The non-thermal Universe
The Universe is filled with highly relativistic particles which acquire their energies in non-thermal processes in the most violent astrophysical environments. Cosmic particle accelerators impart huge energies on charged particles, even beyond 1020 eV as seen in cosmic rays, which in turn produce high-energy gamma rays and neutrinos through their interactions with the interstellar medium. The properties of the cosmic accelerators and the details of the acceleration mechanisms are still largely unknown, and pose some of the most fundamental, unsolved problems in modern astrophysics.
As the cosmic accelerators can be equally sources of energetic cosmic rays, gamma rays, and neutrinos, the study of all three messengers over a range of energies is a powerful approach to study complementary aspects of the sources. Photons and neutrinos travel in straight lines, thus allowing the identification and imaging of source regions. High-energy photons can be absorbed close to their sources and on their way to us, but once they reach Earth they are easily detectable via the air showers they create in the atmosphere. Neutrinos can reach us from far-off objects, and even from the inner, shrouded parts of a source region, but their detection rate is small due to their very low interaction cross sections. Cosmic rays can readily be detected via air showers, but being charged they are deflected in magnetic fields, and are not pointing back to their sources. This greatly obscures their origin. However, for cosmic rays at the highest energies the magnetic deflection (at least for protons) is expected to become small so that pointing back to sources and 'charged particle astronomy' is possible. The energies of all these astroparticles (cosmic rays, gamma rays and neutrinos) extend to scales well beyond those of terrestrial accelerators, offering us a means to study particle interactions in an otherwise unattainable energy region.
Our broad experimental programme with cosmic rays, high-energy neutrinos and gamma rays as messengers from the non-thermal Universe will be detailed in the following. The experimental activities are flanked by an intense theoretical research linking the various results achieved with cosmic messengers and advancing our understanding of the non-thermal Universe.
Since the discovery of cosmic rays about 100 years ago the quest to identify and understand the sources of these high-energy particles has been one of the driving forces in astroparticle physics. The energy spectrum of cosmic rays exhibits a number of characteristic features such as the knee at 1015.5 eV, the ankle at 1018.5 eV, and a suppression above 1019.7 eV. The origin of these prominent features is not yet understood and subject of intense research. They could be related to different galactic and extragalactic source populations and also reflect the propagation of charged particles in the cosmic environment of magnetic fields and background radiations. The existence of ultra-high energy cosmic rays of 1020 eV, a factor of more than 107 higher than the reach of current terrestrial accelerators, challenges conventional theories of particle acceleration in cosmic environments.
In cosmic ray measurements, the key observables are the energy dependence of the particle flux, mass composition, the arrival direction distribution, and the fraction of gamma rays and neutrinos in the observed particle flux. While a good understanding of the all-particle flux and the overall arrival direction distribution has been reached in recent years, the existing data on the cosmic ray composition are insufficient to discriminate among different model scenarios. No composition information is available for the energy range of the flux suppression and composition data at lower energy are subject of large systematic uncertainties. Therefore the most important challenge for the next years is the reliable determination of the cosmic ray composition up to the upper end of the spectrum. This will require both the measurement of composition-sensitive observables of ultra-high energy showers and an improvement of the reliability of air shower simulations.
From the technical point of view there are two central challenges:
- a reduction of the theoretical and experimental systematic uncertainties of the composition measurements is needed to obtain consistent and discriminating constraints on different model scenarios of cosmic ray acceleration and propagation, and
- efficient and economic detection techniques must be developed to enable us to collect higher statistics of cosmic rays at the upper end of the cosmic ray spectrum, and to improve the composition-sensitivity of shower measurements.
High-energy neutrino astrophysics
High-energy neutrinos can reach us from cosmological distances and from regions in the Universe that are opaque to electromagnetic radiation at all wavelengths. Their measurement therefore provides unique information about acceleration and particle interaction processes in astrophysical environments. In particular, neutrinos are a diagnostic of the nature of the particles accelerated in these environments, as they are only produced in interactions of high-energy protons and heavier nuclei. Neutrino telescopes built for the search for astrophysical neutrino sources can also address key questions in particle physics, such as the measurements of fundamental properties of neutrinos, like their mixing angle, mass difference, and potentially even their mass hierarchy. Neutrino telescopes are also used for searches for signatures from beyond-standard-model particles like sterile neutrinos, magnetic monopoles, and Dark Matter particles.
For the near future there are three particular challenges in neutrino astrophysics:
- to solidify the evidence for the astrophysical origin of the high-energy neutrinos observed in the first batch of data and, possibly, to identify neutrino sources in the sky,
- to lower the energy threshold of IceCube to a few GeV, while maintaining an energy resolution that allows oscillations studies, and tackling the mass hierarchy problem. The density of the instrumentation in the ice and its quality have to be increased, and the ice and the event reconstruction have to be understood sufficiently well to analyse the complex oscillation patterns of atmospheric neutrinos in the Earth. And
- to find economic ways to scale the existing telescope to about 10 km3 volume. While the IceCube detector at the South Pole saw the first ever evidence for an astrophysical neutrino signal, the currently instrumented target volume is likely too small to do high-statistics precision measurements of the properties of this astrophysical signal and to decompose it into contributions from individual sources. An evolution of the state-of-the-art technologies for detectors and their deployment or new approaches (e.g. radio/acoustic neutrino detection) are necessary to reach such volumes.
High-energy gamma rays are produced in the most energetic processes in the Universe, in objects such as exploding stars and near supermassive black holes. The primary scientific drivers in gamma-ray astronomy are
- to constrain models on the formation and evolution of stars and galaxies,
- to understand the acceleration and propagation of cosmic rays in a wide range of different environments, and
- to study the formation of relativistic plasma outflows, so-called jets, in the vicinity of stellar-mass and supermassive black holes and thereby understand how black holes grow.
Gamma-ray observations are also powerful to test predictions of New Physics beyond the standard model (i.e. the violation of Lorentz Invariance), and might be the key to unambiguously identify the nature of Dark Matter, see more here.
The experimental techniques of gamma-ray astronomy are well established. Over the past decade, arrays of imaging Cherenkov telescopes like H.E.S.S., MAGIC and VERITAS have greatly increased the sensitivity to gamma rays in the range from 30 GeV to 100 TeV. The ground-based arrays are complemented by the Fermi-LAT space instrument, which is observing the whole sky at energies from 50 MeV to more than 300 GeV. The Cherenkov Telescope Array (CTA) will be the major new ground-based gamma-ray observatory to be built in 2015 to 2019. The improvements of CTA are a factor of ten in sensitivity compared to the current instruments and unprecedented precision on angular and energy scales. CTA is a global endeavour and will serve a world-wide physics community as an open observatory. DESY groups are prominent players in CTA and are involved in the gamma-ray projects H.E.S.S., MAGIC, VERITAS and Fermi-LAT.
The major challenge for the PoF 3 period is to build and operate CTA. The technical challenges for CTA are to construct in collaboration with industry a complex astronomical instrument at a remote location. The safe and reliable operation of the large number of telescopes is ensured through a sophisticated array control system. The telescope calibration has to be much more precise and stable, as many measurements with CTA will be limited by systematic uncertainties (and not by the statistical uncertainties, as for the current instruments). The open access to CTA data will require that software products, data infrastructure and user support are made available for the world-wide physics and astronomy community on a scale which is completely new for the gamma-ray community.
The operation of the existing gamma-ray instruments will be continued at least until CTA is partially operational in 2017. Especially the physics output of HESS II (four telescopes of 12 and one with 28 m diameter) will be maximised to capitalise on the existing large observatories at X-rays (Chandra, XMM) and low-energy gamma rays (INTEGRAL, Fermi).
The interpretation of the high-quality observational gamma-ray data requires matching observations at larger wavelengths and the availability of reliable theoretical models. The multiwavelength and multi-messenger aspects and collaboration with astronomers and astroparticle physicists to better understand cosmic particle accelerators will become ever more important.