Quasars are the most enigmatic objects in astrophysics. Although being subject of intense study for thirty years, many fundamental aspects are still poorly understood. During the last decade several discoveries opened new lines of research. Quasars dominated by non-thermal emission were found to emit large fractions of their radiation in the gamma-ray regime. Rapid variability implies very small diameters as well as highly efficient acceleration, radiation, and cooling mechanisms. Variations on long time-scales are related to morphological changes of jets on parsec scales. In at least one source, those long-term variations may be periodic. All of these results modified our understanding of the fundamental and phenomenological aspects of Quasar models in a significant way. Quantitative understanding suffers from the limited sensitivity of current gamma-ray instrumentation, from the insufficient sampling that can be obtained with any isolated facility, and from the complexity of the physical processes involved. We suggest to coordinate the efforts of the empirical studies by the leading European teams, making use of the unprecedented opportunities that will be available in Europe in the forthcoming years thanks to a unique set of new gamma-ray instruments and facilities (INTEGRAL, AGILE, HESS, and MAGIC). A complementary effort shall be undertaken by leading European theoretical teams to combine the state of the art models of MHD plasma jets, particle acceleration and radiation processes. It is also proposed to explore the high temporal frequency regime of parameter space and to study the rare and very fast events that may ultimately directly trace particle acceleration in AGN. The low duty cyle requires large collaborative efforts, novel statistical treatments of huge data sets, and advances in numerical simulations of diffuse particle acceleration. It will greatly benefit from new instrumental developments from some of the team members.
Funding SchemeNET - Research network contracts
10025 Pino Torinese