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Session 46 - Interstellar Scattering and Scintillation as Tools in Radio Astronomy.
Topical, Oral session, Tuesday, June 09
Sierra/Padre,
Refractive effects have been noticed in pulsar observations for more than twenty years, although they have not always been identified as such. These effects include pulsar flux variations on time scales of days to years, tilted scintles and fringe patterns in dynamic spectra, and elongation of VLBI images of pulsars. Of particular interest are those phenomena, such as fringing or multiple-imaging, that indicate an excess of fluctuation power at the refractive size scale compared to that expected from a Kolmogorov extrapolation from the diffractive size scale. The frequency of occurrence of such events has not been well characterized. Recently, Rickett, Lyne, and Gupta (1997, MNRAS, 287, 739) underscored the remarkable sensitivity of dynamic spectra observations for probing multiple-imaging effects. They found an instance of multiple imaging in which the flux density from the secondary ray path was only 0.0016 times the flux density of the primary ray path, and the separation between the ray directions was more than 10 times larger than the diffractive image size! How prevalent are such remarkable events? We have applied a similar secondary spectrum analysis to a data set of dynamic spectra obtained by one of us (RSF) in 1991-92 at the NRAO 42-m telescope. A total of 22 pulsars were observed at 8 quarterly epochs over these two years at both 800 MHz and 1330 MHz. We have analyzed these data paying particular attention to weak features in the secondary spectra. We will present the results of this analysis in an effort to quantify how often strong refractive effects are present in pulsar scintillation data.
