Photon Emission in a Cascade from Relativistic Protons Initiated by Residual Thermal Photons in Gamma-Ray Bursts

Gamma-ray bursts are generally considered to be the result of internal shocks generated in an inhomogeneous relativistic outflow that arises from a fireball. In such shocks, the Fermi acceleration of protons is naturally expected to be at least as efficient as that of electrons. We investigate the consequences of proton acceleration in the residual thermal photon field of a fireball, especially those on the emission spectra of photons. In contrast to most other studies, we do not invoke a direct electron acceleration in shock waves. We show that the residual radiation field of the fireball can ignite the photopion production and subsequent cascade, and that the photons emitted in this process further enhance the photon-initiated cascade. We find that Fermi accelerated protons with $\gtrsim 10^{13}$ eV efficiently bring their energy into pion production and subsequent photon emission. The particles cascading from the pions emit photons over a wide energy range. The photons emitted from electron--positron pairs distribute continuously from the GeV range down to the X-ray range, while muons can emit gamma-rays by synchrotron radiation with a break at around 1--10 MeV. We also discuss several radiation processes which may possibly produce a break feature in the MeV range, as observed in addition to muon synchrotron radiation.Comment: 25 pages, 9 fugures. to appear in Publications of the Astronomical Society of Japan 5

Electric field screening by a proton counterflow in the pulsar polar cap

We propose a new mechanism to screen the electric field in the pulsar polar cap. Previous studies have shown that if an electron beam from the stellar surface is accelerated to energies high enough to create electron-positron pairs, the required electric field parallel to the magnetic field lines is too strong to be screened out by the produced pairs. We argue here that if non-relativistic protons are supplied from the magnetosphere to flow towards the stellar surface, they can provide an anode to screen out such a strong electric field. Injected electron-positron pairs yield an asymmetry of the electrostatic potential around the screening point. The required pair creation rate in this model is consistent with the conventional models.Comment: 11 pages, 4 figures, accepted for publication in Astronomy & Astrophysic