Spectral distributions in compact radio sources I. Imaging with VLBI data

We discuss a technique for mapping the synchrotron turnover frequency distribution using nearly simultaneous, multi–frequency VLBI observations. The limitations of the technique arising from limited spatial sampling and frequency coverage are investigated. The errors caused by uneven spatial sampling of typical multi– frequency VLBA datasets are estimated through numerical simulations, and are shown to be of the order of 10%, for pixels with the deconvolution SNR ∼ 7. The fitted spectral parameters are corrected for the errors due to limited frequency coverage of VLBI data. First results from mapping the turnover frequency distribution in 3C345 are presented.