Histogram of the source density map of the RASS-BSC (left panel), and RASS-FSC (right panel), per square degree. The mean and median source density of each map are plotted in blue dot-dot-dot-dash and in red dashed lines, respectively. The upper x-axis shows the associated probability of association within 5′ (see text). The sources are drawn from the whole sky so the solid angle is 4π steradian.