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The intra-hour variable quasar J1819$+$3845: 13-year evolution, jet polarization structure and interstellar scattering screen properties

We examine the long-term evolution of the intra-hour variable quasar, J1819+3845, whose variations have been attributed to interstellar scintillation by extremely local turbulent plasma, located only 1-3pc from Earth. The variations in this source ceased some time between June 2006 and February 2007. The evolution of the source spectrum and the long-term lightcurve, and the persistent compactness of the source VLBI structure indicates that the cessation of rapid variability was associated with the passage of the scattering material out of the line of sight to the quasar. We present an analysis of the linear polarization variations and their relation to total intensity variations. The proper motion of polarized features in the quasar jet is found to be subluminal. Systematic time delays between Stokes I, Q and U, in combination with the structure of the source obtained from 8.4GHz VLBI data, confirm the estimate of the screen distance: 1-2pc, making the screen one of the nearest objects to the Solar System. We determine the physical properties of this scattering material. The electron density in the scattering region is extremely high with respect to the warm ionized ISM, with an estimated density of $n_e \sim 97 \, l_0^{1/3} {ΔL}_{100}^{-1/2}$cm$^{-3}$, where $l_0$ is the outer scale of the turbulence in AU and $ΔL = 100 ΔL_{100}$ AU is the depth of the scattering region. If this plasma is in pressure balance with the local magnetic field, one expects a ~2 rad/m^2 rotation measure change associated with the passage of this material past the quasar. We examine the rotation measures of sources and the diffuse polarized emission in the surrounding region. We place a limit of 10 rad/m^2 on the RM change. The variability of sources near J1819+3845 is used to deduce that the screen must therefore be either very small (~100 AU) or patchy.