Observation of orbital modulation of the VHE emission from the binary system LS 5039 with H.E.S.S.

The binary system LS 5039 was serendipitously with the High Energy Stereoscopic system (H.E.S.S.) during the scan of the inner galactic plane in 2004. Deeper observations were carried out in 2005, and brought clear evidence for TeV emission perodicity. This is the highest energy periodic source known so far. The observed flux modulation is attributed to a modulated absorption of the VHE gammaray emission of the compact object through pair creation on the stellar photosphere. Spectral modulation is also observed in this system; this might have several origins such as modulation of particle acceleration or reprocessing of high energy photons towards lower energy through cascading. We will present detailed studies of the source variability (flux and spectral shape), the timescales compared to other wavelengths, and briefly review the implications for existing emission models.