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

TL;DR

This paper models pulsar emissions in high-energy regimes, explaining observed spectral features and light curve behaviors through advanced magnetospheric models involving curvature radiation and energy-dependent particle trajectories.

Contribution

It introduces a refined pulsar emission model incorporating an extended slot gap and current sheet, with a two-step accelerating E-field, to explain VHE observations and light curve evolution.

Findings

Model reproduces flux decrease in the first light-curve peak.

Explains narrowing of pulses with increasing energy.

Predicts larger curvature radii for regions with second peak.

Abstract

The Fermi Large Area Telescope has revolutionised the $\gamma$-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. 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 $\gamma$-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.