Interstellar scattering toward Cygnus X-3: Measurements of anisotropy and of the inner scale

We present 18 cm wavelength Very Long Base Interferometry (VLBI) observations of Cyg X-3 obtained 1989 June 6 along with 18 and 20 cm Very Large Array (VLA) A-array data obtained 1985 February 8. In both data sets, the flux density was relatively high, and the intrinsic source size small compared to the scattering, ideal conditions for a detailed study of interstellar scattering. We find that the scattering disk is anisotropic, with an axial ratio 1.31 +/- 0.02 along position angle 52.0 deg +/- 1.5 deg. This is the first unambiguous measurement of anisotropic scattering in the interstellar medium. We also find a significant break in the phase structure function, the first measurement of a break in any source. The data exclude a steep turbulent spectrum, but are consistent with a shallow turbulent spectrum with an inner scale of 300 km, with a net uncertainty of approximately 50%. We tentatively identify the scattering plasma with the H II region DR 11, for which we estimate a visual extinction of 4 mag, and an emission measure toward Cyg X-3 of 6000 cm(exp -6)pc. We make the case that DR 11 is part of a shell of H II regions centered on the Cyg OB2 association (which is approximately 1.8 kpc distant) that is 2 deg (60 pc) in radius and expanding at 10 km/s. Our inner scale is consistent with that expected for an H II region, and the amount of scattering is consistent with the emission measure. We discuss how the anisotropy can be used to confirm the identification with DR 11 and to distinguish between different models of turbulence by comparison with measurements of the polarization of starlight along the line of sight.