Disentangling the System Geometry of the Supergiant Fast X-Ray Transient IGR J11215-5952 with Swift

IGR J11215-5952 is a hard X-ray transient source discovered in 2005 April with the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) and a member of the new class of high-mass X-ray binaries, the supergiant fast X-ray transients (SFXTs). While INTEGRAL and Rossi X-Ray Timing Explorer observations have shown that the outbursts occur with a periodicity of ~330 days, Swift data have recently demonstrated that the true outburst period is ~165 days. IGR J11215-5952 is the first discovered SFXT displaying periodic outbursts, which are possibly related to the orbital period. The physical mechanism responsible for the X-ray outbursts in SFXTs is still debated. The main hypotheses proposed to date involve the structure of the companion wind or gated mechanisms related to the properties of the compact object. We test our proposed model which explains the outbursts from SFXTs as being due to the passage of the neutron star inside the equatorially enhanced wind from the supergiant companion. We performed a Guest Investigator observation with Swift that lasted 20 ks and several follow-up Target of Opportunity (ToO) observations, for a total of ~32 ks, during the expected "apastron" passage (defined assuming an orbital period of ~330 days), between 2008 June 16 and July 4. The characteristics of this "apastron" outburst are quite similar to those previously observed during the "periastron" outburst of 2007 February 9. The mean spectrum of the bright peaks can be fitted with an absorbed power-law model with a photon index of 1 and an absorbing column of ~10²² cm^-2. This outburst reached luminosities of ~10³⁶ erg s^-1 (1-10 keV), comparable with those measured in 2007. The light curve can be modeled with the parameters obtained by Sidoli et al. for the 2007 February 9 outburst, although some differences can be observed in its shape. The properties of the rise to this new outburst and the comparison with the previous outbursts allow us to suggest that the true orbital period of IGR J11215-5952 is very likely 164.6 days, and that the orbit is eccentric, with the different outbursts produced at the periastron passage, when the neutron star crosses the inclined equatorial wind from the supergiant companion. Based on a ToO observation performed on 2008 March 25-27, we can exclude that the period is 165/2 days.