2003A&A...397...53T

We present new measurements of the diffuse radio halo emission from the Coma cluster of galaxies at 2.675GHz and 4.85GHz using the Effelsberg 100-m telescope. After correction for the contribution from point sources we derive the integrated flux densities for the halo source (Coma C), S_2.675GHz=(107±28)mJy and S_4.85GHz=(26±12)mJy. These values verify the strong steepening of the radio spectrum of Coma C at high frequencies. Its extent strongly depends on frequency, at 4.85GHz it is only marginally visible. The measurement at 4.85GHz is the first flux density determination for Coma C at this high frequency. In order to quantify the spectral steepening we compare the expectations for the spectrum of Coma C with the observations, resorting to basic models for radio halo formation. The in-situ acceleration model provides the best fit to the data. From equipartition assumptions we estimate a magnetic field strength B_eq=0.57(1+k)^0.26µG in the intracluster medium of Coma, where k is the energy ratio of the positively and negatively charged particles. As a by-product of the 2.675GHz observation we present a new flux density for the diffuse emission of the extended source 1253+275 (S_2.675GHz=112±10). This measurement provides a smaller error range for the power-law fit to the spectrum (α=1.18±0.02) compared to previous investigations and yields an equipartition magnetic field strength of B_eq=0.56(1+k)^0.24µG.