VLA Polarization Observations of the Extragalactic Source 1741−038 during an Extreme Scattering Event

The extragalactic radio source 1741-038 underwent an extreme scattering event in 1992. Near the middle of the event and 1.5 yr after the end of the event we obtained Faraday rotation observations of this source. The results are consistent with no change in rotation measure during the scattering event. Many models of extreme scattering events suggest that the events are due to strong refraction by an intervening plasma irregularity in the Galactic interstellar medium. If the irregularity is moving with a typical interstellar velocity of 100 km s -1 , and located 1 kpc distant, we constrain the mean parallel component of the magnetic field within the refractor to be ≲40 μG. While the general interstellar field is substantially less than this value (about 2 μG), the field strength within a shock-induced irregularity is expected to be greatly increased over typical interstellar values. The constraint on the mean parallel field within the refractor is much tighter (≲4 μG) if the refractor is associated with the North Polar Spur.