Discovery of the 3.9 days orbital periodicity in Very High Energy γ -rays

Recent observations of the binary system LS 5039 with the High Energy Stereoscopic System (H.E.S.S.) re- vealed that its Very High Energy (VHE) γ -ray emission is modulated at the 3.9 days orbital period of the system. The bulk of the emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the com- pact object. The flux modulation provides the first indication of γ -ray absorption by pair production on the intense stel- lar photon field. This implies that the production region size must be not significantly greater than the gamma-gamma photosphere size (∼1 AU), thus excluding the large scale collimated outflows or jets (extending out to ∼1000 AU). A hardening of the spectrum is also observed at the same epoch between 0.2 and a few TeV which is unexpected un- der a pure absorption scenario and could rather arise from variation with phase in the maximum electron energy and/or the dominant VHE γ -ray production mechanism. This first- time observation of modulated γ -ray emission allows pre- cise tests of the acceleration and emission models in binary systems.