A Revisit of the Two-Pole Caustic Model for GeV Light Curves of Gamma-Ray Pulsars

TL;DR

This paper revisits and revises the two-pole caustic model to better explain the GeV light curves of gamma-ray pulsars, incorporating photon-photon pair production constraints, and successfully applies it to four pulsars observed by Fermi.

Contribution

The paper introduces a revised two-pole caustic model that accounts for pair production limits on gap extension and applies it to multiple pulsars, improving light curve reproduction.

Findings

The model successfully reproduces the main features of observed light curves.

Effective azimuthal gap extension is about 230 degrees.

Radial distances of radio pulses are estimated for four pulsars.

Abstract

The GeV light curve of a pulsar is an important probe to detect acceleration regions in its magnetosphere. Motivated by the recent reports on the observations of pulsars by {\it Fermi} Large Area Telescope (LAT), we restudy the two-pole caustic model and revise it to investigate the properties of the light curves in the GeV band. In the revised model, although acceleration gaps can extend from the star surface to the light cylinder along near the last open field lines, the extension of the gaps along the azimuthal direction is limited because of photon-photon pair production process. In such gaps, high-energy photons are emitted uniformly and tangentially to the field lines but cannot be efficiently produced along these field lines where the distances to the null charge surface are larger than $\sim0.9$ times of the distance of the light cylinder, and the effective azimuth extension of the gaps is about $230^\circ$. The model is applied to the four pulsars Vela, PSR J1028-5819, PSR J0205+6449, and PSR J2021+3651 whose light curves obtained with {\it Fermi} have been recently released. The model is successful in reproducing the general feature of the light curves for the four pulsars, and the radial distances of the radio pulse for the four pulsars are estimated.