A study on pulsar inner-gap sparking comparing inverse Compton scattering and curvature radiation processes

Abstract

The inner gap model (RS model) is a relatively successful model to understand the radiation behavior of radio pulsars. In this paper, the inner gap theory is reinvestigated by tak-ing into account the inverse Compton scattering (ICS) process of the outgoing high energy particles with the thermal photons in pulsar magnetosphere. The ICS process plays two roles in inner gap physics. Firstly, the energy loss of the outgoing particles due to the ICS process is an efficient mechanism to prevent the particles from attaining very large Lorentz factors. Secondly, the scattered photons in the ICS process may play an important role in inner gap sparking, so that the basic properties of the gap (such as the height and the potential drop across the gap) change greatly. In this paper, we compare the gap 'sparking' conditions due to the ICS process and the curvature radiation (CR) process. Our conclusion is that in most cases, the ICS process will form a pair cascade earlier than CR, which results in a new type of inner gap. We then study this new type of gap in detail under the assumption of the multipole magnetic fields near the neutron star surface, just as what is assumed in the RS model. The following conclusions are drawn: (1) The gap properties not only depend on the pulsar magnetic field B and the rotation period P, but may also depend on the star surface temperature T if the temperature is high enough. This may present an interpretation to the 'nulling' phenomenon. (2) The 'binding energy problem' in the RS model is greatly weakened, because the electric field in the gap is greatly lower than that in the RS model. (3) The Lorentz factors of the outgoing particles are decreased, which are in a wide range of 10³-10⁶ for P - 1. This poses a challenge to the RS emission theory, in which γ ∼ 10⁶ is required.