The presence of interstellar scintillation in the 15 GHz interday
  variability of 1158 OVRO-monitored blazars
  We have conducted the first systematic search for interday variability in a
large sample of extragalactic radio sources at 15 GHz. From the sample of 1158
radio-selected blazars monitored over a $\sim$10 year span by the Owens Valley
Radio Observatory 40-m telescope, we identified 20 sources exhibiting
significant flux density variations on 4-day timescales. The sky distribution
of the variable sources is strongly dependent on the line-of-sight Galactic
H$\alpha$ intensities from the Wisconsin H$\alpha$ Mapper Survey, demonstrating
the contribution of interstellar scintillation (ISS) to their interday
variability. 21% of sources observed through sight-lines with H$\alpha$
intensities larger than 10 rayleighs exhibit significant ISS persistent over
the $\sim$10 year period. The fraction of scintillators is potentially larger
when considering less significant variables missed by our selection criteria,
due to ISS intermittency. This study demonstrates that ISS is still important
at 15 GHz, particularly through strongly scattered sight-lines of the Galaxy.
Of the 20 most significant variables, 11 are observed through the
Orion-Eridanus superbubble, photoionized by hot stars of the Orion OB1
association. The high-energy neutrino source TXS0506$+$056 is observed through
this region, so ISS must be considered in any interpretation of its short-term
radio variability. J0616$-$1041 appears to exhibit large $\sim$20% interday
flux density variations, comparable in magnitude to that of the very rare class
of extreme, intrahour scintillators that includes PKS0405$-$385, J1819$+$3845
and PKS1257$-$326; this needs to be confirmed by higher cadence follow-up
observations.
