Emergence and disappearance of microarcsecond structure in the scintillating quasar J1819+3845

The 4.8-GHz light curves of the scintillating intraday variable quasar J1819+3845 during 2004–2005 exhibit sharp structure, down to a time-scale of 15 min, that was absent from light curves taken prior to this period and from the 2006 light curves. Analysis of the light curve power spectra show that the variations must be due to the emergence of new structure in the source. The power spectra yield a scattering screen distance of 3.8 ± 0.3 pc for a best-fitting vISS= 59 ± 0.5 km s^−1 or 2.0 ± 0.3 pc for the scintillation velocity reported by Dennett-Thorpe & de Bruyn. The scattering medium is required to be exceptionally turbulent, with C2N≳ 0.7 ΔL^−1pc m^(−20/3) for scattering material of thickness ΔLpc pc along the ray path. The 2004 power spectrum can be explained in terms of a double source with a component separation 240 ± 15 μas in 2004.