Research: an overview
High-energy particle astrophysics is a rapidly growing field of research that connects particle physics, cosmology and astrophysics. It aims to answer questions such as:
- What is the Universe made of?
- What is Dark Matter and how can we detect it?
- What are Cosmic Rays? Where do they come from? What accelerates them?
- Are gravitational waves detectable and what can they tell us?
As the field of particle astrophysics develops, it opens up new doors in astronomy. Nowadays light or, more generally, electromagnetic waves are not the only messengers from distant objects in the Universe, as we begin to observe very high-energy cosmic rays, neutrinos, or gravitational waves.
In Bochum we mainly focus on CRs interactions in the vicinity of candidate sources (like AGNs, SN remnants, GRBs. See below...) and we aim to explain the mechanisms that produce and accelerate particles to the extreme energies observed in the CRs spectrum.
The spectrum of cosmic rays
The observed spectrum of cosmic rays shows that particles can be accelerated up to ~1021 eV. Its shape is very well described by a broken power law. The so-called "knee" can be identified by a change in the slope around ~1015 eV) whereas energies above the "ankle" (around ~3×1018 eV) are associated to extragalactic Ultra High-Energy Cosmic Rays (UHECR).
Sources of CRs
What mechanisms are responsible for particle acceleration? And what are the sources of CRs in the highest energy tail of the spectrum? Particles are believed to be accelerated at magnetic inhomogenities either in magnetic clouds (2nd order Fermi acceleration) or at shock fronts (1st order Fermi acceleration).
Simple spectral power laws are predicted by these models and the shape of the CR spectrum is therefore accounted for.
Non relativistic shocks are generated for example in SN explosions that are commonly believed to accelerate particles up to the knee of the spectrum (~1015 eV).
The mechanism responsible for particles acceleration in the "knee-ankle" energy range is still not known. One model assumes that particles come from SN that explode into their own winds. Other possibilities include binary systems or pulsars in the Galaxy.
AGNs and GRBs are instead associated with relativistic shocks that have the potential to accelerate particles up to the highest energies (~1021 eV).
Multimessenger astrophysics: gamma-rays and neutrinos
By comparing observations from different messengers (photons and neutrinos) and at various energies, we aim to learn more about high-energy cosmic phenomena in the Universe and the violent processes that give rise to them. Astrophysical objects like AGNs, GRBs and SN remnants demand an interdisciplinary, multi-wavelength and multi-messenger approach for their comprehension. Candidate sources of UHECRs are likely to emit energetic neutrinos and gamma rays that may be observed by present and future observatories.
Experiments we are involved in
We take part in the following experiments:
- IceCube: it's the first kilometer scale neutrino telescope, under construction at the South Pole (scheduled for completion in 2011). It aims at detecting Cherenkov light emitted by charged particles (muons, electrons and tauons) produced when very energetic neutrinos (>0.1TeV) interact in the vicinity of the instrumented volume. It is expected to open a completely unexplored window in astronomy and to shed new light on the physics of high-energy astronomical sources.
- HESS: a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma-rays in the 100 GeV to 100 TeV energy range. H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality. The first of the four telescopes of the H.E.S.S. project went into operation in Summer 2002; all four were operational in December 2003, and were officially inaugurated on September 28, 2004.
- Cherenkov Telescopoe Array (CTA): the future system of Cherenkov telescopes, covering an energy range between several tens of GeV up to above 10 TeV. A large field of view up to 10 degrees will make a mapping of high-energy photon sources possible and a good resolution significantly smaller than 0.1 degree will enable detailed studies of extended sources.