Modeling of the gamma-ray pulsed spectra of Geminga, Crab, and Vela with synchro-curvature radiation

TL;DR

This paper models gamma-ray pulsar spectra using synchro-curvature radiation, fitting observed Fermi-LAT data for Geminga, Crab, and Vela, and highlights the importance of particle dynamics in spectral features.

Contribution

It introduces a physical synchro-curvature model for pulsar gamma-ray spectra and fits it to observational data, providing insights beyond phenomenological approaches.

Findings

Model reproduces observed spectra well except above a few GeV.

Most low-energy radiation (< GeV) originates from early particle trajectory stages.

Inverse Compton scattering may dominate at higher energies, explaining residuals.

Abstract

$\gamma$-ray spectra of pulsars have been mostly studied in a phenomenological way, by fitting them to a cut-off power-law function. Here, we analyze a model where pulsed emission comes from synchro-curvature processes in a gap. We calculate the variation of kinetic energy of magnetospheric particles along the gap and the associated radiated spectra, considering an effective particle distribution. We fit the phase-averaged and phase-resolved Fermi-LAT spectra of the three brightest gamma-ray pulsars: Geminga, Crab, and Vela, and constrain the three free parameters we leave free in the model. Our best-fit models well reproduce the observed data, apart from residuals above a few GeV in some cases, range for which the inverse Compton scattering likely becomes the dominant mechanism. In any case, the flat slope at low-energy (< GeV) seen by Fermi-LAT both in the phase-averaged and phase-resolved spectra of most pulsars, including the ones we studied, requires that most of the detected radiation below ~ GeV is produced during the beginning of the particle trajectories, when radiation mostly come from the loss of perpendicular momentum.