Gamma-ray absorption in the binary system LS I +61 303

Abstract

LS I +61 303 is a gamma-ray binary, composed of a compact object of unknow nature and a massive star, which has been detected in the range of very high energy (VHE) gamma-rays. These gamma-rays can interact with the optical photons coming from the star. In each interaction, they are absorbed through pair creation [gamma gamma gives] e-e+. This absorption depends strongly on the possible geometries of the system and on the energy of the emitted gamma-ray photon. Therefore, we can study the modulation of the transmission with the orbital phase and with the photon energy. In this work we have made a Fortran code to calculate the transmission of LS I +61 303 system assuming that VHE gamma-rays are produced at the position of the compact object and that we have a pointlike massive star. We have done this study for two inclinations, one corresponding to a neutron star compact object and the other to a black hole. We have observed that the maximum absorption occurs slightly before periastron and it is more intense for large inclinations. The energy range with more absorption is between 100 GeV and 10 TeV. We have compared our results with Dubus (2006) and we have observed a signifcant difference on the absorption for energies above 10 TeV. We have studied the orbital phase of the maximum absorption as a function of the inclination. We have concluded that, if the observed light curve only depended on the transmission, we could know, through this dependence, the nature of the compact object