Simulating high energy orbital light curves from massive star binary systems

Massive star binaries are stellar systems consisting of high-mass luminous stars which possess pronounced winds. Such systems have recently been detected also in the gamma-ray band, up to TeV energies. The proposed project aims at understanding the physics of high energy emission of massive star binary systems with a particular emphasis on the role of relativistic hadrons in such systems. Key is extracting information encoded in the orbitally varying light curves and phase resolved spectra observed by today`s synergy of high performance ground- and space-based high photon energy instruments and experiments. For such purpose the development of a fully time-dependent particle propagation and emission code is planned that will allow to model the high energy light curves and spectra with high precision, and extends the so far employed approximation of a steady-state situation. As a result meaningful constraints on the relativistic particle content and particle acceleration scenarios in massive star environments is expected, as well as important implications for the role of massive star binaries as sources of galactic cosmic rays and their expected neutrino emission.

Massive star binaries are stellar systems consisting of high-mass luminous stars which possess pronounced winds. When the winds collide they form a collision region which is delimited by two shock fronts where wind particles are expected to be accelerated to very high (relativistic) energies. Some of these systems have been detected not only from the radio to the X-ray energy regime but also extended into the ?-ray band. This project contributed to a deeper understanding of the physics of the high-energy emission from massive star binary systems with a particular emphasis on the role of relativistic protons (cosmic rays). For this purpose we developed two simulation programs. Firstly, we combined a description of the dynamics of the magnetized wind fluid with a kinetic description for the propagation and emission of relativistic particles. Secondly, to focus on cosmic ray protons, we followed the acceleration process of individual such particles within the dynamically evolved magnetized wind fluid by using a modified Monte Carlo method. Using these tools we found that colliding wind binary systems are liable to show a significant variation in their high-energy spectra in the course of a strongly elliptical orbit. Spectra can exhibit multiple emission components for phases of low separation of the stars, contrasted by longer periods of one-component spectra for phases of larger stellar separation. The detection of ?-rays from very close such binary systems favors the involvement of cosmic ray protons in the production of these photons. The acceleration process of protons to relativistic energies in these environments has to take into account the built-up cosmic ray pressure on the dynamics of the shock fronts. Acceleration and injection efficiencies are likely to vary along the shock fronts. Our results will likely impact strategies for observing colliding wind binary systems with the currently built Cherenkov Telescope Array.