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 epo chs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux 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, comp act component, essentially identical to the structure seen outside the even t. However, the source's 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 sour ce's diameter is inconsistent with a simple refractive model in which a smo oth refractive lens drifted across the line of sight to PKS 1741-038. We al so see no evidence for ESE-induced substructure within the source or the fo rmation of multiple images, as would occur in a strongly refractive lens. H owever, a model in which the decrease in flux 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 or der I AU, the level of scattering within an ESE lens may be a factor of 10( 7) larger than that in the ambient medium. A filamentary structure could re duce the difference between the strength of scattering in the lens and ambi ent medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are abl e to predict the amount of pulse broadening that would result from a compar able lens passing in front of a pulsar. The pulse broadening would be no mo re than 1.1 mu s, consistent with the lack of pulse broadening detected dur ing ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of si ght toward PKS 1741-038 is consistent with a turbulent origin for the struc tures responsible for ESEs. The source PKS 1741-038 lies near the radio Loo p I and is seen through a local minimum in 100 mu m emission.