The modulation of the gamma-ray emission from the binary LS 5039

TL;DR

This paper models the phase-dependent gamma-ray emission from LS 5039, explaining observed variations through anisotropic inverse Compton scattering and predicting future observations, suggesting a pulsar wind nebula environment.

Contribution

It provides a detailed calculation of the phase-dependent gamma-ray spectrum of LS 5039, incorporating anisotropic scattering and pair production effects, and offers predictions for upcoming gamma-ray observations.

Findings

Good fit to HESS and EGRET data at most phases

Magnetic field at periastron estimated at 0.8 G

Predicted modulation in GLAST energy range

Abstract

Gamma-ray binaries, composed of a massive star and compact object, have been established as a new class of sources of very high energy (VHE) photons. The gamma-rays are produced by inverse Compton scattering of the stellar light by VHE electrons accelerated in the vicinity of the compact object. The VHE emission from LS 5039 displays an orbital modulation. The inverse Compton spectrum depends on the angle between the incoming and outgoing photon in the electron rest frame. Since the angle at which an observer sees the star and electrons changes with the orbit, a phase dependence of the spectrum is expected. The phase-dependent spectrum of LS 5039 is calculated, assuming a continuous injection of electrons. The shape of the electron distribution depends on the injected power-law and on the magnetic field intensity. Anisotropic scattering produces hard emission at inferior conjunction, when attenuation due to pair production of the VHE gamma-rays on star light is minimum. The computed lightcurve and spectra provide good fits to the HESS and EGRET observations, except at phases of maximum attenuation where pair cascade emission may be significant for HESS. Detailed predictions are made for a modulation in the GLAST energy range. The magnetic field intensity at periastron is 0.8+-0.2 G. Anisotropic inverse Compton scattering plays a major role in LS 5039. The derived magnetic field intensity, injection energy and slope suggest a rotation-powered pulsar wind nebula. Gamma-ray binaries are promising sources to study the environment of pulsars on small scales.