Integrated spectra of the halo and relic sources in A 521. Radio halo spectrum (blue): the empty circle is the GMRT  153 MHz flux (this work), filled circles are measurements from B08 and D09; the power-law fit to the data weighted for the uncertainties (dashed blue line) corresponds to α = 1.85. Radio relic spectrum (magenta): filled squares are the flux density values taken from G08, the empty square is the GMRT  153 MHz flux density (this work); the magenta dashed line is the power-law fit (weighted for the uncertainties) corresponding to α = 1.45. The curved dashed and solid blue lines are the fit of homogeneous re-acceleration models, with a constant acceleration efficiency in the radio-emitting volume (constant ratio of turbulent and thermal energy densities). The magnetic field is assumed to scale with radius (azimuthally constant) as ρ^2/3 (short-dashed) and as ρ (solid); flatter radial distributions of the magnetic field strength produce more synchrotron power at lower frequencies.