  Recent observations of the binary system LS5039 with the High Energy
Stereoscopic System (H.E.S.S.) revealed that its Very High Energy (VHE)
gamma-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 compact object. The flux
modulation provides the first indication of gamma-ray absorption by pair
production on the intense stellar 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 under a pure
absorption scenario and could rather arise from variation with phase in the
maximum electron energy and/or the dominant VHE gamma-ray production mechanism.
This first-time observation of modulated gamma-ray emission allows precise
tests of the acceleration and emission models in binary systems.
