The diffuse radio emission from the Coma cluster at 2.675 GHz and 4.85 GHz

We present new measurements of the diffuse radio halo emission from the Coma cluster of galaxies at 2.675 GHz and 4.85 GHz 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), $S_{2.675 GHz}=(107\pm 28)mJy$ and $S_{4.85 GHz}=(26\pm 12)mJy$. These values verify the strong steepening of the radio spectrum of Coma C at high frequencies. Its extent strongly depends on frequency, at 4.85 GHz it is only marginally visible. The measurement at 4.85 GHz 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 $B_{eq}=0.57 (1+k)^{0.26}μG$ 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 2.675 GHz observation we present a new flux density for the diffuse emission of the extended source 1253+275 ($S_{2.675GHz}=112\pm 10$). This measurement provides a smaller error range for the power--law fit to the spectrum ($α=1.18\pm 0.02$) compared to previous investigations and yields an equipartition magnetic field strength of $B_{eq}=0.56 (1+k)^{0.24}μG$.

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