Gaussian-fitted positions and velocities of individual maser features of the ²⁸SiO v = 1, J = 2−1 emission, overlaid on the velocity-integrated intensity map of the λ7 mm emission (all 19 epochs of data spanning 22 months, in grey-scale; Matthews et al. 2010). The cross indicates the estimated position for Source I. Weak λ3 mm emission not visible in Fig. 1 was recovered in the north and south arms and west of the base of the south arm.