Parameters from the parabolic fits to the masers of our three epochs and the masers of Sanna et al. (2012) as a function of time (with t = 0 corresponding to our first epoch: 2001 December 2). We have obtained linear fits for the position (x₀, y₀) of the tip of the bow shock in the four available epochs, which are shown with solid lines in the corresponding plots (the same fits are shown in the plots of the left and right columns). The graphs of the right column show the four epochs, and the left column graphs show our three epochs with an expanded time-axis.