# High-energy pulsar light curves in an offset polar cap $B$-field geometry

## TL;DR

This study models gamma-ray pulsar light curves using various magnetic and electric field configurations, revealing that field structure and emission geometry significantly influence pulsar visibility and pulse shapes, with implications for understanding pulsar magnetospheres.

## Contribution

It introduces detailed geometric models incorporating offset polar cap magnetic fields and variable electric fields, improving fits to observed gamma-ray pulsar light curves, especially for the Vela pulsar.

## Key findings

- Retarded vacuum dipole field with outer gap model best fits Vela data.
- Larger slot gap E-fields improve light curve fits and suggest stronger actual E-fields.
- Offset polar cap parameter ε influences emission models differently based on emissivity assumptions.

## Abstract

The light curves and spectral properties of more than 200 $\gamma$-ray pulsars have been measured in unsurpassed detail in the eight years since the launch of the hugely successful Fermi Large Area Telescope (LAT) $\gamma$-ray mission. We performed geometric pulsar light curve modelling using static, retarded vacuum, and offset polar cap (PC) dipole $B$-fields (the latter is characterized by a parameter $\epsilon$), in conjunction with standard two-pole caustic (TPC) and outer gap (OG) emission geometries. In addition to constant-emissivity geometric models, we also considered a slot gap (SG) $E$-field associated with the offset-PC dipole $B$-field and found that its inclusion leads to qualitatively different light curves. We therefore find that the assumed $B$-field and especially the $E$-field structure, as well as the emission geometry (magnetic inclination and observer angles), have a great impact on the pulsar's visibility and its high-energy pulse shape. We compared our model light curves to the superior-quality $\gamma$-ray light curve of the Vela pulsar (for energies $>100$ MeV). Our overall optimal light curve fit (with the lowest $\chi^2$ value) is for the retarded vacuum dipole field and OG model. We found that smaller values of $\epsilon$ are favoured for the offset-PC dipole field when assuming constant emissivity, and larger $\epsilon$ values are favoured for variable emissivity, but not significantly so. When we increased the relatively low SG $E$-fields we found improved light curve fits, with the inferred pulsar geometry being closer to best fits from independent studies in this case. In particular, we found that such a larger SG $E$-field (leading to variable emissivity) gives a second overall best fit. This and other indications point to the fact that the actual $E$-field may be larger than predicted by the SG model.