Left – GMRT  153 MHz contours of the central field around the cluster A 521. Contours start at ±3σ_153  MHz =  ±2.7 mJy/b and are spaced by a factor of two. The resolution is 34.7′′ × 20.9′′, PA 59.0°. The dashed circle represents the area of 1 Mpc containing the central radio halo (same as in Fig. 1 of B08). Right – GMRT  153 MHz contours of the central diffuse emission in A 521 (halo, relic), after subtracting source A. Contours start at ±3σ_153  MHz =  ±4.35 mJy/b and are spaced by a factor of two, overlaid on the VLA 1400 MHz image, in greyscale (from D09; the lowest level of greyscale corresponds to the 3σ_1400  MHz = 90  μJy/b, the highest corresponds to 0.55 mJy/b, and the peak flux density is 15.5 mJy/b). The two images have similar angular resolutions: 38.0′′ × 35.0′′, PA 0° at 153 MHz and 30.0′′ × 30.0′′, PA 0° at 1400 MHz.