Synchrotron emissivity of an electron spectrum with a power-law slope, q, extending between an injection momentum, ${\widehat{p}}_{\mathrm{inj}}$, and a maximum momentum of ${\widehat{p}}_{\max }={10}^6$. Each spectrum is normalized to contain a total energy density of ϵ = 1 erg cm⁻³ and emits in a magnetic field with B = 1 μG. The color shows emissivity and contour lines are spaced apart by half an order of magnitude.