Pignataro, G. V.G. V.PignataroBONAFEDE, AnnalisaAnnalisaBONAFEDEBERNARDI, GianniGianniBERNARDIRiseley, C. J.C. J.RiseleyDALLACASA, DanieleDanieleDALLACASAVENTURI, TizianaTizianaVENTURI2024-05-292024-05-2920240004-6361http://hdl.handle.net/20.500.12386/35144Aims. Recent X-ray and Sunyaev-Zeldovich (SZ) observations have detected thermal emission between early-stage merging galaxy clusters. The main purpose of this work is to investigate the properties of the non-thermal emission in the interacting cluster pairs Abell 0399-Abell 0401 and Abell 21-PSZ2 G114.9. Methods. These two unique cluster pairs have been found in an interacting state. In both cases, their connection along a filament is supported by an SZ effect detected by the Planck satellite and, in the special case of Abell 0399-Abell 0401, the presence of a radio bridge has been already confirmed by LOFAR observations at 140 MHz. Here, we analyse new high-sensitivity, wideband (250-500 MHz) uGMRT data of these two systems and describe an injection procedure to place limits on the spectrum of Abell 0399-Abell 0401 and on the radio emission between Abell 21-PSZ2 G114.9. Results. In both cases, the low-surface-brightness diffuse emission is not detected in Band 3 (250-500 MHz). For the A399-A401 pair, we are able to constrain the steep spectral index of the bridge emission to be α > 2:2 with a 95% confidence level between 140MHz and 400 MHz. We also detect a small patch of the bridge with a flatter spectral index, which may suggest a variable spectral index distribution across the bridge area. For the A21-PSZ2 G114.9 pair, we are able to place an upper limit on the flux density of the bridge emission with two different methods, finding at the central frequency of 383MHz a conservative value of f1u < 260 mJy at a 95% confidence level, and a lower value of f2u < 125 mJy at an 80% confidence level, based on visual inspection and a morphological criterion. Conclusions. Our work provides a constraint on the spectrum in the bridge A399-A401 that disfavours shock acceleration as the main mechanism for the radio emission. The methods that we propose for the limits on the radio emission in the A21-PSZ2 G114.9 system represent a first step towards a systematic study of these sources.STAMPAenArticle10.1051/0004-6361/2023462432-s2.0-85184952294https://www.aanda.org/articles/aa/full_html/2024/02/aa46243-23/aa46243-23.htmlhttps://api.elsevier.com/content/abstract/scopus_id/85184952294FIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation