The modulation of the gamma-ray emission from the binary LS 5039
Laboratoire d'Astrophysique de Grenoble, UMR 5571 CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble, France
Accepted: 2 October 2007
Context.Gamma-ray binaries have been established as a new class of sources of very high energy (VHE, >100 GeV) photons. These binaries are composed of a massive star and a compact object. The gamma rays are probably 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.
Aims.The inverse Compton spectrum depends on the angle between the incoming and outgoing photons in the rest frame of the electron. Since the angle at which an observer sees the star and electrons changes with the orbit, a phase dependence of the spectrum is expected.
Methods.A procedure for computing anisotropic inverse Compton emission is explained and applied to the case of LS 5039. The spectrum is calculated assuming the continuous injection of electrons close to the compact object: the shape of the steady-state distribution depends on the injected power law and on the magnetic-field's intensity.
Results.Compared to the isotropic approximation, anisotropic scattering produces harder and fainter emission at inferior conjunction, crucially at a time when the attenuation due to pair production of the VHE gamma-rays on star light is minimum. The computed light curve and spectra are very 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 G.
Conclusions.The anisotropy in inverse Compton scattering plays a major role in LS 5039. A simple model reproduces the observations, constraining the magnetic field intensity and injection spectrum. The comparison with observations, the derived magnetic field intensity, injection energy, and slope together suggest emission from a rotation-powered pulsar wind nebula. These results confirm gamma-ray binaries as promising sources for studying the environment of pulsars on small scales.
Key words: radiation mechanisms: non-thermal / stars: individual (LS 5039) / gamma rays: theory / X-rays: binaries
© ESO, 2008