Gamma-ray telescopes have reported some surprising observations of multiTeV photons from distant active galactic nuclei (AGN), which show no significant attenuation due to pair production off the extragalactic background light (EBL). Furthermore, some sources are believed to have such a high intrinsic density of low-energy photons that the very high energy (VHE) photons should have been absorbe...

MeV seed photons produced in shocks in a variable ultra-relativistic outflow gain energy by the Fermi mechanism, because the photons Compton scatter off relativistically colliding shells. The Fermi-modified high-energy photon spectrum has a non-universal slope and a universal cutoff. A significant increase in the total radiative efficiency is possible. In some gamma ray bursts, most of the powe...

We have calculated the opacities and secondary production mechanisms of high energy photons arising in gamma-ray burst internal shocks, using exact cross-sections for the relevant processes. We find that for reasonable choices of parameters, photons in the range of 10’s to 100’s of GeV may be emitted in the prompt phase. Photons above this range are subject to electron-positron pair production ...

When a brass compensator is set in a treatment beam, beam hardening may take place. This variation of the energy spectrum may affect the accuracy of dose calculation by a treatment planning system and the results of dose measurement of brass compensator intensity modulated radiation therapy (IMRT). In addition, when X-rays pass the compensator, scattered photons are generated within the compens...

Recent results, obtained by the H1 and ZEUS collaborations, are presented on differential cross sections , for inclusive prompt photon production in DIS and for photoproduction of prompt photons accompanied by a hadronic jet. Also presented are cross sections of normalised scaled momentum distribution of charged final state hadrons, measured by H1 in DIS ep collisions at high Q 2 in the Breit f...

Many quantum gravity theories imply that the vacuum is filled with virtual black holes. This paper explores the process in which high energy photons interact with virtual black holes and decay into gravitons and photons of lower energy. The effect requires violation (or modification) of Lorentz invariance and implies that high energy photons cannot propagate over arbitrarily large distances. Fo...

A solution is presented for the spectrum of high-energy GRB photons confined to a quasithermal baryonic photosphere. The solution is valid in the steady-state limit assuming the region under consideration is optically thick to the continuously injected photons. It is shown that for a high luminosity photosphere, the non-thermal electrons resulting from γ-ray Compton cooling lose their energy by...

Models where relativistic jets from merger or accretion induced collapse in compact binary systems produce cosmological gamma ray bursts (GRBs) also predict that GRBs are accompanied by delayed emission of high energy photons, TeV neutrinos, X-rays, optical photons and radio waves. Their emission mechanisms are similar to those responsible for their emissions by blazars during gamma ray flares....