The effect of different magnetospheric structures on predictions of gamma-ray pulsar light curves

Abstract

The field of -ray pulsars has been revolutionised by the launch of the Fermi Large Area Telescope, increasing the population from 7 to 161 detected pulsars. The light curves of these y-ray pulsars exhibit a variety of profile shapes, and also different relative phase lags with respect to their radio pulses. We investigated the impact of different magnetospheric structures on the predicted y-ray pulsar light curve characteristics. We performed geometric light curve modelling utilising the static dipole, retarded vacuum dipole, and a symmetric offset dipole field (characterised by a parameter E), in conjunction with standard emission geometries, i.e., the two-pole caustic (with the slot gap its physical representation), and outer gap models (assuming uniform emissivity). This offset dipole field is a heuristic model that mimics deviations from the static dipole (corresponding to E = 0). We also considered a slot gap electric field for this case, which modulates the y-ray emissivity. We solved the particle transport equation and found that the particle energy only became large enough to yield significant curvature radiation at large altitudes above the stellar surface, given the relatively low electric field (in the case of the Vela pulsar). Therefore, the particles did not always attain the radiation-reaction limit (where the acceleration rate balances the radiation loss rate). The B-field structure and emission geometry determined the pulsar’s visibility and its pulse shape. For the symmetric offset dipole field we observed a small but noticeable effect in the phase plots and light curves for larger E (for both the constant and variable emissivity cases). We noted that the inclusion of the slot gap electric field led to qualitatively different light curves compared to those produced by the geometric models. We fitted our model light curves to the superior-quality y-ray light curve of the Vela pulsar (for energies > 100 MeV) for each B-field and geometric model combination using a X2 fitting method. We found an overall optimal fit for the retarded vacuum dipole field and outer gap model combination. For the retarded vacuum dipole field, the two-pole caustic model was statistically disfavoured compared to all other model combinations, since the Vela light curve possesses low off-peak emission. For the static dipole field, neither geometric model was significantly preferred. We lastly found that the offset dipole field favour smaller values of E for constant emissivity and larger E values for variable emissivity, but not significantly so.