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High resolution image reconstruction of pulsar B0834+06 scattered by the interstellar medium

  • Author(s): Gao, Jianjian
  • et al.
Abstract

The scattered radio image of a pulsar, as a result of the radio wave passing through the turbulent interstellar plasma, is a valuable probe of the plasma turbulence. However the scattering angles are often so small, typically a few milli-arcsec, that the radio image cannot be resolved even with very long baseline interferometry (VLBI). We used several different methods to reconstruct the scattered brightness image of pulsar B0834+06. We first combined the secondary spectrum technique with VLBI astrometry and successfully mapped the scattered brightness image of pulsar B0834+06 at 327 MHz with an angular resolution 100 times finer than would have been possible with VLBI alone. We find that the scattering occurs in a compact region about 420 pc from the Earth. This image has two components, both essentially linear and nearly parallel. The primary feature is highly inhomogeneous on spatial scales as small as 0.05 AU, and extremely anisotropic. The second feature (called offset feature) is much fainter and is displaced from the axis of the primary feature by about 9 AU. We find that the velocity of the scattering plasma is 16 ± 10 km s⁻¹ approximately parallel to the axis of the linear feature. Another technique is then presented which allows reconstruction of the core scattered brightness image in two dimensions from individual reversed sub-arc, providing an estimate of the axial ratio of the anisotropic turbulence. we obtained well-estimated 2 dimensional core image, and successfully estimated the half power width in both parallel and perpendicular direction (̃ 3.85 mas and ̃ 1.3 mas respectively) with axial ratio ̃ 3. Based on previous knowledge that this brightness image is highly elongated, we take it to the extreme and find a very good scattered brightness model : a 1-dimensional curvi-line model for primary feature, and a two straight-line linear model for offset feature. We also found that offset feature is frequency dependent. Considering all those interesting properties of this plasma turbulence, we came up with two possible geometry models for the physical plasma turbulence behind the scattered brightness image, the parallel geometry model and the orthogonal geometry model. Although no direct proof is available, we believe orthogonal model fits our observation better. It doesn't require the pulsar to be underneath the center of the primary feature, and it's easy to explain the similarity of those two data sets 22 months apart. A prism-shaped screen provides an explanation of the 1 ms offset feature and also helps to explain the frequency dependence of the offset feature

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