Discovery of an eccentric 30 day period in the supergiant X-ray binary SAX J1818.6-1703 with INTEGRAL

Context. SAX J1818.6-1703 is a flaring transient X-ray source serendipitously discovered by BeppoSAX in 1998 during an observation of the Galactic centre. The source was identified as a high-mass X-ray binary with an OB supergiant companion (SGXB). Displaying short and bright flares and an unusually very low quiescent level implying an intensity dynamical range as large as 10(3-4), the source was classified as a supergiant fast X-ray transient (SFXT). Aims. The mechanism triggering the different temporal behaviour observed between the classical SGXBs and the recently discovered class of SFXTs is still debated. The discovery of long orbits (> 15 d) should help to discriminate between emission models and bring constraints. Methods. We analysed archival INTEGRAL data on SAX J1818.6-1703. We built short- and long-term light curves and performed a timing analysis in order to study the temporal behaviour of SAX J1818.6-1703 on different time scales. Results. INTEGRAL revealed an unusually long orbital period of 30.0 +/- 0.2 d and an elapsed accretion phase of similar to 6 d in the transient SGXB SAX J1818.6-1703. This implies an elliptical orbit and constraints the possible supergiant spectral type between B0.5-1I with eccentricities e similar to 0.3-0.4 (for the average fundamental parameters of supergiant stars). During the accretion phase, the source behaved like a classical SGXB. The huge variations of the observed X-ray flux can be explained through accretion of macro-clumps formed within the stellar wind. Our analysis strengthens the model which predicts that SFXTs behave as SGXBs but with different orbital parameters, thus different temporal behaviour.