Constraints on Emission and Field Geometry of Gamma-ray Pulsars from Phase Resolved Spectroscopy and Light Curve Modeling

The Fermi Large Area Telescope (LAT) has revolutionized the field of gamma-ray pulsar astronomy. At this time, the LAT has detected gamma-ray pulsations from 117 pulsars, many of which are undetected at radio wavelengths or are millisecond pulsars newly discovered in previously unassociated Fermi sources. These high quality observations have opened a new window to understanding the generation mechanisms of high-energy pulsar emission. In particular, the excellent statistics allow for careful modeling of the light curve features as well as for phase-resolved spectral modeling, the results of which contain information about the structure and geometry of pulsar magnetospheres and emission zones. We present phase resolved spectroscopy and light curve modeling for a selection of gamma-ray pulsars. We combine our results to infer the emission geometry and estimate the magnitude of the accelerating parallel electric field and its dependence on emission radius. Simulated light curves, which we generate using geometrical representations of the outer gap and slot gap emission models within the magnetic field structures of the vacuum retarded dipole and force-free magnetosphere, are used to model the LAT light curves and find the best fit values of the magnetic inclination angle, viewing angle, maximum emission radius, and gap width. To estimate the accelerating field we assume that the high-energy emission is dominated by curvature radiation, so that the spectral cutoff energy depends on the parallel electric field and curvature radius, and that the geometry is determined by the best fit light curves for each model. We find that while the observed light curves can be fit comparably well for most combinations of the emission and field geometries, the force-free field produces larger phase lags between the radio and modeled gamma-ray peaks than are observed. However, the straighter field lines, and hence larger field line radii of curvature, of the force-free model are needed to achieve physically realistic values for the accelerating field. As pulsar magnetosphere modeling proceeds to explore solutions between vacuum and force-free, these physical diagnostics can be used to constrain the true field structure.