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We report the discovery of fast, frequency-dependent intensity variationsfrom the scintillating intra-day variable quasar J1819+3845 at a wavelength of21cm which resemble diffractive interstellar scintillations observed inpulsars. The timescale (down to 20 min) and the bandwidth (frequencydecorrelation bandwidth of 160 MHz) of the observed variations jointly implythat the component of the source exhibiting this scintillation must possess abrightness temperature well in excess of the inverse Compton limit. A specificmodel in which both the source and scintillation pattern are isotropic impliesa brightness temperature 0.5 x 10^13 z_{pc} K, where previous estimates placethe distance to the scattering medium in the range z_{pc}=4-12pc, yielding aminimum brightness temperature >20 times the inverse Compton limit. Anindependent estimate of the screen distance using the 21cm scintillationproperties alone indicates a minimum screen distance of z approx 40pc and abrightness temperature above 2 x 10^14 K. There is no evidence for anisotropyin the scattering medium or source from the scintillation characteristics, butthese estimates may be reduced by a factor comparable to the axial ratio if thesource is indeed elongated.Comment: A&A in press, 18 pages, 9 fig

Diffractive interstellar scintillation of the quasar J1819+3845 at ${\vec{\lambda}}$21 cm