No abstract

Stars twinkle to the eye through atmospheric turbulence, but planets, because of their larger angular size, do not. Similarly, scintillation due to the local interstellar medium will modulate the radio flux of gamma-ray-burst afterglows and may permit indirect measurements of their angular sizes. The amplitude of refractive scintillation is of order ten percent at ten gigahertz unless the source size is much larger than the expected size, of order ten microarcseconds. Diffractive scintillation is marginally possible, depending sensitively on the source size, observing frequency, and scattering measure of the interstellar medium.PACS

We have made high-resolution, high-sensitivity dynamic spectra of a sample of strong pulsars at 430 MHz with the Arecibo radiotelescope. For 4 pulsars we find faint but sharply delineated features in the secondary spectra. These are examples of the previously observed "crisscross" or "multiple drift slope" phenomenon presumed to be due to multiple imaging of the pulsar by the interstellar medium. The unprecedented resolution and dynamic range of our observations allow a deeper level of analysis. Distances to the dominant scattering screen along the line of sight are determined and are shown to agree well with those inferred from other scintillation phenomena. Multiple imaging of the pulsar by the ISM is required. A compact central image surrounded by a faint scattering halo, roughly circularly symmetric, is consistent with the data. Scattering from filaments may also be consistent. The angular extent of the scattering material parallel to the direction of the pulsar velocity is ∼ 5 mas, corresponding to a linear extent of ∼ 2 AU. Further observations of these features should allow better discrimination between models and an identification of the scattering structures.

No abstract

We present a fitting function to describe the statistics of flux modulations caused by interstellar scintillation. The function models a very general quantity: the cross-correlation of the flux observed from a compact radio source of finite angular size observed at two frequencies and at two positions or times. The formula will be useful for fitting data from sources such as intraday variables and gamma-ray burst afterglows. These sources are often observed at relatively high frequencies (several gigahertz), where interstellar scattering is neither very strong nor very weak, so that asymptotic formulae are inapplicable.