The research activity of Felix Aharonian has covered a wide range of astrophysical and astronomical fields including the basic theory of radiation processes in relativistic plasmas, theoretical and observational studies concerning the origin of cosmic rays both of Galactic and extragalactic nature, and extensive observational work in gamma-ray astronomy.
As for the theoretical work mostly related to his early research career, we stress his exact derivation of the kernel of the Compton scattering in the general relativistic case, the radiation spectrum of electron-positron plasmas and the “blue shift” of the corresponding annihilation line, the formation of a Maxwellian tail in thermal plasmas, the energy spectrum from pairs with photon- photon interactions and the first detailed calculations of their cascades in radiation fields. Recently he and his colleagues have developed, for the first time, a complete theory of magnetic bremsstrahlung of relativistic particles in a highly turbulent medium, the so-called jitter radiation.
It is to be highly appreciated how the nominee has systematically focused his theoretical expertise onto the interpretations of extreme astrophysical conditions as found particularly in young Supernova Remnants and Pulsars, and in Active Galactic Nuclei with relativistic jets, all sites of choice for efficient particle acceleration and ensuing high energy phenomena.
One specific focus of his research is constituted by the century-old issue concerning the origin of Cosmic Rays, a field in which Felix Aharonian is a recognized authority. We stress in particular his studies concerning the acceleration of Cosmic Rays to above 100 TeV in Supernova Remnants within our Galaxy, the propagation of the high energy tail in the intergalactic space, and the prediction of gamma rays from giant molecular clouds as Cosmic Ray barometers, In fact, Felix Aharonian was among the first to recognize the great potentiality of high energy gamma-ray observations from sources that qualify as likely accelerators of Cosmic Rays. The validity of his anticipations is now well established and his latest contribution has just been published (Aharonian et al. 2012, Nature, Abrupt acceleration of a 'cold' ultrarelativistic wind from the Crab pulsar, 432, 507).
In order to scale up the efficiency for the detections of very high energy gamma rays sources, Felix Aharonian has pioneered the “stereoscopic atmospheric Cherenkov imaging technique” by neatly formulating the basic principles (Aharonian et al. 1993, Experimental Astronomy, The system of imaging atmospheric Cherenkov telescopes – The new prospects for VHE gamma ray astronomy, 2, 331) and assessing the expected performance of Cherenkov telescope arrays. These ideas have been put to test first with the prototypical array HEGRA at La Palma (Canary Islands), and then with the H.E.S.S. (High Energy Stereoscopic System) array in Namibia. The very successful observations carried out under his coordination have in fact opened a new window in the observation of the Universe.
The about 150 papers detail the many exciting results of such observations conducted by the HERA and H.E.S.S. collaborations of which he is a leading member. Here we just mention that a most interesting result, with important cosmological implications, is the derivation of an upper limit on the background light at optical/near IR wavelengths which severely constrains the contributions from sources other than galaxies. All these achievements have been recognized by two distinguished international science awards assigned to the H.E.S.S. collaboration, the 2006 EU Descartes Prize and 2010 Rossi Prize (A.Aharonian, W.Hofmann and H.Voelk). The innovation of the “stereoscopic” approach, also adopted by other instruments such as MAGIC and VERITAS, is at the heart of the future Cherenkov Telescope Array (CTA), a project involving a large international collaboration.
His overall scientific production includes about 400 articles in peer reviewed journals. In addition to those already mentioned in square brackets, we have selected a limited number that are highly significant, well cited and representative of his wide research interests as outlined above:
1. High energy electrons and positrons in cosmic rays as an indicator of the existence of a nearby cosmic tevatron, Astronomy and Astrophysics, 294, L41 (1995): Aharonian F. A., Atoyan, A. M., Voelk H. J.
2. TeV gamma rays from BL Lac objects due to synchrotron radiation of extremely high energy protons, New Astronomy, 5, 377 (2000): Aharonian F. A.
3. Proton-synchrotron radiation of large-scale jets in active galactic nuclei, Monthly Notices of the Royal Astronomical Society, 332, 215 (2002): Aharonian F. A.
4. Fermi Bubbles: Giant, Multibillion-Year-Old Reservoirs of Galactic Center Cosmic Rays, Physical Review Letters, 106, 101102 (2011): Crocker R. M., Aharonian F.
5. High-energy particle acceleration in the shell of a supernova remnant, Nature, 432, 75 (2004): Aharonian, F.A. et al.
6. Very high energy gamma rays from the direction of Sagittarius A*, Astronomy and Astrophysics, 425, L13 (2004), Aharonian, F.A. et al.
7. High-Energy Gamma Rays from the Massive Black Hole in the Galactic Center, The Astrophysical Journal, 619, 306 (2005): Aharonian F.A., Neronov A.
8. A low level of extragalactic background light as revealed by γ-rays from blazars, Nature, 440, 1018 (2006): Aharonian, F.A. et al.
9. Discovery of very-high-energy γ-rays from the Galactic Centre ridge, Nature 439, 695 (2006): Aharonian, F.A. et al.
10. The H.E.S.S. Survey of the Inner Galaxy in Very High Energy Gamma Rays, The Astrophysical Journal, 636, 777 (2006) : Aharonian, F A. et al.
In addition, he is the author of a well-known monograph on “Very high energy cosmic gamma radiation: a crucial window on the extreme Universe” (2004, World Scientific).
Apart from gamma ray astronomy he is also involved in major experiments in other domains such as KM3NeT (a high energy neutrino project in the Mediterranean Sea), ROTSE (a network of optical telescopes for the GRBs afterglow) and ASTRO-H (a major future JAXA-NASA-ESA X- ray mission).
Felix Aharonian is an outstanding astrophysicist whose many contributions to high-energy astrophysics and to the physics of cosmic accelerators are widely recognized. He is a rare example of a scientist that combines a deep understanding of both the theoretical and observational aspects, and whose basic ideas and intuitions have revolutionized the field of ground-based gamma ray astronomy and opened a new window for the observations of extremely energetic cosmic phenomena.