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 } { \mu G } 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 _ { 2.675 GHz } = 112 \pm 10 ) .
This measurement provides a smaller error range for the power–law fit to the spectrum ( \alpha = 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 } { \mu G } .