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Register a new userMulti-frequency imaging in VLBI
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S. Likhachev
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The new technique, multi-frequency imaging (MFI) is developed. In VLBI, Multi-Frequency Imaging (MFI) consists of multi-frequency synthesis (MFS) and multi-frequency analysis (MFA) of the VLBI data obtained from observations on various frequencies. A set of linear deconvolution MFI algorithms is described. The algorithms make it possible to obtain high quality images interpolated on any given frequency inside any given bandwidth, and to derive reliable estimates of spectral indexes for radio sources with continuum spectrum. Thus MFI approach makes it is possible not only to improve the quality and fidelity of the images and also essentially to derive the morphology of the observed radio sources.
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We report results of pentachromatic VLBI survey for 18 GHz-peaked spectrum sources. Spectral fitting at every pixel across five frequencies allows us to illustrate distribution of optical depth in terms of free-free absorption or synchrotron self absorption. Quasars and Seyfert 1 sources show one-sided morphology with a core at the end where the optical depth peaks. Radio galaxies and Seyfert 2 show symmetric double-sided jets with a optically thick core at the center.
We present Global VLBI and HSA images of the gravitational lens B2016+112 at 18, 6 and 3.6 cm. Previous VLBI observations showed that images A and B (which are clearly lensed images of a single background source) and the elongated region C are each divided into components. Our new high-resolution maps reveal more components in images A and B, clearly demonstrating their expected opposite parities. According to the scenario of Koopmans et al. (2002), the arc-like region C consists of two merging, partial images (``C1-C2) of just a small region of the same background source, seen with high lens magnification. We have determined the spectra and relative positions of the components within all four images in order to test this scenario. We find that the outer north-west components in images A and B do indeed have radio spectra similar to the components seen in C1 and C2.
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