A model of unpulsed very high energy gamma rays from the crab nebula and pulsar

Abstract

The angular resolution of gamma-ray detectors does not allow one to separate the nebula from the pulsar in the Crab. It is generally assumed that the steady emission of gamma rays comes from the nebula. Using the "outer magnetospheric gap" model, we propose an alternative mechanism in which the steady emission of gamma rays could come from a compact region, a couple of light cylinder radii beyond the pulsar. In that region, the magnetic field has been transformed from a dipole field to a swirling field. In the model, two mono-energetic beams of gap primary e+(e-) if Ω · Bs > 0 (<0) with a Lorentz factor ∼ 4 × 107 are streaming out from the outer gaps through the light cylinder. The gap primary e+(e-) beam from the first outer gap can produce ∼10 TeV primary very high energy (VHE) gamma rays through inverse Compton scattering with the IR photons from the second gap, and vice versa. The primary VHE gamma rays are sufficiently energetic to produce secondary e± pairs by collision with the same pulsed IR photons. Since the mean free path of these conversion processes is comparable to the local radius of curvature, the pitch angles of the secondary pairs are large enough to result in isotropic secondary VHE gamma rays with Eγ ∼ TeV. Comparisons between the model spectra and the observed data are discussed in the text.