1997A&A...321...55D

We present new measurements of the diffuse radio emission from the Coma cluster of galaxies at 1.4 GHz using the Effelsberg 100-m-telescope. Even at that high frequency, the halo source Coma C has an extent down to noise of ∼80' corresponding to 3Mpc (H₀=50km/s/Mpc) in Coma. The radio map reveals clear similarities with images of the extended X-ray halo of the Coma cluster. However, the radio halo appears to be displaced from the X-ray halo by ∼3-4arcmin. After subtracting the contributions from point sources we obtained an integrated diffuse flux density of S_1.4GHz=640±35mJy from Coma C. We derive relations between the various observationally determined spectral indices and the spectral index of the synchrotron emissivity, which allow one to achieve a rough estimate concerning the consistency of the presently available data at different frequencies and to place constraints on the emissivity index distribution. In the halo's core region, the inferred emissivity index between 0.3GHz and 1.4GHz appears to be in the range 0.4-0.75 implying that there must be some very effective mechanism for particle acceleration operating in the intracluster medium. We discuss implications of our measurements and of the spectral index information on current theories for radio halo formation. We stress the importance of having more measurements of Coma C at frequencies above 1.4GHz, in order to be able to derive constraints on the physics of the formation process of the radio halo.