Pulsar observations with RadioAstron
All radio signals from cosmic sources are distorted by the plasma turbulence in the interstellar medium (ISM). Understanding of this turbulence is therefore essential for the proper interpretation of astronomical radio observations. The properties and characteristics of this turbulence can best be studied by observing point-like radio sources, where the results are not infuenced by the extended structure of the source, but instead are directly attributable to the effect of the ISM itself. Pulsars are such sources. Dispersion and scattering affect radio emission from pulsars. Whereas dispersion in the plasma column introduces delays in arrival time that depend upon frequency and results in smearing of the pulse, scattering by density inhomogeneities causes angular broadening, pulse broadening, intensity modulation or scintillation, and distortion of radio spectra in the form of dffraction patterns.

The space VLBI mission RadioAstron offers the unique possibility to observe pulsars at low frequencies and on baselines with projected lenghts of up to 350,000 km. On such long baselines the scattering disk of the pulsar is completely resolved and scattering properties of the interstellar medium can be investigated in detail. We are publishing a series of papers on pulsars observed with RadioAstron. The Figure below shows the visibility magnitudes as a function of delay for five different baselines with their lengths
given in parentheses in units of 1,000 km. The three lower plots show the signal from the scattering disk at zero delay, stron on the short ground-ground baseline and weaker on the ground-space baselines. The two upper plots show that the scattering disk is completely resolved. Only a distribution of visibility magnitudes along delay remains. This distribution gives the unique information on the scattering parameters.

Visibility amplitudes