We present LOw Frequency ARray observations of the Coma Cluster field at 144 MHz. The cluster hosts one of the most famous radio halos, a relic, and a low surface brightness bridge. We detect new features that allow us to make a step forward in the understanding of particle acceleration in clusters. The radio halo extends for more than 2 Mpc, which is the largest extent ever reported. To the northeast of the cluster, beyond the Coma virial radius, we discover an arc-like radio source that could trace particles accelerated by an accretion shock. To the west of the halo, coincident with a shock detected in the X-rays, we confirm the presence of a radio front, with different spectral properties with respect to the rest of the halo. We detect a radial steepening of the radio halo spectral index between 144 and 342 MHz, at ∼30′ from the cluster center, that may indicate a non-constant re-acceleration time throughout the volume. We also detect a mild steepening of the spectral index toward the cluster center. For the first time, a radial change in the slope of the radio–X-ray correlation is found, and we show that such a change could indicate an increasing fraction of cosmic-ray versus thermal energy density in the cluster outskirts. Finally, we investigate the origin of the emission between the relic and the source NGC 4789, and we argue that NGC 4789 could have crossed the shock originating the radio emission visible between its tail and the relic.

Diffuse radio emission at the centre of galaxy clusters has been observed both in merging clusters on scales of Mpc, called giant radio haloes, and in relaxed systems with a cool-core on smaller scales, named mini haloes. Giant radio haloes and mini haloes are thought to be distinct classes of sources. However, recent observations have revealed the presence of diffuse radio emission on Mpc scales in clusters that do not show strong dynamical activity. RX J1720.1+2638 is a cool-core cluster, presenting both a bright central mini halo and a fainter diffuse, steep-spectrum emission extending beyond the cluster core that resembles giant radio halo emission. In this paper, we present new observations performed with the LOFAR Low Band Antennas (LBA) at 54 MHz. These observations, combined with data at higher frequencies, allow us to constrain the spectral properties of the radio emission. The large-scale emission presents an ultra-steep spectrum with $\alpha _{54}^{144}\sim 3.2$. The radio emission inside and outside the cluster core have strictly different properties, as there is a net change in spectral index and they follow different radio-X-ray surface brightness correlations. We argue that the large-scale diffuse emission is generated by particles re-acceleration after a minor merger. While for the central mini halo we suggest that it could be generated by secondary electrons and positrons from hadronic interactions of relativistic nuclei with the dense cool-core gas, as an alternative to re-acceleration models.

Context. MS 0735.6+7421 is a galaxy cluster that hosts a central radio galaxy with a very steep spectrum. The spectrum is produced by one of the most powerful known jetted active galactic nuclei (AGN). The radio plasma, ejected at nearly light speed from the central AGN, has displaced the intra-cluster medium, leaving two pairs of cavities observable in the X-ray. The cavities are associated with two different outbursts and have distributed energy to the surrounding medium. While the age of the cavities has previously been estimated from the X-rays, no confirmation from radio data is available. Furthermore, the radio spectrum has only been derived from integrated flux density measurements so far, and the spatial distribution that would help us to understand the nature of this source is still lacking. Aims. We perform for the first time a detailed, high-resolution spectral study of the source at radio frequencies and investigate its duty cycle. We compare this with previous X-ray estimates. Methods. We used new observations at 144 MHz produced with the LOw Frequency ARray (LOFAR) together with archival data at higher frequencies (235, 325, 610, 1400, and 8500 MHz), to investigate the spectral properties of the source. We also used radiative models to constrain the age of the source. Results. At the LOFAR frequency, the source presents two large outer radio lobes that are wider than at higher frequencies, and a smaller intermediate lobe that is located south-west of the core. A new inspection of X-ray data allowed us to identify an intermediate cavity that is associated with this lobe. It indicates a further phase of jet activity. The radio lobes have a steep spectrum even at LOFAR frequencies, reaching α144610 = 2.9 in the outer lobes and α144610 = 2.1 in the intermediate lobe. Fitting the lobe spectra using a single injection model of particle ageing, we derived a total age of the source between 170 and 106 Myr. This age agrees with the buoyancy and sound-crossing timescales derived from X-ray data. The resolution of the spectral age map we performed allows us to reconstruct the duty cycle of the source. In three phases of jet activity, the AGN was active for most of the time with only brief quiescent phases that ensured the repeated heating of the central gas. Finally, we estimated the minimum energy inside the outer lobes. We find that a source of additional pressure support must be present to sustain the bubbles against the pressure of the external medium.