GeV-TeV -ray light curves expected in the IC e pair cascade model for massive binaries: application to LS 5039

Department of Experimental Physics, University of Łódź, 90-236 Łódź, ul. Pomorska 149/153, Poland e-mail: bednar@fizwe4.fic.uni.lodz.pl

Received: 18 August 2006
Accepted: 6 November 2006

Abstract

Context. TeV gamma-ray emission from two massive binaries of the microquasar type, LS 5039 and LS I +61^o 303, show clear variability with their orbital periods.

Aims. Our purpose is to calculate the GeV and TeV γ-ray light curves from the massive binary LS 5039, which are expected in the specific inverse Compton e pair cascade model. This model has successfully predicted the basic features of the high energy γ-ray emission from LS 5039 and LS I +61^o 303.

Methods. In the calculations we apply the Monte Carlo code, which follows the IC e pair cascade in the anisotropic radiation of the massive star.

Results. The γ-ray light curves and spectra are obtained for different parameters of the acceleration scenario and the inclination angles of the binary system. We find that the GeV and TeV γ-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. This model also predicts a broad maximum in the TeV γ-ray light curve between the phases ~0.4–0.8 consistently with the observations of LS 5039 by the HESS telescopes. Moreover, we predict an additional dip in the TeV light curve for large inclination angles ~60^o. This feature could serve as a diagnostic for independently measuring the inclination angle of this binary system indicating also the presence of a neutron star in LS 5039.