In this paper, we have covered several methodological and scientific aspects of studying synchrotron spectrum of the parsec-scale regions in AGN. The main conclusions can be stated as follows:
1) We have discussed a technique that can be used for mapping the turnover frequency distribution and obtaining spectral information from multi-frequency VLBA data. A feasibility study shows that multi-frequency VLBA observations can be used for spectral imaging and continuous spectral fitting.
2) Multi-frequency VLBA observations made with up to 10 minute
separations between the scans at each frequency can provide a
satisfactory spatial sampling and image sensitivity for sufficiently
bright sources with intermediate (mas) structures. The
fractional errors from comparing the data at different frequencies
should not exceed 10% for emission with SNR
, in this case.
4) A procedure for broadband synchrotron spectrum fitting has been introduced for mapping the distribution of spectral parameters of radio emission from parsec-scale jets. Corrections based on the local curvature of the fitted spectra are introduced, in order to compensate for the incomplete frequency coverage in cases where the true turnover frequency is outside of the range of observing frequencies.
5) From a 4-frequency VLBA observation of 3C345, the first map of the turnover frequency distribution are produced. The maps indicate possible locations of the relativistic channel and strong shock fronts inside the jet. The magnetic field distribution derived from the turnover frequency and flux distributions is consistent with the plane shocks existing in the immediate vicinity of the source core. The extended emission appears to have a very low turnover frequency for which the existing data do not warrant a good estimate, limiting the conclusions to deducing certain information from the gradients of the turnover frequency which are visible in the extended jet. The observed gradients are consistent with the patterns of velocity distribution and density gradients typical for Kelvin-Helmholtz instabilities propagating in a relativistic jet. A more detailed study, with observations made at lower frequencies, is required for making conclusive statements about the nature of the observed gradients of the turnover frequency.
Acknowledgements
We would like to thank anonymous referee and I. Pauliny-Toth for many constructive comments on the paper. A substantial part of this work has been completed during the author's fellowship at the National Radio Astronomy Observatory (NRAO). The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities Inc.
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