Three modes of pulsar inner gap

Abstract

It is found that two radiation processes can cause pulsar inner gap breakdown, namely, the curvature radiation (CR) and the inverse Compton scattering (ICS) of high-energy particles off the thermal photons in the neutron star vicinity. The ICS photons are usually more energetic than the CR photons, so that the ICS process is the dominant mechanism to cause pulsar inner gap breakdown. For the ICS process, there are two preferred frequencies for the scattered γ-rays: (1) ω<inf>s</inf> = 2γω<inf>B</inf>, which is attributed to the resonant scattering, and (2) ω<inf>s</inf> ∼ γ²(2.82kT), which is mainly the contribution of the thermal-peak photons, which are usually more energetic. Thus we obtain three modes for pulsar inner gap sparking, namely, the "resonant ICS mode," the "thermal-peak ICS mode," and the "CR mode." A more energetic mode is more likely to dominate the gap-sparking process if the mean free path of the electrons (or positrons) is smaller than the gap height, which is equal to the mean free path of the γ-rays before being absorbed by the magnetic fields. The mean free paths of the electrons (or positrons) for the two ICS modes are functions of the surface temperature of a neutron star, so that we can obtain two "critical temperatures" to distinguish the three modes of pulsar inner gap. If the surface temperatures of the neutron stars fluctuate around the "critical temperatures," the gap properties will change suddenly, which will result in a switching effect in pulsar radiation. The observed "mode changing" phenomenon of some pulsars can be explained by switching between the resonant ICS mode and the thermal-peak ICS mode. The young pulsars near the "birth line" are dominated by the resonant ICS mode, while the pulsars near the "death line" and the millisecond pulsars near the "appearance line" are more likely dominated by the thermal-peak ICS mode. © 1997. The American Astronomical Society. All rights reserved.