Abstract
Despite the diversity and energy dependence of γ-ray pulse morphologies in Crab, Vela and Dragonfly, the phaseograms of these three canonical young pulsars can be organised within a single four-beam geometric template. Using Fermi Large Area Telescope data, we fit the 60 MeV--3 GeV phaseograms with a mechanism-agnostic, geometry-first parametric model that incorporates phase-dependent Doppler shifts and constrains the three-dimensional locations and bulk motions of four emission sites. In each pulsar, the phaseogram admits a decomposition into two altitude-separated beam pairs. The lower-altitude pair is produced by plasma with bulk motion close to azimuthal corotation, sharpening the main peaks. The higher-altitude pair shows a radially outward bulk-motion component, suggestive of inertial effects in a toroidally dominated magnetic field, and contributes bridge/shoulder emission and ripple-like modulations overlapping the main peaks. As a posteriori, the lower-altitude pair is consistent with curvature-dominated outer-magnetospheric emission, while the higher-altitude pair is consistent with synchrotron-dominated emission from a current-sheet-like outflow. Higher-altitude site heights vary from ≃0.7 (Crab, ≈1~kyr) to ≃1.1--1.4 light-cylinder radii (Vela and Dragonfly, ≈10~ kyr). This unified four-beam, observation-driven geometry maps an altitude-dependent azimuthal tilt of pulsed γ-ray emission, providing an observationally anchored framework amenable to systematic tests and readily extensible to other young pulsars.
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