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Supergiant fast X-ray transients (SFXTs) are X-ray binary systems with asupergiant companion and likely a neutron star, which show a fast ($\sim 10^3$s) and high variability with a dynamic range up to $10^{5-6}$. Given theirextreme properties, they are considered among the most valuable laboratories totest accretion models. Recently, the orbital parameters of a member of thisclass, IGR J08408-4503, were obtained from optical observations. We used thisinformation, together with X-ray observations from previous publications andnew results from X-ray and optical data collected by INTEGRAL and presented inthis work, to study the accretion mechanisms at work in IGR J08408-4503. Wefound that the high eccentricity of the compact object orbit and the large sizeof the donor star imply Roche lobe overflow (RLO) around the periastron. It isalso likely that a fraction of the outer layers of the photosphere of the donorstar are lost from the Lagrangian point $L_2$ during the periastron passages.On the basis of these findings, we discuss the flaring variability of IGRJ08408-4503 assuming the presence of an accretion disc. We point out that IGRJ08408-4503 may not be the only SFXT with an accretion disc fueled by RLO.These findings open a new scenario for accretion mechanisms in SFXTs, sincemost of them have so far been based on the assumption of spherically symmetricaccretion.

Accretion disc by Roche lobe overflow in the supergiant fast X-ray transient IGR J08408-4503