Diffuse synchrotron emission in clusters of galaxies has been known since the seventies but many aspects of its origin are still puzzling. Clusters are the latest and largest objects to form in our Universe during the most violent phase of structure formation. While we know that some of the energy released during the formation process is channeled into particle acceleration and turbulences, we are still unable to grasp the details of the related non- thermal emission. Thanks to the radio data provided by new instruments, such as LOFAR, we expect to understand these phenomena in detail. However, recent observations have challenged our “classical view”, revealing details on the non-thermal component that cannot be explained by the previously thought mechanisms. Therefore, there are several open questions that this field needs to tackle to bring our understanding to a deeper level:
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What is the origin of the radio-emitting electrons and magnetic fields filling megaparsec volumes?
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As diffuse shock acceleration has been challenged by recent observations, what is then the main mechanism that accelerate particles in clusters? What are the important plasma process in the shock formation in high-beta plasma like the intra- cluster medium?
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Where are the seed electrons for turbulent re-acceleration coming from? What are the main characteristics of the turbulence field in clusters?
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What is the connection between radio jets from active galactic nuclei and fossil electrons that provide seed electrons to shock acceleration and turbulent acceleration? What are the nature and origin of radio ghosts and radio phoenix?
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Cosmic-ray protons must contribute to the relativistic particle population in clusters, what is then their exact role?
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Is high-energy non-thermal emission in X-rays, gamma rays or neutrinos detectable? What can we learn from current non-detections?
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What can we learn from observations of the thermal part of intra-cluster medium in X-rays and with Sunyaev-Zel’dovich effect? And from optical observations?
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What is the connection between the different kinds of synchrotron diffuse emission observed in clusters (halos, mini-halos, relics)? What is the connection of these with the dynamical state and evolutionary stage of clusters?
The main goal of this workshop is to bring together the community working on non-thermal emission in clusters from different angles – theory, simulations and observations – in order to unveil the origin of this emission and its connection with the structure formation process. With the increasing amount of data available nowadays and that will soon come online thanks to LOFAR and other SKA-precursors, it is timely to understand how to gain a deeper insight.
This workshop will be considered successful if, at the end of the week, we will have answers to the above questions, or a precise shared strategy on how to reach those answers through common efforts from the different angles. We will ask ourselves what is needed by theorists, simulators and observers at a practical level to answer a particular questions, and encourage the formation of new collaborations to reach these goals in the next few years.
