Radio versus X-ray surface brightness correlation for the halo in Abell 2256 at 144 MHz (Left), 675 MHz (Middle), and 1.5 GHz (Right). The surface brightness is extracted from square-shaped boxes (width of 23 kpc) and in a “common region” visible across all three frequencies. The X-ray surface brightness is extracted from the XMM-Newton (0.5−2 keV band) image smoothed with a Gaussian FWHM of 6″ to have a good signal-to-noise ratio in the fainter outer regions. The radio surface brightness is extracted from radio maps at 20″ resolution. The Linmix best-fit relations for the halo data points are depicted as black dashed lines. Squared-shape boxes represent cells where both the radio and X-ray surface brightness is greater than 3σ. The 2σ upper limits are indicated by arrows. The lower panel shows the residuals of log I_R and log I_X with respect to the Linmix best fit line for the halo region only. Green data points are extracted from the wedge arc.