The production of orbitally modulated UHE photons in LS 5039

Abstract

Gamma-ray binaries present emission that is variable and can reach UHE. The processes behind the acceleration of the particles that produce this very energetic radiation are yet to be understood. We probe the properties of the particle accelerator and the UHE photon emitter in the gamma-ray binary LS 5039. From the properties of the binary system and the UHE radiation detected by HAWC, we used analytical tools to investigate how these properties constrain the emission and acceleration regions, namely the role of synchrotron losses, particle confinement, and the accelerated particle spectrum, and propose an acceleration scenario that can relax the derived constraints. The modest target densities for hadronic processes and the overall gamma-ray orbital variability favor IC scattering of ultraviolet photons from the massive companion star by highly relativistic e-. The acceleration of the highest energy e- implies a constraint on synchrotron cooling in the accelerator, which can set an upper limit on its magnetic field. Moreover, the detected variability requires very strong particle confinement in both the acceleration and emitter, which sets a lower limit on their magnetic fields that is barely consistent with the synchrotron cooling constraint from acceleration. Synchrotron losses may be higher in the emitter if it is separated from the accelerator, but this requires a very hard particle injection spectrum. An accelerator based on an ultrarelativistic outflow can alleviate these requirements. A scenario for LS 5039 as that proposed by Derishev et al., in which an ultrarelativistic magnetized outflow accelerates leptons injected within the outflow by gammagamma absorption, provides a viable mechanism to accelerate very energetic e-. This mechanism relaxes the acceleration and confinement requirements by reducing the impact of synchrotron cooling, and can generate the required e- spectrum.