Context :

Aims : We have investigated highly atypical morphological parsec-scale changes in the flat spectrum extragalactic radio source 2023+335 which are coincident with an extreme scattering event ( ESE ) seen at radio wavelengths during the first half of 2009 .

Methods : We used ( i ) 15.4 GHz Very Long Baseline Array ( VLBA ) observations of the quasar 2023+335 obtained at 14 epochs between July 2008 and Nov. 2012 as part of the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments ( MOJAVE ) program , ( ii ) earlier archival VLBA observations of the source performed at 1.4 , 2 , 8 , 15 , 22 , and 86 GHz to analyze the properties of the proposed turbulent screen toward 2023+335 , and ( iii ) data sets from the Owens Valley Radio Observatory ( OVRO ) and University of Michigan Radio Astronomy Observatory ( UMRAO ) single-dish monitoring programs performed at 15 and 14.5 GHz , respectively , to study integrated flux density changes .

Results : 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 \aas@@fstack { \circ } 4 ) 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 \sim - 20 \degr , 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 \approx 40 \degr , 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 \sim 0.14 yr , we determined that the foreground screen has a double-lens structure , with proper motion ( \sim 6.8 mas yr ^ { -1 } ) , and angular size ( \sim 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 .

Conclusions :