Probing the High-energy $\gamma$-ray Emission Mechanism in the Vela Pulsar via Phase-resolved Spectral and Energy-dependent Light Curve Modeling

Abstract

Recent kinetic simulations sparked a debate regarding the emission mechanism responsible for pulsed GeV $\gamma$-ray emission from pulsars. Some models invoke curvature radiation, while other models assume synchrotron radiation in the current-sheet. We interpret the curved spectrum of the Vela pulsar as seen by H.E.S.S. II (up to $\sim$100 GeV) and the $Fermi$ Large Area Telescope (LAT) to be the result of curvature radiation due to primary particles in the pulsar magnetosphere and current sheet. We present phase-resolved spectra and energy-dependent light curves using an extended slot gap and current sheet model, invoking a step function for the accelerating electric field as motivated by kinetic simulations. We include a refined calculation of the curvature radius of particle trajectories in the lab frame, impacting the particle transport, predicted light curves, and spectra. Our model reproduces the decrease of the flux of the first 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 explain the first of these trends because we find 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 that the spectral cutoff of the second peak is correspondingly larger. However, an unknown azimuthal dependence of the $E$-field as well as uncertainty in the precise spatial origin of the GeV emission, precludes a simplistic discrimination of emission mechanisms.

24 pages, 19 figures, accepted by ApJ

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Cited by in corpus (8)

• The Third Fermi Large Area Telescope Catalog of Gamma-ray Pulsars
• Discovery of a Radiation Component from the Vela Pulsar Reaching 20 Teraelectronvolts
• Synthetic pulsar lightcurves from global kinetic simulations and comparison with the Fermi-LAT catalog
• Observational connection of non-thermal X-ray emission from pulsars with their timing properties and thermal emission
• The Vela pulsar and its pulsar wind nebula Vela-X using 13 years of Fermi-LAT Observations
• Towards Modelling AR Sco: Generalised Particle Dynamics and Strong Radiation-Reaction Regimes
• Towards Modelling AR Sco: Calibration -- Reproducing High-Energy Pulsar Emission and Testing Convergence to Aristotelian Electrodynamics
• Multi-wavelength emission in resistive pulsar magnetospheres