PhotonPhoton Optical Depth for GammaRay Bursts
Abstract
The photonphoton optical depth for gammaray bursts at cosmological distances is potentially so large that accurate calculations are needed to evaluate the conditions for which high energy photons can escape. One must explicitly include the rest frame emission pattern and determine the photon field at each point along the path of a highenergy 'test' photon to the observer. Relativistic expanding shell models have a key advantage over static relativistic wind models. For an expanding shell there is a sudden decrease of the optical depth with increasing Lorentz factor when the surface that contributes photons to the path of the test photon no longer subtends more than a critical angle regardless of how close the test photon is to the shell. As a result, expanding shell models require smaller relativistic flow rates than stationary surfaces. The minimum Lorentz factor that models must provide is roughly 100 for a 100 MeV photon to escape. We have made two crucial assumptions which require further study. The spectrum has been assumed to be a power law and a twocomponent power law or a power law with a highenergy cutoff would decrease the required gamma. The expanding shell model uses a infinitely thin emitting surface and one with a finite thickness could increase the required gamma.
 Publication:

American Institute of Physics Conference Series
 Pub Date:
 1992
 Bibcode:
 1992AIPC..265..158F
 Keywords:

 Cosmology;
 Gamma Ray Bursts;
 Gamma Ray Observatory;
 Photons;
 Distance;
 Emission Spectra;
 Power Spectra;
 Spaceborne Astronomy;
 Astrophysics