CLEAN imaging systematics of M87 radio jet

Abstract: The spectral index images of the jet in the nearby radio galaxy M87 have previously been shown with Very Long Baseline Interferometric arrays at 2-43 GHz. They exhibit flattening of the spectra at a location of inner (central) spine and toward outer ridges. This could imply optical depth effects, lower energy cutoff or stratification of the emitting particles energy distribution. In this paper we employ simulations of multifrequency VLBI observations of M87 radio jet with various model brightness distributions. CLEAN deconvolution errors produce significant features in the observed images. For intensity images they result in the appearance of the inner ridge line in the intrinsically edge brightened jet models. For spectral index images they flatten the spectra in a series of stripes along the jet. Another bias encountered in our simulations is steepening of the spectra in a low surface brightness jet regions. These types of the imaging artefacts do not depend on the model considered. We propose a methods for the compensation of the systematics using only the observed data.