Unstable GRB photospheres and e^± annihilation lines

We propose an emission mechanism of prompt gamma-ray bursts (GRBs) that can reproduce the observed nonthermal spectra with high radiative efficiencies, >150%. Internal dissipation below a photosphere can create a radiation-dominated thermal fireball. If e^± pairs outnumber protons, radiative acceleration of e^± pairs drives the two-stream instabilities between pairs and protons, leading to "proton sedimentation" in the accelerating pair frame. Pairs are continuously shock heated by proton clumps, scattering the thermal photons into a broken-power-law shape, with a nonthermal energy that is comparable to the proton kinetic energy, consistent with observations. Pair photospheres become unstable around the radius of the progenitor star where strong thermalization occurs, if parameters satisfy the observed spectral (Yonetoku) relation. Pair annihilation lines are predicted above continua, which could be verified by GLAST.