsible for part of the galactic cosmic radiation. In the nearby Large Magellanic Clound, H.E.S.S. identified several extremely luminous sources. Further, galaxies with active nuclei and starburst galaxies appear as faint objects. Due to its wide-angle view, HAWC is especially suited to study extended objects like pulsar wind nebulae. To understand the various objects, a multi-wavelength analysis is required. In this context, special attention is paid to the infrared emission from dust which is a direct tracer of the interstellar matter with which cosmic rays interact to produce gamma rays.
Cherenkov Telescopes and Water Cherenkov Detectors
High-energy gamma rays from space – a trillion times more energetic than visible light – do not reach the Earth’s surface. Nevertheless, they can be sighted at the ground with the atmosphere serving as a detector. When entering the atmosphere, the gamma quanta collide with atomic nuclei producing cascades of electrically charged secondary particles, so-called particle showers. These emit faint bluish and extremely short flashes of light (Cherenkov light) which can be detected in dark nights with large reflector telescopes that are equipped with fast photosensors. To trace the exact direction the particle showers come from, they are observed stereoscopically by several telescopes simultaneously.
H.E.S.S. consists of five telescopes, four of them being identical with 107 m2 mirror area each. They form the corners of a square of side 120 m. A camera – a matrix of 960 photosensors – is placed in the focus of each mirror. H.E.S.S. was the first instrument that was able to produce true images of astrophysical gamma-ray sources. In the centre of the array, a fifth, huge telescope with 614 m2 mirror area and a camera with 2048 pixels has been operational since 2012. It strongly enhances the sensitivity of the system and extends the observable energy range to lower energies.
Preparations are underway for a next generation observatory with dramatically improved performance. The Cherenkov Telescope Array (CTA) will consist of two arrays, in Chile and La Palma, with around 120 telescopes of three different sizes. CTA will bring much better resolution, higher sensitivity, a much wider energy range, and a collection area of many square kilometres at the highest energies. The MPIK instrumentation effort is on novel cameras for the different telescope types.
At high-altitude sites, the shower particles can be observed directly – and around the clock – using water-filled detectors, where they also produce Cherenkov light. The main detector of HAWC consists of a dense array of 300 tanks at an altitude of 4100 m. The tanks are filled with high-purity water and equipped with light sensors. They are surrounded by an array of 350 more loosely arranged smaller ‘outrigger‘ tanks, which significantly improve the characterization of particle showers hitting the boundary area of the main array.
The Early Universe – Elementary Particles at Highest Energies
Immediately after the Big Bang, the extremely hot matter of quarks and gluons formed a so-called quark-gluon plasma. Also in very high-energy collisions of the nuclei of heavy elements, a quark-gluon plasma of the size of an atomic nucleus can be generated for an extremely short time span. In order to better understand this state, the particles formed in energetic proton-lead and lead-lead collisions are investigated with LHCb.
Another, theoretically treated topic is the connection between cosmic inflation and high-energy particle physics. Hints on highest-energy processes in the early Universe may also come from gravitational waves.