Extreme Scattering Events: An Observational Summary
  We review observational constraints on the structures responsible for extreme scattering events, focussing on a series of observations of the quasar PKS 1741-038. VLA observations were conducted to search for changes in the rotation measure and H I absorption during the ESE, while VLBI observations sought ESE-induced changes in the source's image. No RM changes were found implying B_|| < 12 mG, and no H I opacity changes were found implying N(H I) < 6.4 x 10^{17} cm^{-2}. No multiple imaging was observed, but the diameter of the source increased by 0.7 mas, contrary to what is predicted by simple refractive lens modeling of ESEs. We summarize what these limits imply about the structure responsible for this ESE. 
T. Joseph W. Lazio (NRL), Alan L. Fey (USNO), R. A. Gaume (USNO)