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

Abstract

The light curves and spectral properties of more than 200 γ-ray pulsars have been measured in unsurpassed detail in the eight years since the launch of the hugely successful Fermi Large Area Telescope (LAT) γ-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 ε), 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 γ-ray light curve of the Vela pulsar (for energies > 100 MeV). Our overall optimal light curve fit (with the lowest χ 2 value) is for the retarded vacuum dipole field and OG model. We found that smaller values of ε are favoured for the offset-PC dipole field when assuming constant emissivity, and larger ε 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.