The Extreme Scattering Event toward PKS 1741−038: VLBI Images

We report multiepoch VLBI observations of the source PKS 1741[038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum ux density of the ESE, the other two were as the ux density of the source was returning to its nominal value. The fourth epoch was at the maximum ux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the sources diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the sources diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741[038. We also see no evidence for ESEinduced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in ux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741[038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107 larger than that in the ambient medium. A lamentary structure could reduce the di erence between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from lamentary structures, they occur when the lamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a