A common four-beam geometry reveals altitude-stratified GeV pulses in canonical young pulsars

TL;DR

This study uses Fermi LAT data to model gamma-ray pulse profiles of young pulsars with a four-beam geometric template, revealing altitude-stratified emission regions and their physical characteristics.

Contribution

It introduces a geometry-first, mechanism-agnostic model that decomposes pulsar gamma-ray emission into altitude-separated beam pairs, linking observed features to magnetospheric structures.

Findings

Pulse profiles can be decomposed into two altitude-separated beam pairs.

Lower-altitude emission is consistent with curvature-dominated outer magnetosphere.

Higher-altitude emission suggests synchrotron processes in a current sheet-like outflow.

Abstract

Despite the diversity and energy dependence of $\gamma$-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 \textit{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 $\simeq 0.7$ (Crab, $\approx 1$~kyr) to $\simeq 1.1$--$1.4$ light-cylinder radii (Vela and Dragonfly, $\approx 10$~kyr). This unified four-beam, observation-driven geometry maps an altitude-dependent azimuthal tilt of pulsed $\gamma$-ray emission, providing an observationally anchored framework amenable to systematic tests and readily extensible to other young pulsars.