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S. Boldyrev (U. Chicago), C. R. Gwinn (U. California, Santa Barbara)
Observations of pulsar signals provide a valuable tool for investigating electron density distribution in the interstellar medium. The pulsar intrinsic signal is narrow in time, while the observed signal is broad and asymmetric with a sharp rise and a slow decay. The pulse broadening is caused by the random refraction the waves experience while propagating in the interstellar medium.
The shapes of the signals, and their scalings with the wave length and with the pulsar distances have been investigated for 30 years, however, two puzzles have remained. First, observational pulse scaling with the pulsar distance is anomalously strong; it is consistent with the standard model only when non-stationary statistics of electron fluctuations along the line of sight are assumed. Second, the observational pulse shapes are consistent with the standard model only under assumption that the scattering material is concentrated in a thin slab between the pulsar and the Earth.
We show that both paradoxes are resolved at once if one assumes stationary and uniform, but non-Gaussian statistics of the electron-density distribution in the interstellar medium. Such statistics must be of Lévy type, and the propagating ray should exhibit a Lévy flight rather than the Gaussian random walk assumed in the standard model. A Lévy distribution has a divergent second moment, therefore, the standard theory cannot apply, i.e., the observed pulse shapes should be related to the full shape of the electron distribution function, not to its second moment. We develop a theory of wave propagation in such a non-Gaussian random medium, and demonstrate its good agreement with observations.
The work was partly supported by the NSF Center for magnetic self-organization in astrophysical and laboratory plasmas, U. of Chicago.
References:
Boldyrev S. & Gwinn C. R., Phys. Rev. Lett, 91 (2003) 131101; ApJ 584 (2003) 791.
The author(s) of this abstract have provided an email address for comments about the abstract: boldyrev@uchicago.edu
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Bulletin of the American Astronomical Society, 36 #2
© 2004. The American Astronomical Soceity.