Gamma-Ray Burst without Baryonic and Magnetic Load?

We show that, contrary to common belief, internal shocks can arise in an accelerating radiation-dominated jet if it is confined even weakly to a converging opening angle because the acceleration declines. The radiation-dominated internal shock (RDIS) enables a very efficient yet highly nonthermal emission by Fermi-like photon acceleration, keeping the electron-positron ($e^{\pm}$) pair photosphere and inertia up to a high Lorentz factor >1000. In gamma-ray bursts (GRBs), a weak confinement would persist beyond the progenitor star or surrounding matter because of the fast cocoon accompanying the breakout jet. The simplest model predicts few high-energy cosmic rays and neutrinos, and a correlation between the early afterglow and the GeV-TeV prompt emission. The central engine allows a less fine-tuned baryon load than previously thought, even including pure-leptonic unmagnetized outflows.