Extreme intra-hour variability of the radio source J1402+5347 discovered
  with Apertif
  The propagation of radio waves from distant compact radio sources through
turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a
phenomenon called interstellar scintillation. Such scintillations are a unique
probe of the micro-arcsecond structure of radio sources as well as of the
sub-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 or
longer are commonly seen at centimetre wavelengths and are thought to result
from the line-of-sight integrated turbulence in the interstellar plasma of the
Milky Way. So far, only three sources were known that show more extreme
variations, with modulations at the level of some dozen percent on timescales
shorter 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 discovery
with Apertif of a source (J1402+5347) showing extreme (~50%) and rapid
variations 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 axis
of about 20,000 km. The canonical theory of refractive scintillation constrains
the 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 distance
of 32 pc. If the scintillations are associated with Alkaid, then the angular
size of J1402+5347 along the minor axis of the scintels must be smaller than
~10 micro arcsec yielding an apparent brightness temperature for an isotropic
source of >~ 10^ 14K. }
