Radial profile of the X_CO factor as a function of the galactocentric radius in bins of 100 pc. Reddish data points refer to the X_CO factor obtained with method I, while the green to bluish represent X_CO obtained with method II. The uncertainty of each bin is given by the colour scale and the error bars represent the standard error ($\sigma /\sqrt{N}$). Red and blue dotted lines refer to the mean X_CO obtained using methods I and II, respectively. The solid black line shows the Galactic value, while the two black dotted lines show 2.5 times the Galactic value, respectively.