Final Report Summary - GAMMARAYBINARIES (Exploring the gamma-ray sky: binaries, microquasars and their impact on understanding particle acceleration, relativistic winds and accretion/ejection phenomena in cosmic sources.) Light extends beyond the visible spectrum: at low energies to infrared and radio waves, at high energies to ultraviolet light, X-rays and, finally, gamma rays. At the highest energies, the light that we detect usually originated in the vicinity of neutron stars and black holes. High-energy gamma-ray emission is the result of poorly understood processes that challenge our intuitions built upon everyday phenomena. The latest generation of gamma-ray observatories have renewed our vision of the high-energy sky, offering exciting opportunities to understand how particles are accelerated in various environments.The project focused on non-thermal emission from compact binary sources, in which a black hole or neutron star orbits around a companion star. These systems typically show highly relativistic outflows and the goal is to understand why, when and how they form using the newly available information from the gamma-ray part of the spectrum.We found that gamma-ray emission occurs in a wide variety of binary systems, composed of pulsars, white dwarfs or black holes. Three of these discoveries were showcased in the journal Science. The observed gamma-ray emission is often variable on the orbital period of the binary. We explored how gamma-ray modulations constrain the size and location of the high-energy particles in the binary. This effort culminates in the development of a suite of tools to combine hydrodynamical simulations of relativistic outflows with emission models. The processes are similar to those at work in other types of astrophysical sources (supernova remnants, pulsar wind nebulae, active galactic nuclei), but binaries strain our theories by testing them on scales and in environments that were generally not foreseen, requiring much more highly efficient particle acceleration than typically assumed.
