GeV-TeV $\vec{\gamma}$-ray light curves expected in the IC e$^\pm$ pair cascade model for massive binaries: application to LS 5039

Abstract

TeV gamma-ray emission from two massive binaries of the microquasar type, LS 5039 and LS I +61$^{\rm o}$ 303, show clear variability with their orbital periods. Our purpose is to calculate the GeV and TeV $\gamma$-ray light curves from the massive binary LS 5039 which are expected in the specific Inverse Compton $e^\pm$ pair cascade model. This model successfully predicted the basic features of the high energy $\gamma$-ray emission from LS 5039 and LS I +61 303. In the calculations we apply the Monte Carlo code which follows the IC $e^\pm$ pair cascade in the anisotropic radiation of the massive star. The $\gamma$-ray light curves and spectra are obtained for different parameters of the acceleration scenario and the inclination angles of the binary system. It is found that the GeV and TeV $\gamma$-ray light curves should be anti-correlated. This feature can be tested in the near future by the simultaneous observations of LS 5039 with the AGILE and GLAST telescopes in GeV energies and the Cherenkov telescopes in the TeV energies. Considered model also predicts a broad maximum in the TeV $\gamma$-ray light curve between the phases $\sim 0.4-0.8$ consistently with the observations of LS 5039 by the HESS telescopes. Moreover, we predict additional dip in the TeV light curve for large inclination angles $\sim 60^{\rm o}$. This feature could serve as a diagnostic for independent measuring of the inclination angle of this binary system indicating also on the presence of a neutron star in LS 5039.