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The propagation of radio waves from distant compact radio sources throughturbulent interstellar plasma in our Galaxy causes these sources to twinkle, aphenomenon called interstellar scintillation. Such scintillations are a uniqueprobe of the micro-arcsecond structure of radio sources as well as of thesub-AU-scale structure of the Galactic interstellar medium. Weak scintillations(i.e. an intensity modulation of a few percent) on timescales of a few days orlonger are commonly seen at centimetre wavelengths and are thought to resultfrom the line-of-sight integrated turbulence in the interstellar plasma of theMilky Way. So far, only three sources were known that show more extremevariations, with modulations at the level of some dozen percent on timescalesshorter than an hour. This requires propagation through nearby (d <~10 pc)anomalously dense (n_e ~10^2 cm^-3) plasma clouds. Here we report the discoverywith Apertif of a source (J1402+5347) showing extreme (~50%) and rapidvariations on a timescale of just 6.5 minutes in the decimetre band (1.4 GHz).The spatial scintillation pattern is highly anisotropic, with a semi-minor axisof about 20,000 km. The canonical theory of refractive scintillation constrainsthe scattering plasma to be within the Oort cloud. The sightline to J1402+5347,however, passes unusually close to the B3 star Alkaid (eta UMa) at a distanceof 32 pc. If the scintillations are associated with Alkaid, then the angularsize of J1402+5347 along the minor axis of the scintels must be smaller than~10 micro arcsec yielding an apparent brightness temperature for an isotropicsource of >~ 10^ 14K. }

Extreme intra-hour variability of the radio source J1402+5347 discovered with Apertif