Mach number derived from radio observations (ℳ_radio) plotted against that from the ICM temperature (ℳ_X). The results of A2255 are indicated by the red cross (assuming radio spectral index: ) and the brown lower limit (). The black crosses show the results of other systems (see text for the detail). The gray dashed line indicates the linear correlation as a reference. Here we note that not all Mach numbers inferred from radio observations based on injected spectral index. Therefore, future low-frequency radio observations can change the Mach numbers displayed here (see text for a detail).