Gamma-ray spectral properties of mature pulsars: a two-layer model

We use a simple two-layer outer gap model, whose accelerator consists of a primary region and a screening region, to discuss the gamma-ray spectrum of mature pulsars detected by Fermi. By solving the Poisson equation with an assumed simple step-function distribution for the charge density in these two regions, the distribution of the electric field and the curvature radiation process of the accelerated particles can be calculated. In our model, the properties of the phase-averaged spectrum can be completely specified by three gap parameters, i.e., the fractional gap size in the outer magnetosphere, the gap current in the primary region, and the gap size ratio between the primary region and the total gap size. We discuss how these parameters affect the spectral properties. We argue that although the radiation mechanism in the outer gap is a curvature radiation process, the observed gamma-ray spectrum can substantially deviate from the simple curvature spectrum because the overall spectrum consists of two components, i.e., a primary region and a screening region. In some pulsars, the radiation from the screening region is so strong that the photon index from 100 MeV to several GeV can be as flat as similar to 2. We show that the fitting fractional gap thickness of the canonical pulsars increases with the spin-down age. We find that the total gap current is about 50% of the Goldreich-Julian value and the thickness of the screening region is a few percent of the total gap thickness. We also find that the predicted gamma-ray luminosity is less dependent on the spin-down power (L-sd) for pulsars with L-sd greater than or similar to 10(36) erg s(-1), while the gamma-ray luminosity decreases with the spin-down power for pulsars with L-sd less than or similar to 10(36) erg s(-1). This relation may imply that the major gap closure mechanism is a photon-photon pair-creation process for pulsars with L-sd greater than or similar to 10(36) erg s(-1), while it is a magnetic pair-creation process for pulsars with L-sd less than or similar to 10(36) erg s(-1).