Millisecond pulsars and pulsar wind nebulae as sources of gamma rays and cosmic rays

Thesis (Ph.D. (Space Physics)--North-West University, Potchefstroom Campus, 2008.

Our main aim was to implement a fully 3D General Relativistic (GR) frame-dragging polar cap (PC) millisecond pulsar (MSP) model as a numerical code, and constrain it using multiwavelength and cosmic-ray data. We also modelled pulsar wind nebulae (PWN) as sources of gamma rays and cosmic rays. Detailed modelling of the unscreened and screened MSPs PSR J0437-4715 and PSR B1821-24 yielded low curvature radiation (CR) cut-off energies and highlighted the need for 4th generation Cherenkov telescopes and / or space telescopes such as GLAST in order to observe CR emission from gamma-ray MSPs. We also constrained their gamma-ray efficiencies using EGRET upper limits. We studied the behaviour of the basic MSP model for a mesh of (P, P, x-> C)_sPace-- and found that cut-off energies, efficiencies, energy fluxes, and integral fluxes generally decrease with larger P and smaller P, and that their maxima occur for on-beam radiation. Accurate knowledge of pulsar geometry is therefore crucial when making predictions regarding MSP visibility. In addition, screening complicates the matter. We furthermore found that beaming correction factors may be significantly larger than unity, and that if these factors are unknown, actual trends may be concealed. We numerically confirmed the theoretical expectation that the gamma-ray efficiency is a 7% for MSPs with unscreened PCs. Using a population of 59 MSPs, we found that roughly half of these are expected to be visible for GLAST. We performed a population study of MSPs in the globular cluster (GC) 47 Tucanae and obtained "geometry-averaged" results with relatively small uncertainties. The EGRET upper limit at 1 GeV constrained the number of visible GC pulsars to N « 27. Larger values of N imply a reduction in the model-predicted pulsed integral flux, even more severely so in the case of a GLAST LAT non-detection of the cumulative flux. We also calculated the unpulsed inverse Compton scattering (ICS) and synchrotron radiation (SR) fluxes due to particles escaping from the ensemble of MSPs and found that the ICS flux will probably not be visible for H.E.S.S., while telescopes such as Chandra and Hubble should yield upper limits for the SR component of diffuse radiation. We modelled the PWN GO.9+0.1, and found optimal fits for the pulsar's initial spin-down luminosity and injection spectral break energy using multiwavelength data. From these followed predictions for the putative embedded pulsar's present-day spin-down power and PC magnetic field strength. We furthermore participated in ongoing collaborative research involving pulsars and PWN as sources of cosmic rays. We concluded that while PSR J0437-4715 is not expected to make a significant contribution to the local interstellar spectrum (LIS), both Geminga and B0656+14 may make a non-negligible contribution to the local cosmic ray positron spectrum. We predicted an anisotropy in the positron LIS due to the latter two pulsars of up to a few percent, which will probably only be testable by the PAMELA mission. We lastly obtained constraints on the birth periods of and diffusion coefficients in the direction of these pulsars. Future work include considering multipolar magnetic fields, including an ^-component in the accelerating E-field, reconsidering electro dynamic boundary conditions, and performing magneto-spheric particle-in-cell (PIC) simulations.

Doctoral