VLBA observations of a rare multiple quasar imaging event caused by
  refraction in the interstellar medium
  We report on the first detection of the theoretically-predicted rare
phenomenon of multiple parsec-scale imaging of an active galactic nucleus
induced by refractive effects due to localized foreground electron density
enhancements, e.g., in an AU-scale plasma lens(es) in the ionized component of
the Galactic interstellar medium. We detected multiple imaging in the low
galactic latitude (b=-2 deg) quasar 2023+335 from the 15.4 GHz MOJAVE
observations when the source was undergoing an ESE. While the parsec-scale jet
of the source normally extends along PA -20 deg, in the 28 May 2009 and 23 July
2009 images a highly significant multi-component pattern of secondary images is
stretched out nearly along the constant galactic latitude line with a local PA
40 deg, indicating that the direction of relative motion of the plasma lens is
close to orbital. Weaker but still detectable imaging patterns at similar
position angles are sporadically manifest at several other epochs. Modeling the
ESE that occurred in early 2009 and lasted ~0.14 yr, we determined that the
foreground screen has a double-lens structure, with proper motion (~6.8
mas/yr), and angular size (~0.27 mas). We also found that the angular
separation between the two brightest sub-images roughly follows a
wavelength-squared dependence expected from plasma scattering. Furthermore, by
analyzing archival non-simultaneous VLBA observations covering a wide frequency
range from 1.4 to 86 GHz, we found that the scattered angular size of the VLBI
core follows a $\nu^{-1.89}$ dependence, implying the presence of a turbulent,
refractive dominated scattering screen that has a confined structure or is
truncated transverse to the line of sight toward 2023+335.
