Astron. Astrophys. 355, 1168-1180 (2000)

4. Conclusions

We investigated refraction and polarization transfer in an ultrarelativistic pulsar plasma embedded in an infinitely strong magnetic field. In agreement with the standard models of electron-positron cascade, the plasma density was assumed to be zero near the magnetic axis and beyond the boundary of the open field line tube. In addition, the plasma density distribution was taken to be nonaxisymmetric. This does not conflict with the customary model of magnetosphere structure. With such density distribution wave mode coupling introduced by refraction results in an antisymmetric profile of circular polarization with the sense reversal near the pulse centre. The observed frequency evolution of the antisymmetric V-profiles (the increase of with the frequency) corresponds to the increasing role of refraction at high frequencies. At low frequencies, which are believed to originate at high altitudes in the open field line tube, rotation effect is essential rather than refraction. Wave mode coupling introduced by magnetosphere rotation leads to symmetric profiles of circular polarization. The rays emitted sufficiently close to the edge of the open field line tube can intensely deviate out of the tube causing depolarization or even sense reversal at the wings of V-profile. The observed irregularities in the swing of position angle, which usually accompany high circular polarization, are also naturally explained by polarization-limiting effect.

We also suggested the interpretation of the morphological features of pulsar total-intensity profiles on the basis of propagation effects in the magnetosphere. It is shown that refraction in the plasma, whose number density decreases towards the magnetic axis and towards the edge of the open field line tube, can account for the angular separation of profile components. The spectral evolution of total-intensity profiles is well explained by the increasing efficiency of refraction at high frequencies.

In our investigation the plasma was assumed to be cold, although this is not the case in pulsar magnetospheres. It should be noted that the dispersive curves for the waves in the hot plasma are qualitatively similar to those in the case of the cold plasma (Lyubarskii 1995). The quantitative description of the waves in the hot plasma requires using the concrete form of the particle distribution function, which is still obscure. In addition, the exact distribution of the plasma number density across the open field line tube is also unknown. So the obtained results can allow some quantitative modification, while the qualitative picture is believed to remain the same.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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