Off‐Axis Emission from Relativistic Plasma Flows

We show that there is no universal law describing how the spectra and luminosity of synchrotron and inverse Compton radiation from relativistic jets change with increasing observation angle. Instead, the physics of particle acceleration leaves pronounced imprints in the observed spectra and allows for a freedom in numerous modifications of them. The impact of these effects is the strongest for high-energy radiation and depends on the details of the particle acceleration mechanism(s). Thus, the observed impact is sensitive to the models' details, and one can use that sensitivity to discriminate between various (but maybe rather similar) models. Generally, the beam patterns of relativistic jets in the GeV-TeV spectral domain are much wider than the inverse Lorentz factor. The off-axis emission in this energy range appears to be brighter and has a much harder spectrum and a much higher cutoff frequency compared to the values derived from Doppler-boosting considerations alone. The implications include the possibility of explaining high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic jet sources, such as blazars, and the prediction of GeV-TeV afterglow from transient jet sources, such as gamma-ray bursts. We also discuss the phenomenon of beam pattern broadening as applied to neutrino emission.