Real-time detection of an extreme scattering event: constraints on
  Galactic plasma lenses
  Extreme scattering events (ESEs) are distinctive fluctuations in the
brightness of astronomical radio sources caused by occulting plasma lenses in
the interstellar medium. The inferred plasma pressures of the lenses are $\sim
10^3$ times the ambient pressure, challenging our understanding of gas
conditions in the Milky Way. Using a new survey technique, we have discovered
an ESE while it was in progress. We report radio and optical follow-up
observations. Modelling of the radio data demonstrates that the lensing
structure is a density enhancement and that the lens is diverging, ruling out
one of two competing physical models. Our technique will uncover many more
ESEs, addressing a long-standing mystery of the small-scale gas structure of
the Galaxy.
