Context :

Aims : Very Large Array observations at 1477 MHz revealed the presence of a radio mini-halo surrounding the faint central point-like radio source in the Ophiuchus cluster of galaxies .
In this work we present a study of the radio emission from this cluster of galaxies at lower radio frequencies .

Methods : We observed the Ophiuchus cluster at 153 , 240 , and 614 MHz with the Giant Metrewave Radio Telescope .

Results : The mini-halo is clearly detected at 153 and 240 MHz , the frequencies at which we reached the best sensitivity to the low-surface brightness diffuse emission , while it is not detected at 610 MHz because of the too low signal-to-noise ratio at this frequency .
The most prominent feature at low frequencies is a patch of diffuse steep spectrum emission located at about 5′ south-east from the cluster centre .
By combining these images with that at 1477 MHz , we derived the spectral index of the mini-halo .
Globally , the mini-halo has a low-frequency spectral index of \alpha _ { 240 } ^ { 153 } \simeq 1.4 \pm 0.3 and an high-frequency spectral index of \alpha _ { 1477 } ^ { 240 } \simeq 1.60 \pm 0.05 .
Moreover , we measure a systematic increase of the high-frequency spectral index with radius : the azimuthal radial average of \alpha _ { 1477 } ^ { 240 } increases from about 1.3 , at the cluster centre , up to about 2.0 in the mini-halo outskirts .

Conclusions : The observed radio spectral index is in agreement with that obtained by modeling the non-thermal hard X-ray emission in this cluster of galaxies .
We assume that the X-ray component arises from inverse-Compton scattering between the photons of the cosmic microwave background and a population of non-thermal electrons which are isotropically distributed and whose energy spectrum is a power law with index p .
We derive that the electrons energy spectrum should extend from a minimum Lorentz factor of \gamma _ { min } \lesssim 700 up to a maximum Lorentz factor of \gamma _ { max } \simeq 3.8 \times 10 ^ { 4 } with an index p = 3.8 \pm 0.4 .
The volume-averaged strength for a completely disordered intra-cluster magnetic field is B _ { V } \simeq 0.3 \pm 0.1 \mu G .