Regions used to obtain the α − I_X correlation (top). The radio spectral index values were extracted between 144 MHz and 675 MHz. Each box has a width of 20″ corresponding to a physical size of about 23 kpc. The spectral index versus X-ray surface brightness relation across the halo in Abell 2256 (bottom). There is a strong anticorrelation between these two quantities (the black dashed line shows the fitting excluding the core region while the red dashed line includes the core region).