Rotation measure image of the radio galaxy A401B. The angular resolution is 3.0′′  ×  3.0′′. The contours refer to the total intensity image at 8465 MHz. Contour levels are drawn at: 0.07 0.15 0.5 1 and 3 mJy/beam. The histogram shows the rotation measure distribution for all significant pixels. The plots show the position angle Ψ_Obs as a function of λ² at different source locations.