Evidence for Refractive Scintillation of Two Extragalactic Sources Seen through the Galactic Disk

Extragalactic radio sources were qualitatively examined to determine if there are any cases in which apparent variations at 2.7 GHz are uncorrelated with variations at 8.1 GHz, as might be expected for extrinsically induced refractive scintillation. Two sources exhibiting this behavior were identified, NRAO 150 and 2013 + 370, both of which happen to lie within 2 deg of the Galactic plane. The calculated refractive scintillation properties, based on intrinsic VLBI sizes and scattering measurements in the Galactic disk, are in reasonable agreement with the observed fluctuations. The light curves of these sources are interpreted as significantly affected by refractive scintillation in the interstellar medium. The findings suggest that extragalactic sources seen through extended paths in the Galactic disk may be modulated by refractive scintillation even at gigahertz frequencies.