Non-Thermal High Energy Emission from the Massive Colliding Wind Binary Systems

The collision of strong stellar winds in massive binary systems creates powerful shocks, which accelerates a small amount of particles to relativistic energies at the shock interface. This process is important in fundamental astrophysics in two ways; i) some particles may contribute to cosmic-rays observed around the Earth, whose origin is not well known, and ii) it provides a good laboratory for particle acceleration physics as the shock occurs steadily in a predictable environment. Some relativistic particles collide with stellar photons or ambient material near the binary system and emit non-thermal X-rays and/or γ-rays, which are good probes of particle acceleration. This emission had been searched for decades but not found convincingly without sensitive high-energy telescopes.The NuSTAR telescope observed the enigmatic supermassive binary system η Carinae multiple times after 2014 and found conclusive evidence of non-thermal emission from the star in the extremely hard X-ray band. This emission is prominent between 20-50 keV, below which thermal emission from shock colliding plasma dominates. It is relatively stable throughout the binary orbit, but it disappears near periastron when the wind colliding activity shuts off. This variation indicates that the non-thermal X-ray emission originates from the head-on wind-wind collision. The flat spectrum is consistent with inverse-Compton of stellar UV photons by accelerated electrons. The spectrum smoothly connects to a gamma-ray spectrum of a Fermi source detected around η Carinae, suggesting that acceleration occurs up to the GeV energy.This result provides the strongest evidence so far that particle acceleration occurs at the wind colliding shock of a massive colliding wind binary system. NuSTAR also found a similar extremely hard X-ray emission from the prototypical massive colliding wind binary system WR140, while it did not from the nearby massive binary system, γ² Velorum, which may be detected in γ-rays with Fermi. We introduce these observing results with NuSTAR and discuss the condition of particle acceleration at the wind-wind colliding shocks.