Swift/XRT monitoring of the supergiant fast X-ray transient IGR J18483-0311 for an entire orbital period

IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging to the new class of high-mass X-ray binaries called supergiant fast X-ray transients. This system is one of the two members of this class, together with IGR J11215-5952, where both the orbital (18.52 d) and spin period (21 s) are known. We report on the first complete monitoring of the X-ray activity along an entire orbital period of a supergiant fast X-ray transient. These Swift observations, lasting 28 d, cover more than one entire orbital phase consecutively. They are a unique data set, which allows us to constrain the different mechanisms proposed to explain the nature of this new class of X-ray transients. We applied the new clumpy wind model for blue supergiants developed by Ducci et al. to the observed X-ray light curve. Assuming an eccentricity of e = 0.4, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from 10¹⁸ g to 5 × 10²¹ g.