We find that in a typical fireball of cosmological gamma-ray burst (GRB) a neutron component is initially present and has the energy comparable with that of the proton-electron component. Velocity decoupling of the neutron component is accompanied by pion production in inelastic proton-neutron collisions and generation of gamma-quanta in subsequent decay of neutral pions. These quanta are blueshifted into 100 GeV domain.
Emission of 100 GeV photons precedes the main pulse of gamma-radiation by a time interval approximately equal to the duration of the burst itself and contains of the total GRB energy. This is more than sufficient for detection of the faintest bursts by means of modern Cherenkov telescopes. The observation of prompt energetic gamma-rays will allow direct studying of physical conditions in fireballs.
Ultra-high energy quanta are also produced during the main GRB pulse due to inverse Comptonization of synchrotron radiation by relativistic electrons in shock waves. We show that the comptonized photons carry at least 10 per cent of the total energy and typically are well above 1 TeV. However, they are absorbed by infrared background radiation and cannot be observed from a cosmological distance.
At the same time, the ultra-high energy radiation may be absorbed in the close vicinity of the source due to interaction with ultraviolet and soft X-ray quanta from the same source scattered in an ambient medium. In this case almost all the energy initially contained in the ultra-hard radiation is reprocessed into a softer spectral range corresponding to the two-photon absorption threshold. The observation as well as the absence of the absorption/reprocessing of the ultra-high energy photons would place a rather strong limits on the density of interstellar medium near the source and the Lorentz factor of GRB fireball.
Fifth Huntsville Gamma Ray Burst Symposium
Hunsville, Alabama, USA
18-22 October, 1999