- Conference date: 20−22 Sep 1993
- Location: College Park, Maryland (USA)
Because of photon‐photon absorption on the microwave and infrared/optical background radiation fields, VHE (Very High Energy) γ‐rays from sources at cosmological distances (≳100 Mpc) will not be directly visible. The radiation from electromagnetic cascades initiated by the absorption of these γ‐rays, however, is in principle observable. It is commonly assumed that this radiation can only be detected as a diffuse background since even small intergalactic magnetic fields would be sufficient to deflect secondary pairs and their subsequent radiation out of the line of sight to the source. We have examined the effects of pair propagation and find that this is not the case for sources at cosmological distances. A pair deflection length short enough to ensure that most of the cascade does not propagate in a straight line it also short enough to ensure that a significant fraction of its gets deflected back towards the observer. In the ‘‘worst’’ (most likely) case of instant randomization of pair velocities, the cascade develops as an isotropically emitting halo about the source.
The TeV halo emission comes from a region ≲50 Mpc in radius which, for source distance d=1000 Mpc, corresponds to an apparent emission region of angular size ≲3°. The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the VHE source. Limiting or determining the intensity of pair halos provides a bound on or measure of the time integrated VHE power of possible sources like AGN. Additionally, the gamma‐radiation from pair halos contains unique cosmological information about the universal IR/O background and distances to sources. Finally, the synchrotron emission from a pair halo may also be important and could be used as another means of identifying halos.
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