2006A&A...446..185M

We report the discovery of fast, frequency-dependent intensity variations from the scintillating intra-day variable quasar J1819+3845 at λ21cm which resemble diffractive interstellar scintillations observed in pulsars. The observations were taken with the Westerbork Synthesis Radio Telescope on a dozen occasions in the period between Aug. 2002 and Jan. 2005. The data were sampled at both high temporal and high frequency resolution and have an overall simultaneous frequency span of up to 600MHz. In constructing the light curves and dynamic spectra the confusion from background sources has been eliminated. The timescale (down to 20min) and the bandwidth (frequency decorrelation bandwidth of 160MHz) of the observed variations jointly imply that the component of the source exhibiting this scintillation must possess a brightness temperature well in excess of the inverse Compton limit. A specific model in which both the source and scintillation pattern are isotropic implies a brightness temperature 0.5x10¹³z_pcK, where previous estimates place the distance to the scattering medium in the range z_pc=4-12pc, yielding a minimum brightness temperature >20 times the inverse Compton limit. An independent estimate of the screen distance using the 21cm scintillation properties alone indicates a minimum screen distance of z≃40pc and a brightness temperature above 2x10¹⁴K. There is no evidence for anisotropy in the scattering medium or source from the scintillation characteristics, but these estimates may be reduced by a factor comparable to the axial ratio if the source is indeed elongated. The observed scintillation properties of J1819+3845 at 21cm are compared with those at 6cm, where a significantly larger source size has been deduced for the bulk of the emission by Dennett-Thorpe & de Bruyn (2003A&A...404..113D). However, opacity effects within the source and the different angular scales probed in the regimes of weak and strong scattering complicate this comparison.