Two Modes of Partially Screened Gap

TL;DR

This paper explores two distinct mechanisms, Curvature Radiation and Inverse Compton Scattering, that cause breakdowns in the Partially Screened Gap model of pulsars, explaining observed mode-changing and nulling phenomena.

Contribution

It introduces a dual-mode PSG model with CR and ICS mechanisms, providing a unified explanation for pulsar mode-changing and nulling behaviors.

Findings

Two gap breakdown modes: PSG-off (CR) and PSG-on (ICS).

Different plasma properties in each mode explain pulsar phenomena.

The model links magnetic field strength and polar cap temperature to gap behavior.

Abstract

The analysis of X-ray observations suggest an ultrastrong ($B\gtrsim 10^{14} \,{\rm G}$) surface magnetic field at the polar cap of pulsars (Szary, 2013). On the other hand, the temperature of the polar caps is about a few millions Kelvin. Based on these two facts we use the Partially Screened Gap (PSG) model to describe the Inner Acceleration Region (IAR). The PSG model assumes that the temperature of the actual polar cap is equal to the so-called critical value, i.e. the temperature at which the outflow of thermal ions from the surface screens the gap completely. We have found that, depending on the conditions above the polar cap, the generation of high energetic photons in IAR can be caused either by Curvature Radiation (CR) or by Inverse Compton Scattering (ICS). Completely different properties of both processes result in two different scenarios of breaking the acceleration gap: the so-called PSG-off mode for the gap dominated by CR and the PSG-on mode for the gap dominated by ICS. The existence of two different mechanisms of gap breakdown naturally explains the mode-changing phenomenon. Different characteristics of plasma generated in the acceleration region for both modes also explain the pulse nulling phenomenon.