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Session 120 - The Local Interstellar Medium.
Oral session, Thursday, January 18
Corte Real, Hilton

[120.01] Evidence for Anisotropic Radio Scattering in the Interstellar Medium

A. S. Trotter, J. M. Moran (CfA), L. F. Rodr\'\iguez (UNAM)

The extragalactic radio source NGC 6334B is the most strongly scattered object known, with an angular size of \theta \approx 3\arcsec at 20 cm. Its angular size scales as the square of the observing wavelength, \theta \propto \lambda^2, and its phase structure function, as measured by interferometric visibilities, has a power-law dependence on baseline. Both of these properties are expected if NGC 6334B is being scatter-broadened by an intervening screen of turbulent interstellar plasma. The line of sight to the source intercepts the northern lobe of the bipolar outflow NGC 6334A, which is most likely the source of the scattering. We present observations of NGC 6334B with the VLA at 1.4, 2, 6, 20 and 90 cm, and with the VLBA at 1.4, 2, 3.6 and 6 cm. We performed anisotropic model fits to visibility amplitudes and to closure amplitudes. All the data are consistent with a scattering screen model for this source. The disk is heavily resolved on all but the shortest VLBA baselines, and was undected by the VLA at 90 cm, where the scattering should be particularly severe (\theta \sim 10\arcmin). Moreover, the VLA observations give strong indication that the scattering disk of NGC 6334B is anisotropic, with an axial ratio of \sim 1.2 at 6 and 20 cm, and \sim 1.7 at 2 cm. This anisotropy is most likely due to the effects of a coherent magnetic field permeating the scattering medium. Our fits imply a power-law index in the turbulent density fluctuation power spectrum of 3.7 < \alpha < 4.0, where \alpha = 11/3 corresponds to Kolmogorov turbulence, and \alpha=4 is expected if the interferometer baselines are shorter than the inner (dissipative) scale of turbulence. Measurements of nearby high velocity (80 km\:s^-1) H_2O maser features showed no sign of scatter broadening, implying that they are most likely physically associated with the scattering region.

Program listing for Thursday