The scatter-broadened image of Cygnus-X-3

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Abstract
We have made observations of the highly scatter-broadened image of Cygnus X-3, using MERLIN at both 408 and 1692 MHz. At each frequency the shape of the image is close to that of an elliptical Gaussian, but the ellipticity and the position angle are frequency-dependent. We interpret the variable elongation of the scattering disc as being due to scale-dependent anisotropy in the scattering medium arising from variations in the direction of the projected interstellar magnetic field. From the scale on which the anisotropy changes we derive a value for the outer scale of turbulent density fluctuations, rout ∼ few × 1011 km ∼ 0.01 pc. Cyg X-3 is the third source for which the scattering disc has been shown to be markedly elliptical for at least one wavelength. This suggests that anisotropy in the turbulent ISM may be a widespread phenomenon, and emphasizes the importance of extending scattering theory to include its effects on observations. At both frequencies our interferometer data show an excess of correlated flux density at large baselines compared with that expected from a pure Gaussian image profile. This excess visibility power is due to weak image granulation and was predicted for a power-law spectrum of turbulence by Goodman & Narayan. From the amplitude of the excess visibility power we constrain the exponent in the power-law spectrum to be rin of the density power spectrum in the scattering screen in front of Cyg X-3: (i) from the wavelength-dependence of scatter-broadening; (ii) from the visibility as a function of those baselines where the source is moderately resolved; and (iii) from the excess power in the visibility on baselines where the source is highly resolved. From a combination of the results from methods (ii) and (iii) we cautiously constrain rin, for this line of sight to lie in the range 102 < rin < 105.5 km. Future MERLIN and VLBI measurements should be able to place better constraints on the spectrum of density fluctuations, using the analysis methods we present here. However, in view of the subtlety of the effects being sought, obtaining data of the requisite quality will always present a significant observational challenge.

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