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tokens: good λ 2 fits for the other RMs measured at different locations as well (some of them are shown in Figure 3 ) and also in the other five data sets not presented in Figure 2 . We omitted the jet and RMs at ≈900 mas from the core at 2 GHz for better visualization; those data are presented in Figure 3 . At lower frequencies, Faraday rotation is observable in more outward regions of the jet due to longer cooling times of the jet plasma. At higher frequencies, Faraday rotation is observable closer to the compact upstream emission thanks to better angular resolution and less depolarization. We note that the RM distributions are patchy at all frequencies because significant linear polarization is detected only in some parts of the jet, possibly due to substantial depolarization in the other parts. We also note that it is unlikely that those patchy RMs are artifacts because we found that the RMs in different epochs at the same observing frequency are detected in similar locations of the jet (see Appendix C ). Figure 2. Color map of the RM distribution, overlaid on contours of the total intensity of the M87 jet in three VLBA data sets (out of eight) at 2 (a), 5 (b), and 8 GHz (c). Contours start at 0.79, 0.54, and 0.53 mJy per beam for the 2 GHz, 5 GHz, and 8 GHz maps, respectively, and increase by factors of 2. The RM colorscale in units of rad m −2 is shown at the top-right corner. Beam sizes are illustrated by the gray shaded ellipses. All maps are rotated clockwise by 23° relative to astronomical R.A.–decl. coordinates for better visualization. EVPA as function of λ 2 , along with the best-fit λ 2 laws at the locations indicated by the black dashed arrows, is shown in (d)–(f). We note that all RMs measured at different locations show good λ 2 fits (see Figure 3 ). We omitted the jet and RMs at ≈900 mas from the core at 2 GHz for better visualization (see Figure 3 ). Figure 3. Same as Figure 2 but including HST -1 at ≈900 mas from the core at 2 GHz and showing EVPA– λ 2 diagrams for various jet locations where significant Faraday rotation is detected. 3.2. Radial RM Profile To obtain a radial RM profile along the jet, we calculated spatially binned RM by taking the weighted mean of all values in each separated region of the map with similar RM values. A priori, taking a weighted mean over a part of a map assumes that all individual pixels are independent from each other, which is not the case here. Pixels values are correlated across the extension of a resolution element (here, the synthesized beam). Thus, we first calculated a mean value, then its formal error (which assumes all pixels to be uncorrelated), and then multiplied this formal error by , where n is the number of the pixels used for taking the mean, Σ RM the size of the map region with RM values, and Σ FWHM the area within the FWHM of the synthesized beam. We present the mean distance from the black hole, the binned RM values, and corresponding RM errors in Table 2 . This data is used