Is the surface brightness spectrum of the coma cluster radio halo inverted

Comparison of published maps of the Coma Cluster radio halo at 430 and 1400 MHz (Hanisch, 1980) reveals that the spectrum of the surface brightness is inverted or flat over most of the extent of the 1400 MHz diffuse emission. This is very unusual for a highly extended source thought to emit via the synchrotron mechanism. Various explanations are considered, including a relatively flat electron-energy spectrum, free-free emission, emission by numerous compact sources, as well as instrumental effects. Constraints provided by searches for the Sunyaev-Zel'dovich effect tend to disqualify explanations (including free-free emission) in which the flat surface brightness spectrum continues to microwave frequencies. Verification of the high surface brightness at 1400 MHz is required.