Timing and spectral study of the Be XRB IGR J11305-6256: Swift discovers the orbital period and a soft X-ray excess

IGR J11305-6256 is one of the numerous sources discovered through the INTEGRAL scan of the Galactic Plane. Thanks to the Swift-BAT survey, that allows the frequent sampling of any sky region, we have discovered in the hard X-ray emission of this source a modulation with a period of ~120.83 d. The significance of this periodic modulation is ~4 standard deviations in Gaussian statistics. We interpret it as the orbital period of the binary system. We derive an orbital separation between IGR J11305-6256 and its companion star of ~286 R$_{\odot}$ corresponding to ~19 times the radius of the companion star. The broadband XRT-BAT (0.3-150 keV) spectrum is described either by the sum of a black-body and a cut-off power-law or by a partially absorbed cut-off power-law. The temporal and spectral characteristics of the source indicate its possible association with the class of persistent, but faint, Be X-ray binary systems.