Gamma-ray absorption in the massive TeV Binary LS 5039

Abstract

The recent detections of binary systems emitting very high-energy (VHE) gamma-rays has introduced a new class of binaries: gamma-ray binaries. These binaries, composed of a compact object and a massive star, are distinguished by a non-thermal emission with a peak above 1 MeV in the spectral energy distribution (SED). For massive companions with effective temperature of 20 000{40 000 K, the stellar photon energies are in the range for pair-production with TeV photons. Therefore, the VHE emission can be e ciently absorbed. The absorption is modulated due to its dependence on the orbital geometry because of the angular dependence of the pair production cross-section and the energy threshold. We show that the total cross-section is energy dependent, and for T* = 39 000 K the gamma ray photons with hundreds of GeV are highly absorbed. We have written a Fortran code to perform numerical calculations of the opacity of the gamma ray absorption due to pair production for the gamma ray binary LS 5039, considering the point-like approximation for the companion. The results obtained for a system inclination i = 60º show the minimum absorption when the compact object is far from the star and close to the observer, and the maximum near the periastron. Our results are compared with Dubus (2006), where a complete treatment considering the massive star size is shown, finding minor differences. We also compare the available VHE lightcurve with a transmission lightcurve at 3 TeV and conclude that the absorption due to pair production is insufficient to explain the observations, implying that cascading or emission away from the compact object have to be invoked.