Non-thermal X-ray and soft gamma-ray radiation from the young pulsars

Abstract

This thesis focuses on the radiation mechanisms of non-thermal X-rays and soft gamma-rays of two types of thousands year old spin-down powered pulsars. The thousands year old pulsars have distinct radiation behaviors from the middle-aged gamma-ray pulsars. In the magnetosphere of the pulsar, the particles are accelerated by the electric field resulting from the rotation of the neutron star. These accelerated particles move along the magnetic field lines and emit GeV gamma-ray curvature photons. For the middle-aged pulsars, most of the curvature photons, whose observed spectra are described well by power law with exponential cut-off, can escape out of the light cylinder. In X-ray band, the middle-aged pulsars usually have black body radiation with a weak non-thermal component described by power law. On the other hand, for the thousands-year-old pulsars, the curvature spectra in GeV band, which obey power law with exponential cut-off, are smeared out by the pair creation or missed by the line of sight. The secondary pairs generated by pair creation processes spiral around the magnetic field lines and emit synchrotron photons, and the young pulsars have stronger non-thermal X-ray and soft gamma-ray radiation than the middle-aged ones.

Seven young pulsars have been studied here, they are the Crab pulsar, PSRs B0540-69, B1509-58, J1846-0258, J1811-1925, J1617-5055 and J1930+1852. These seven fall into two categories: the Crab-like pulsars and soft gamma-ray pulsars. The Crab-like pulsars include the Crab pulsar and the Giant Crab PSR B0540-69, and the soft gamma-ray pulsars include the other five. The main difference between the two types of young pulsars is that the Crab-like pulsars’ spectra peak at E ≤ 1MeV while the soft gamma-ray pulsars’ spectra (in units of MeV/cm2/s) peak at E ∼ 10MeV. Their spectra also have different photon indices in X-ray band. The physics behind is two different pair creations, the photon-photon pair creation and the magnetic pair creation. The former happens when a high energy photon collides with a soft photon, and the latter happens when a high energy photon penetrates through strong perpendicular magnetic field. In the outer gap of the pulsar, a large mount of pairs are generated around the null charge surface via photon-photon pair creation, and the electric field separates the two charges to move in opposite directions. Therefore, there are outflow and inflow of particles in the magnetosphere, whose curvature photons are converted to pairs by photon-photon pair creation and magnetic pair creation respectively. For the Crab-like pulsar, the non-thermal X-rays and soft gamma-rays are emitted by the outgoing secondary pairs generated by photon-photon pair creation in the outer magnetosphere; for the soft gamma-ray pulsar, the radiating secondary pairs are generated below the null charge surface by the magnetic pair creation.