High‐Altitude Emission from Pulsar Slot Gaps: The Crab Pulsar

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Abstract
We present results of a 3D model of optical-to-γ-ray emission from the slot gap accelerator of a rotation-powered pulsar. Primary electrons accelerating to high altitudes in the unscreened electric field of the slot gap reach radiation reaction limited Lorentz factors of ~2 × 107, while electron-positron pairs from lower altitude cascades flow along field lines interior to the slot gap. The curvature, synchrotron, and inverse Compton radiation of both primary electrons and pairs produce a broad spectrum of emission from infrared to GeV energies. Both primaries and pairs undergo cyclotron resonant absorption of radio photons, allowing them to maintain significant pitch angles. Synchrotron radiation from pairs with a power-law energy spectrum from γ = 102 to 105, dominate the spectrum up to ~10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV up to a few GeV. We examine the energy-dependent pulse profiles and phase-resolved spectra for parameters of the Crab pulsar as a function of magnetic inclination α and viewing angle ζ, comparing to broadband data. In most cases, the pulse profiles are dominated by caustics on trailing field lines. We also explore the relation of the high-energy and the radio profiles, as well as the possibility of caustic formation in the radio cone emission. We find that the Crab pulsar profiles and spectrum can be reasonably well reproduced by a model with α = 45° and ζ ~ 100° or 80°. This model predicts that the slot gap emission below 200 MeV will exhibit correlations in time and phase with the radio emission.
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