Multi-frequency synthesis algorithm based on Generalized Maximum Entropy
  Method. Application to 0954+658
  We propose the multi-frequency synthesis (MFS) algorithm with spectral
correction of frequency-dependent source brightness distribution based on
maximum entropy method. In order to take into account the spectral terms of
n-th order in the Taylor expansion for the frequency-dependent brightness
distribution, we use a generalized form of the maximum entropy method suitable
for reconstruction of not only positive-definite functions, but also
sign-variable ones. The proposed algorithm is aimed at producing both improved
total intensity image and two-dimensional spectral index distribution over the
source. We consider also the problem of frequency-dependent variation of the
radio core positions of self-absorbed active galactic nuclei, which should be
taken into account in a correct multi-frequency synthesis. First, the proposed
MFS algorithm has been tested on simulated data and then applied to
four-frequency synthesis imaging of the radio source 0954+658 from VLBA
observational data obtained quasi-simultaneously at 5, 8, 15 and 22 GHz.
