Modeling of gamma-ray pulsar light curves using the force-free magnetic field

TLDR: In this article, the authors present the first results of gamma-ray pulsar light curve modeling using the more realistic eld taken from three-dimensional force-free magnetospheric simulations.

Abstract: Gamma-ray emission from pulsars has long been modeled using a vacuum dipole eld. This approximation ignores changes in the eld structure caused by the magnetospheric plasma and strong plasma currents. We present the rst results of gamma-ray pulsar light curve modeling using the more realistic eld taken from three-dimensional force-free magnetospheric simulations. Having the geometry of the eld, we apply several prescriptions for the location of the emission zone, comparing the light curves to observations. We nd that when the emission region is chosen according to the conventional slot-gap (or two-pole caustic) prescription, the model fails to produce double-peak pulse proles, mainly because the size of the polar cap in force-free magnetosphere is larger than the vacuum eld polar cap. This suppresses caustic formation in the inner magnetosphere. The conventional outer-gap model is capable of producing only one peak under general conditions, because a large fraction of open eld lines does not cross the null charge surface. We propose a novel \separatrix layer" model, where the high-energy emission originates from a thin layer on the open eld lines just inside of the separatrix that bounds the open ux tube. The emission from this layer generates two strong caustics on the sky map due to the eect we term \Sky Map Stagnation" (SMS). It is related to the fact that the force-free eld asymptotically approaches the eld of a rotating split monopole, and the photons emitted on such eld lines in the outer magnetosphere arrive to the observer in phase. The double-peak light curve is a natural consequence of SMS. We show that most features of the currently available gamma-ray pulsar light curves can be reasonably well reproduced and explained with the separatrix layer model using the force-free eld. Association of the emission region with the current sheet will guide more detailed future studies of the magnetospheric acceleration physics. Subject headings: MHD | pulsars: general | gamma-rays: theory | stars: magnetic elds