Investigation of image errors in VLBI images of SN 1993J

All deconvolved images from observational data obtained from very-long-baseline interferometry (VLBI) arrays are subject to image errors. They are caused by noise and incomplete sampling in the u-v plane. To investigate image errors we simulated data on the basis of VLBI observations of SN 1993J taken on 2007 Nov. 3. We computed the brightness distribution of a model of a supernova, calculated the visibility data of the model and made an image with the simulated data. The difference between model image and the image made with the simulated data indicates image errors. We found that image errors are larger by a factor~ 1.6 than the rms fluctuations of the background noise. We investigated the modulation of the brightness around the ridge of the shell and concluded that brightness variations are significant for the 2007 observations and also for those made in 1996, 1997 and 2010.