Integrated spectrum of the radio halo in A 697. The blue empty circle is the GMRT  153 MHz measurement derived in this paper. GMRT  flux density measurements and limits at 330 and 610 MHz (filled circles and red asterisks respectively) are taken from M10, while the WSRT flux densities at 1382 and 1714 MHz are taken from van Weeren et al. 2011. The dashed line is the power-law fit to the data, weighted for the uncertainties (α = 1.5); the solid curved line is from the model of homogeneous re-acceleration, with a constant acceleration efficiency in the radio-emitting volume (constant ratio of turbulent and thermal energy densities). The magnetic field is assumed to decline with radius (azimuthally constant) as ρ. The (red) butterfly and asterisks mark the fluxes at 325 and 610 MHz corrected for the missing flux (M10).