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Session 46 - Interstellar Scattering and Scintillation as Tools in Radio Astronomy.
Topical, Oral session, Tuesday, June 09
Extreme scattering events (ESEs) are a class of dramatic changes in the flux density of radio sources. They are marked by a decrease (\approx 50%) in the flux density near 1 GHz for several weeks to months, bracketed by substantial increases. ESEs are thought to be due to refractive scattering by ionized structures in the Galactic interstellar medium---structures potentially associated with sites of interstellar turbulence.
We are conducting a three-pronged investigation of ESEs: Improved modeling of the lensing structures and ray tracing have been used to make quantitative comparisons with observed light curves. (2) VLBI imaging of the source 1741-038 during an \hboxESE. (3) A wavelet analysis of Green Bank Interferometer light curves in order to assess, in a systematic manner, the probability that a source will undergo an \hboxESE.
We use geometrical optics to study the refraction of a distant background radio source by a plasma lens (Clegg et al. 1998, \itApJ, \bf496, 253\rm). We find general agreement between modelled and observed light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies may result from some combination of lens substructure, anisotropic lens shapes, lenses which only graze the sources, or unresolved substructure within the sources. A typical lens is 0.1 AU in diameter with an electron density of 10^4 cm^-3. A lens may cause angular position wander of 0.1--100 mas at 2.25 GHz.
As 1741-038 underwent its ESE, its structure remained essentially unchanged. There is little evidence for additional angular broadening during the ESE nor is there any evidence for lensing-induced substructure in the source, such as might be caused by strong refraction or substructure within the lens. We detect no evidence of angular wander, though we have limited ability to detect angular wandering and our models predict that the amount of angular wander was only 0.4 mas at 2.2 GHz.
We present wavelet scalograms constructed from GBI light curves. We show the scalograms for various sources and illustrate how these scalograms can be used to identify an \hboxESE.
Program listing for Tuesday