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 103-4, 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 ~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 ~ 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.