A model for the two component gamma-ray spectra observed from the gamma-ray binaries

TL;DR

This paper proposes a model explaining the two-component gamma-ray spectra observed in binary systems LS 5039 and LS I +61 303, attributing them to two electron populations accelerated by double shock structures caused by stellar and pulsar wind interactions.

Contribution

It introduces a novel model linking two gamma-ray spectral components to electron populations accelerated by double shocks in binary systems.

Findings

The model reproduces observed gamma-ray spectra features.

Electron populations differ in maximum energy due to shock properties.

The model aligns with observations at different orbital phases.

Abstract

Observations of two well known binary systems (LS 5039 and LS I +61 303) with the satellite and Cherenkov telescopes revealed the broad band $\gamma$-ray spectra which seems to show two components, the first at GeV energies, showing exponential cut-off at a few GeV, and the second at TeV energies which does not fit well to the extrapolation of spectrum from the GeV energy range. We propose that such two component spectrum is produced by two populations of electrons which appear within the binary system as a result of acceleration on a double shock structure separated by a contact discontinuity. Such structure is created within the binary system as a result of the interaction of the pulsar and massive star winds. The shocks from the side of the pulsar and the massive star have different proprieties which allow acceleration of electrons to different maximum energies. These two populations of electrons produce two component $\gamma$-ray spectra caused by the Inverse Compton (IC) scattering of stellar radiation. The example calculations, performed in terms of the anisotropic IC $e^\pm$ pair cascade model, for the location of the pulsar at the periastron and apastron passages confirm the high energy emission features observed from LS I +61 303.