INTEGRAL observations of IGRJ11215-5952: the first Supergiant Fast X-ray Transient displaying periodic outbursts

The hard X-ray source IGRJ11215-5952, discovered with INTEGRAL during a brief outburst in 2005, has been proposed as a new member of the class of Supergiant Fast X-ray Transients. Analysing archival INTEGRAL observations of the source field,we have discovered two previously unnoticed outbursts (in July 2003 and in May 2004),spaced by intervals of ~330 days, suggesting a possible orbital period.The 5-100keV spectrum is well described by a cut-off power law, with a photon index of 0.5, and a cut-off energy 15-20keV, typical of High Mass X-ray Binaries containing a neutron star. The luminosity is ~3E36erg/s assuming 6.2kpc, the distance of the likely optical counterpart, the blue supergiant HD306414.A fourth outburst was discovered in 2006 with XTE/PCA, 329 days after the third one, confirming the periodic nature of the source outbursts.Follow-up observations with Swift/XRT refined the source position and confirmed the association with HD306414.The 5-100keV spectrum, the recurrent nature of the outbursts, the blue supergiant companion star HD 306414, support the hypothesis that IGRJ11215-5952 is a Supergiant Fast X-ray Transient, and it is the first object of this class of High Mass X-ray Binaries displaying periodic outbursts.