Revealing the particle acceleration in stellar wind shocks of massive binary stars

Binary systems comprising massive stars in relatively close orbits allow the presence of strong interaction between the two winds of the components. When the distance is close enough, an energetic shock is produced due to the collision of the two stellar winds, which can shine at radio wavelengths. These regions have proven to be extremely efficient environments to accelerate particles up to relativistic energies, involving higher mass, photon, and magnetic energy densities than their analogue processes in supernova remnants or interstellar bow-shocks. However, while a few of these systems can exhibit an exceptionally powerful and extreme region (like in the case of Eta Carinae or Apep), the dynamics on the average energy range of the population is poorly understood. And the limits on the physical conditions required for efficient particle acceleration to be produced remains unclear. The radio emission would arise from synchrotron emission produced along the region where the two winds collide, typically describing a bow-shaped structure that can be only resolved with very high-resolution instruments. Additionally, radio monitorings of the light-curve and spectral index along the orbit allow astronomers to fully characterize these systems, narrowing down the properties of the stellar winds and the shocked region. In particular, these studies allow us to identify binary - or higher multiplicity - systems that otherwise could not be identified. These searches are particularly sensitive to binaries with long orbital periods (of the order of tens of years), where other techniques like optical spectroscopy typically fail to identify the additional components. Large-scale studies of these systems thus allow us to reach a better understanding on how these massive stars (of typically O, B, or WR spectral types) behave in binary systems, understand which percentage of these stars are not single systems, and finally better describe the evolution of these massive systems, which are the ultimate progenitors of compact binary mergers producing significant amounts of gravitational waves. This is the project that the PANTERA-Stars (Particle Acceleration and Non-Thermal Emission of Radiation in Astrophysics - Stars) collaboration aims for