Fermi Gamma-Ray Pulsars: Understanding the High-Energy Emission from Dissipative Magnetospheres

Abstract

Based on the Fermi observational data we reveal meaningful constraints for the dependence of the macroscopic conductivity $(σ)$ of dissipative pulsar magnetosphere models on the corresponding spin-down rate, $\dot{\mathcal{E}}$. Our models are refinements of the FIDO (Force-Free Inside, Dissipative Outside) models whose dissipative regions are restricted on the equatorial current-sheet outside the light-cylinder. Taking into account the observed cutoff-energies of all the Fermi-pulsars and assuming that a) the corresponding $γ-$ray pulsed emission is due to curvature radiation at the radiation-reaction-limit regime and b) this emission is produced at the equatorial current-sheet near the light-cylinder, we show that the \emph{Fermi}-data provide clear indications about the corresponding accelerating electric-field components. A direct comparison between the \emph{Fermi} cutoff-energies and the model ones reveals that $σ$ increases with $\dot{\mathcal{E}}$ for high $\dot{\mathcal{E}}$-values while it saturates for low ones. This comparison indicates also that the corresponding gap-width increases toward low $\dot{\mathcal{E}}$-values. Assuming the Goldreich-Julian flux for the emitting particles we calculate the total $γ-$ray luminosity $(L_γ)$. A comparison between the dependence of the Fermi $L_γ$-values and the model ones on $\dot{\mathcal{E}}$ indicates an increase of the emitting particle multiplicity with $\dot{\mathcal{E}}$. Our modeling guided by the \emph{Fermi}-data alone, enhances our understanding of the physical mechanisms behind the high energy emission in pulsar magnetospheres.