The presence of interstellar scintillation in the 15 GHz interday variability of 1158 OVRO-monitored blazars

TL;DR

This study systematically investigates interstellar scintillation as a cause of interday radio variability in a large sample of blazars at 15 GHz, revealing its significant role especially along certain Galactic sight-lines.

Contribution

It provides the first large-scale evidence that interstellar scintillation affects 15 GHz blazar variability and links this effect to Galactic structures like the Orion-Eridanus superbubble.

Findings

20 sources show significant 4-day variability due to ISS.

ISS contribution is higher along sight-lines with strong Hα emission.

ISS remains important at 15 GHz, especially through certain Galactic regions.

Abstract

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.