The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz

open20siACKNOWLEDGEMENTS. NB, AB, CJR acknowledge support from the ERC through the grant ERC- Stg DRANOEL no 714245. HE acknowledges support from the DFG, Germany [427771150]. Basic research in radio astronomy at the Naval Research Laboratory is supported by 6.1 Base funding. AB acknowledges support from the VIDI research programme with project number 639.042.729, which is financed by the Netherlands Organisation for Scientific Research (NWO). GB, RC, FG, and MR acknowledge support from INAF mainstream project ‘Galaxy Clusters science with LOFAR’. AD acknowledges support by the BMBF Verbundforschung under the grant 05A20STA. ACE acknowledges support from STFC grant ST/P00541/1. RJvW acknowledges support from the ERC Starting Grant ClusterWeb 804208. LOFAR, the Low-Frequency Array designed and constructed by ASTRON (Netherlands Institute for Radio Astronomy), has facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy. This research made use of the LOFAR IT computing infrastructure supported and operated by INAF, and by the Physics Dept. of Turin University (under the agreement with Consorzio Interuniversitario per la Fisica Spaziale) at the C3S Supercomputing Centre, Italy. The Jülich LOFAR Long Term Archive and the German LOFAR network are both coordinated and operated by the Jülich Supercomputing Centre (JSC), and computing resources on the supercomputer JUWELS at JSC were provided by the Gauss Centre for supercomputing e.V. (grant CHTB00) through the John von Neumann Institute for Computing (NIC). The scientific results reported in this article are based in part on data obtained from the VLA Data Archive, the GMRT Data Archive and the Chandra Data Archive. This research made use of the following Python packages: APLPY (Robitaille & Bressert 2012), ASTROPY (Astropy Collaboration et al. 2013) and NUMPY (van der Walt, Colbert & Varoquaux 2011).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 LOw Frequency ARray 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 ultrasteep spectrum with α ∼ 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.openBiava N.; De Gasperin F.; Bonafede A.; Edler H.W.; Giacintucci S.; Mazzotta P.; Brunetti G.; Botteon A.; Bruggen M.; Cassano R.; Drabent A.; Edge A.C.; Ensslin T.; Gastaldello F.; Riseley C.J.; Rossetti M.; Rottgering H.J.A.; Shimwell T.W.; Tasse C.; Van Weeren R.J.Biava N.; De Gasperin F.; Bonafede A.; Edler H.W.; Giacintucci S.; Mazzotta P.; Brunetti G.; Botteon A.; Bruggen M.; Cassano R.; Drabent A.; Edge A.C.; Ensslin T.; Gastaldello F.; Riseley C.J.; Rossetti M.; Rottgering H.J.A.; Shimwell T.W.; Tasse C.; Van Weeren R.J