Observational consequences of the Partially Screened Gap

Observations of the thermal X-ray emission from old radio pulsars implicate that the size of hot spots is much smaller then the size of the polar cap that follows from the purely dipolar geometry of pulsar magnetic field. Plausible explanation of this phenomena is an assumption that the magnetic field at the stellar surface differs essentially from the purely dipolar field. Using the conservation of the magnetic flux through the area bounded by open magnetic field lines we can estimate the surface magnetic field as of the order of $10^{14}$G. Based on observations that the hot spot temperature is about a few million Kelvins the Partially Screened Gap (PSG) model was proposed which assumes that the temperature of the actual polar cap equals to the so called critical temperature. We discuss correlation between the temperature and corresponding area of the thermal X-ray emission for a number of pulsars. We have found that depending on the conditions in a polar cap region the gap breakdown can be caused either by the Curvature Radiation (CR) or by the Inverse Compton Scattering (ICS). When the gap is dominated by ICS the density of secondary plasma with Lorentz factors $10^{2}-10^{3}$ is at least an order of magnitude higher then in a CR scenario. We believe that two different gap breakdown scenarios can explain the mode-changing phenomenon and in particular the pulse nulling. Measurements of the characteristic spacing between sub-pulses ($P_{2}$) and the period at which a pattern of pulses crosses the pulse window ($P_{3}$) allowed us to determine more strict conditions for avalanche pair production in the PSG.

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