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

(Abridged) We report multi-epoch VLBI observations of the source PKS 1741-038as it underwent an extreme scattering event. Observations at four epochs wereobtained, and images were produced at three of these. During the event thesource consisted of a dominant, compact component, essentially identical to thestructure seen outside the event. However, the source's diameter increasedslightly at 13 cm during the ESE. An increase in the source's diameter isinconsistent with a simple refractive model. We also see no evidence forESE-induced substructure within the source or the formation of multiple images,as would occur in a strongly refractive lens. However, a model in which thedecrease in flux density during the ESE occurs solely because of stochasticbroadening within the lens requires a larger broadening diameter during the ESEthan is observed. Thus, the ESE toward 1741-038 involved both stochasticbroadening and refractive defocussing within the lens. If the structureresponsible for the ESE has a size of order 1 AU, the level of scatteringwithin an ESE lens may be a factor of 10^7 larger than that in the ambientmedium. A filamentary structure could reduce the difference between thestrength of scattering in the lens and ambient medium, but we conclude that, ifESEs arise from filamentary structures, they occur when the filamentarystructures are seen lengthwise. We predict the amount of pulse broadening thatwould result from a comparable lens passing in front of a pulsar. The pulsebroadening would be no more than 1.1 microseconds, consistent with the lack ofpulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSRJ1643-1224.Comment: 19 pages, LaTeX2e with AASTeX-4.0, 1 LaTeX table and 5 figures in 9 PostScript files, to be published in the ApJ, minor change in Figures 2a, 3a, and 4a to correct a labe