GMRT observations of the Ophiuchus galaxy cluster

Abstract

Aims: Observations with the Very Large Array telescope 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 α₂₄₀¹⁵³ ≃ 1.4±0.3 and an high-frequency spectral index of α₁₄₇₇²⁴⁰ ≃ 1.60±0.05. Moreover, we measure a systematic increase of the high-frequency spectral index with radius: the azimuthal radial average of α₁₄₇₇²⁴⁰ 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 agrees with that obtained by modelling 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 an index p. We derive that the electrons energy spectrum should extend from a minimum Lorentz factor of γ_min ≲ 700 up to a maximum Lorentz factor of γ_max ≃ 3.8 × 10⁴ with an index p = 3.8±0.4. The volume-averaged strength for a completely disordered intra-cluster magnetic field is B_V ≃ 0.3±0.1 μG.