Multifrequency Observations of Radio Pulse Broadening and Constraints on Interstellar Electron Density Microstructure

Abstract

We have made observations of 98 low Galactic latitude pulsars to measure pulse broadening caused by multipath propagation through the interstellar medium. Data were collected with the 305 m Arecibo telescope at four radio frequencies between 430 and 2380 MHz. We used a CLEAN-based algorithm to deconvolve interstellar pulse broadening from the measured pulse shapes. We employed two distinct pulse-broadening functions (PBFs): PBF 1 is appropriate for a thin screen of scattering material between the Earth and a pulsar, while PBF 2 is appropriate for scattering material uniformly distributed along the line of sight from the Earth to a pulsar. We found that some observations were better fitted by PBF 1 and some by PBF 2 . Pulse-broadening times (τ d ) are derived from fits of PBFs to the data and are compared with the predictions of a smoothed model of the Galactic electron distribution. Several lines of sight show excess broadening, which we model as clumps of high-density scattering material. A global analysis of all available data finds that the pulse broadening scales with frequency, ν, as τ d ∝ ν -α , where α ~ 3.9 ± 0.2. This is somewhat shallower than the value α = 4.4 expected from a Kolmogorov medium but could arise if the spectrum of turbulence has an inner cutoff at ~300-800 km. A few objects follow particularly shallow scaling laws (the mean scaling index ⟨α⟩ ~ 3.1 ± 0.1 and ~3.8 ± 0.2, respectively, for PBF 1 and PBF 2 ), which may arise from large-scale refraction or from the truncation of scattering screens transverse to the Earth-pulsar line of sight.