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

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.