Time variation in V_LSR gradient PA and amplitude. a) Each of the four panels refers to the maser source indicated in the upper right corner of the panel, and presents the histogram of the standard deviation (ΔP_s) of the values of the gradient PA, at different epochs, for features persisting over two or three observing epochs. The bin size of all the histograms is 10°. b) Each of the four panels refers to the maser source indicated in the upper right corner of the panel, and presents the histogram of the fractional time variation (ΔΓ/Γ) in the gradient amplitude, for features persisting over two or three observing epochs. The bin size of all the histograms is 0.2.