One of the major scientific goal for Athena, the concept for the next-generation European X-ray telescope recently selected from ESA as second Large Mission, is to determine how baryons assemble and dynamically evolve into galaxy clusters.
The outskirts of galaxy clusters are the place where the connection between the highest peaks in the comic matter density and the large-scale structure is established. Until recently, more than about 70% of the cluster's volume, where most of the mass accretion processes onto the main halo is occurring, has remained essentially unexplored. In these regions, the distribution of the hot gas (traced with both X-ray and SZ signal) is expected to be clumpy and asymmetric, with non-negligible effects from non-thermal processes (like, e.g., turbulence, bulk motions, magnetic fields, particle acceleration) that can be also investigated through radio observations. The overall dark matter distribution has started to be mapped through the weak lensing signal and the galaxy distribution, by which also the evolution of the properties of the accreting galaxies as the environment changes (from low to high density regions) can be traced.
To keep the momentum high, and the discussion lively, on the physics and the problematics concerning the peripheries of the galaxy clusters, the objectives of this EWASS Symposium are the following: 1) Presentation of the state-of-art researches on the clusters' peripheries, both from multi-band (from optical to radio, SZ, and X-ray) observations, and numerical simulations, and theoretical models; 2) Deep discussion on the present limitations affecting the analysis of these regions, and how we can overcome these problems with the next generation of instruments.
Gamma-ray bursts (GRBs), are cosmic flashes of gamma-rays, and are some of the most energetic events ever detected, with luminosities exceeding 1050 erg/sec. GRBs are thus powerful beacons of the early Universe, and extreme physical laboratories. They are intrinsically linked to the lifecycle of stars and galaxies, therefore GRBs can provide complementary insights into galactic and stellar evolution. Multi-wavelength observations of these enigmatic events allow us to go deeper into their underlying physics, but 40 years after their discovery their origin remains elusive.
A great amount of theoretical work has been invested in order to understand what is the central engine and the emission mechanism of GRBs. It has been proposed that GRBs fall into two subcategories, short- and long-duration. However, recently accumulated data suggests that the distinction based on burst duration may not be as strong as originally thought, and all GRBs may be energized by the same type of central engine. Signatures of the central engine may be revealed with GRB observations, in particular in the X-rays, where a treasure load of data from the BAT and XRT instruments aboard the Swift satellite are available from the last 10 years.
Most recently, the question of the central engine has been put aside, while research focuses on the emitting region, the emission mechanisms and the effort to understand all the characteristics of the light curves and the spectra of the bursts. While this has significantly increased our understanding of GRBs, new features and phenomena are still being discovered, such as ultralong duration GRBs, of which one was recently linked to a super-luminous supernova. Furthermore, GRBs are proving to be useful probes of cosmic star formation history and cosmology, and much research is being performed to fully exploit their advantages.
This is an exciting time in the GRB field. The already sizeable GRB community in Europe and worldwide has grown rapidly in the last decade. Various space missions and ground observations are providing a wealth of new data from GeV to radio energies. These are complemented by recent theoretical and numerical advancements in our understanding of the physical mechanisms behind GRBs. In the modern era of multi messenger astronomy, the study of GRBs is likely to intensify as we try to piece together information from the full electromagnetic spectrum, gravitational waves and neutrinos.