Modelling pulsar emission in the high-energy and very-high-energy regimes

Abstract

The Fermi Large Area Telescope has revolutionised the γ-ray pulsar field, increasing the population to over 250 detected pulsars. The majority display spectra with exponential cutoffs in a narrow range around a few GeV. Models predicted cutoffs up to 100 GeV; it was therefore not expected that pulsars would be visible in the very-high-energy (>100 GeV) regime. Subsequent surprise discoveries by ground-based telescopes of pulsed emission from four pulsars above tens of GeV have marked the beginning of a new era, raising important questions about the electrodynamics and local environment of pulsar magnetospheres. I have performed geometric light curve modelling using static, retarded vacuum, and offset polar cap dipole Bfields, in conjunction with standard two-pole caustic and outer gap geometries. I also considered a slot gap E-field associated with the offset polar cap B-field and found that its inclusion leads to qualitatively different light curves. Solving the particle transport equation shows that the particle energy only becomes large enough to yield significant curvature radiation at large altitudes above the stellar surface, given this relatively low E-field. Therefore, particles do not always attain the radiation-reaction limit. Increasing the slot gap E-field by a factor of 100 led to improved light curve fits, as well as curvature radiation reaction at lower altitudes. The overall optimal light curve fit was for the retarded vacuum dipole field and outer gap model. Recent kinetic simulations sparked a debate regarding the emission mechanism of pulsed γ-ray emission from pulsars. Some models invoke curvature radiation, while others assume synchrotron radiation in the current sheet. Detection of the Vela pulsar by H.E.S.S. (20 − 120 GeV) and Fermi provides evidence for a curved spectrum. We posit this to result from curvature radiation via primary particles in the pulsar magnetosphere and current sheet. We present energy-dependent light curves using an extended slot gap and current sheet model and invoking a two-step accelerating E-field as motivated by kinetic simulations. I include a refined calculation of the curvature radius of particle trajectories, impacting the particle transport, predicted light curves, and spectra. The model reproduces the decrease of flux of the first light-curve peak relative to the second one, evolution of the bridge emission, near constant phase positions of peaks, and narrowing of pulses with increasing energy. We can fundamentally explain the first of these trends, since I found that the curvature radii of the particle trajectories in regions where the second γ-ray light curve peak originates are systematically larger than those associated with the first peak, implying a correspondingly larger cutoff for the second peak. An unknown azimuthal dependence of the E-field as well as uncertainty in the precise emission locale preclude a simplistic discrimination of emission mechanisms. Finally, H.E.S.S. recently announced the detection of pulsed emission from the Vela pulsar up to 7 TeV, constraining particle energies to exceed several TeV. I contributed to a paper invoking synchrotron self-Compton emission to model this new radiation component, thus providing a consistent framework to describe the TeV emission from Vela.