Anisotropic Interstellar Scattering toward the Cygnus Region

Abstract

We report on multifrequency VLBA observations of interstellar scattering (ISS) toward four extragalactic sources seen through the Cygnus region, 2005+403, 2008+332, 2021+317, and 2048+312. We are able to successfully determine scattering disk parameters at frequencies up to 5.0 GHz for 2005+403 and 2008+332 and up to 2.3 GHz for 2048+312. We were unable to reliably determine scattering disk parameters for 2021+317 at frequencies from 8.5 GHz down to 1.67 GHz because of its complex intrinsic source structure. The scattering disks are elliptical with axial ratios of about 0.75:1 with little measurable variation between these sources. We interpret our measurements as due to the effects of anisotropic interstellar turbulence. The anisotropy parameters, axial ratio and position angle, for those sources for which we have data at multiple frequencies appear to be frequency-independent making refractive distortions of isotropic turbulence an unlikely explanation. Our estimates of β, the power-law index of the power spectrum of electron density fluctuations, based upon direct model fits to the data on a source-by-source and frequency-by-frequency basis, are consistent with β = 4 for most of the data but are strongly affected by our estimate of the intrinsic structure of the sources. Two of these sources, 2005+403 and 2021+317, exhibit significant intrinsic source structure at frequencies as low as 1.67 GHz, while the other two, 2008+332 and 2048+312, lack discernible source structure at frequencies lower then 2.3 GHz. The frequency scalings of the scattered angular sizes are also consistent with β = 4. We discuss the implications of our measurements for the inner and outer scales of the turbulence.