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

TL;DR

This paper reports the discovery of a 30-day orbital period in the supergiant X-ray binary SAX J1818.6-1703, revealing insights into its long-term behavior and supporting models that unify supergiant fast X-ray transients with classical supergiant X-ray binaries.

Contribution

The study identifies an unusually long orbital period and accretion phase in SAX J1818.6-1703, providing new constraints on the system's orbital parameters and accretion mechanisms.

Findings

Discovered a 30-day orbital period in SAX J1818.6-1703.

Detected an accretion phase lasting about 6 days.

Supported the model linking SFXTs and classical SGXBs through orbital parameters.

Abstract

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. Displaying short and bright flares and an unusually very-low quiescent level implying intensity dynamical range as large as 1e3-4, the source was classified as a Supergiant Fast X-ray Transient. 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. We analysed archival INTEGRAL data on SAX J1818.6-1703. We built short- and long-term light curves and performed timing analysis in order to study the temporal behaviour of SAX J1818.6-1703 on different time scales. 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 average fundamental parameters of supergiant stars). During the accretion phase, the source behaved like classical SGXBs. 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.