The enigmatic gamma-ray binaries
Gamma-rays are a high-energy form of electromagnetic radiation, known to be emitted by stars as they collapse into black holes. Observations by the HESS and MAGIC collaborations have established hundreds of sources of such light with energies of several megaelectronvolts (MeV). The GAMMARAYBINARIES (Exploring the gamma-ray sky: Binaries, microquasars and their impact on understanding particle acceleration, relativistic winds and accretion/ejection phenomena in cosmic sources) project has shed light on how high-energy emission might arise in these systems. Non-thermal processes dominate at such high energies such as relativistic ejections from compact objects. Researchers have identified a few of these gamma-ray sources with binaries composed of a massive star and a compact companion – a neutron star or black hole. High-intensity gamma rays are also emitted by binary star systems where each star generates such a strong stellar wind that they collide with the wind of the other star. A distinguishing feature of gamma-ray binaries is that emission above 1 MeV dominates their spectra, concealing the black body-like component from the companion star. Another characteristic feature is that they all have variable gamma-ray emission, sometimes modulated by the orbital period. Researchers combined hydrodynamical simulations of relativistic outflows with emission models to explore these gamma-ray modulations. Theoretical work that followed the detection of gamma-ray binaries revealed processes similar to those at work in other types of astrophysical sources. However, this effort to explain gamma-ray emission from binaries strained existing theories of high-energy astrophysical processes, by testing them on scales and in conditions that were not foreseen. This is why new observations were most valuable. GAMMARAYBINARIES revealed a variety of binaries emitting gamma-rays and proved that particle acceleration is much more widespread than would have been thought a decade ago.
Keywords
Gamma-ray, particle acceleration, black hole, GAMMARAYBINARIES, non-thermal processes
