Simulation Of Cascade Process In Pulsar Polar Cap

Cascade process in pulsar polar cap is one of the fundamental physical process of pulsars,which is related to the generation of the plasma of pulsar wind.However,the emission from cascade region has not been confirmed in the high energy observations.There are several widespread models about the acceleration region and the detailed emission mechanism,but the origin of the non-thermal X-ray emission of pulsars is also under debate.A group of recently found pulsars,so-called MeV pulsars,obviously differ from the normal gama-ray pulsars.Generally,gama-ray pulsars have spectrums with peak at GeV band,but the MeV pulsars radiate their maximum energy in MeV band,and there are no emissions detected in GeV band.It is believed that GeV emissions from the pulsars mainly come from outer magnetosphere around the light cylinder,produced by the particles accelerated in the outer gap region.But the GeV photons produced in outer magnetosphere can not be attenuated to MeV by the photon-photon(??)pair-creation process.Thus this model can not explain the non-detection of GeV photons from MeV pulsars.The so-called MeV pulsars could be the evidence of the emission from the polar cap cascade.In this thesis,we investigate the possibility that the emissions of MeV pulsars come from the secondary particles produced in the polar cap cascade process.To discuss such a possibility,we simulated the magnetic pair-creation process in the polar cap cascade region,based on the algorithm of Timokhin et al.(2015,2018).We calculated the multiplicity of particles and the luminosity of escaped photons under different parameters such as spin period,magnetic field strength and curvature radius of magnetic field lines.Our simulation indicates that,not considering the Resonant Inverse Compton Scattering(RICS)process,the maximum multiplicity is(?)106,when the magnetic field strength is about 1012G.Considering the RICS process,the magnetic field strength at the maximum multiplicity(?106.5)is slightly higher than the former,which requires pulsars to be young and have a higher surface temperature(T?106K).Then we applied the simulation to MeV pulsars.Comparing the results of simulation with the observation on MeV pulsars,we discussed possible parameters(e.g.curvature radius and the size of cascade region)that produces a consistent model.We found that there is obvious correlation between the spectral peak energy and the curvature radius of magnetic field line near the neutron star surface:when the curvature radius is set ?c=106cm,the pulsars' radiation is consistent with the observations.Thus we suggest that the soft gama-ray emissions of the MeV pulsar are coming from the polar cap cascade region.