One-dimensional pair cascade emission in gamma-ray binaries

Abstract

In gamma-ray binaries such as LS 5039 a large number of electron-positron pairs are created by the annihilation of primary very high energy (VHE) gamma-rays with photons from the massive star. The radiation from these particles contributes to the total high energy gamma-ray flux and can initiate a cascade, decreasing the effective gamma-ray opacity in the system. The aim of this paper is to model the cascade emission and investigate if it can account for the VHE gamma-ray flux detected by HESS from LS 5039 at superior conjunction, where the primary gamma-rays are expected to be fully absorbed. A one-dimensional cascade develops along the line-of-sight if the deflections of pairs induced by the surrounding magnetic field can be neglected. A semi-analytical approach can then be adopted, including the effects of the anisotropic seed radiation field from the companion star. Cascade equations are numerically solved, yielding the density of pairs and photons. In LS 5039, the cascade contribution to the total flux is large and anti-correlated with the orbital modulation of the primary VHE gamma-rays. The cascade emission dominates close to superior conjunction but is too strong to be compatible with HESS measurements. Positron annihilation does not produce detectable 511 keV emission. This study provides an upper limit to cascade emission in gamma-ray binaries at orbital phases where absorption is strong. The pairs are likely to be deflected or isotropized by the ambient magnetic field, which will reduce the resulting emission seen by the observer. Cascade emission remains a viable explanation for the detected gamma-rays at superior conjunction in LS 5039.

8 pages, 7 figures, 1 table, accepted for publication in Astronomy and Astrophysics

References in corpus (11)

• On the formation of TeV radiation in LS 5039
• The modulation of the gamma-ray emission from the binary LS 5039
• Propagation of very high energy gamma-rays inside massive binaries LS 5039 and LSI +61 303
• Microquasars as sources of positron annihilation radiation
• The magnetic field and the location of the TeV emitter in Cygnus X-1 and LS 5039
• Leptonic secondary emission in a hadronic microquasar model
• Pulsar wind zone processes in LS 5039
• Gamma-rays from cascades in close massive binaries containing energetic pulsars
• Non-thermal emission from secondary pairs in close TeV binary systems
• Cascading on extragalactic background light
• Spectral signature of a free pulsar wind in the gamma-ray binaries LS 5039 and LSI +61$\degr$303

Cited by in corpus (9)

• Gamma-ray binaries and related systems
• Thermal X-ray Emission from the Shocked Stellar Wind of Pulsar Gamma-ray Binaries
• Modeling the three-dimensional pair cascade in binaries. Application to LS 5039
• High energy gamma-ray emission from compact galactic sources in the context of observations with the next generation Cherenkov Telescope Arrays
• On the possibility of sub-TeV Gamma-ray emission from Cyg X-3
• Modulation Mechanism of TeV, GeV, and X-ray Emission in LS5039
• Population III microquasars
• Expected gamma-ray emission from x-ray binaries
• High energy emission from galactic jets