NCRA Colloquium

A synchrotron source with a random distribution of velocity vectors for radiating charges will have no systematic phase relation between radiation fields from individual charges and would assumedly give rise to an incoherent emission. However, we show that a partial coherence due to antenna mechanism can be inherently present in any compact synchrotron source, at wavelengths sufficiently longward of the characteristic frequency. This coherence arises naturally, without the need of a specific mechanism and has a number of interesting observational effects. It can account for a number of observed phenomena, including (1) brightness temperatures far exceeding the limit set by the inverse Compton catastrophe, (2) source variability over a wide range of timescales, (3) the limiting spectral index for self-absorbed compact sources, and (4) the correlation between the variability in optical/X-ray and radio wavebands.