Spectrum and Duration of Delayed MeV-GeV Emission of Gamma-Ray Bursts in Cosmic Background Radiation Fields

We generally analyze prompt high-energy emission above a few hundreds of GeV due to synchrotron self-Compton scattering in internal shocks. However, such photons cannot be detected because they may collide with cosmic infrared background photons, leading to electron/positron pair production. Inverse-Compton scattering of the resulting electron/positron pairs off cosmic microwave background photons will produce delayed MeV-GeV emission, which may be much stronger than a typical high-energy afterglow in the external shock model. We expand on the Cheng & Cheng model by deriving the emission spectrum and duration in the standard fireball shock model. A typical duration of the emission is ~ 10^3 seconds, and the time-integrated scattered photon spectrum is nu^{-(p+6)/4}, where p is the index of the electron energy distribution behind internal shocks. This is slightly harder than the synchrotron photon spectrum, nu^{-(p+2)/2}. The lower energy property of the scattered photon spectrum is dependent on the spectral energy distribution of the cosmic infrared background radiation. Therefore, future observations on such delayed MeV-GeV emission and the higher-energy spectral cutoff by the Gamma-Ray Large Area Space Telescope (GLAST) would provide a probe of the cosmic infrared background radiation.Comment: 5 pages, accepted for publication in Ap

The Afterglow of GRB 990123 and a Dense Medium

Recent observations show that the temporal decay of the R-band afterglow from GRB 990123 steepened about 2.5 days after the burst. We here propose a possible explanation for such a steepening: a shock expanding in a dense medium has undergone the transition from a relativistic phase to a nonrelativistic phase. We find that this model is consistent with the observations if the medium density is about $3\times 10^6 {\rm cm}^{-3}$. By fitting our model to the observed optical and X-ray afterglow quantitatively, we further infer the electron and magnetic energy fractions of the shocked medium and find these two parameters are about 0.1 and $2\times 10^{-8}$ respectively. The former parameter is near the equipartition value while the latter is about six orders of magnitude smaller than inferred from the GRB 970508 afterglow. We also discuss possibilities that the dense medium can be produced.Comment: 12 pages, LaTeX, published in ApJ Letter

Radio Emission from Pulsar Wind Nebulae without Surrounding Supernova Ejecta: Application to FRB 121102

In this paper, we propose a new scenario in which a rapidly-rotating strongly-magnetized pulsar without any surrounding supernova ejecta produces fast radio bursts (FRBs) repeatedly via some mechanisms, and meanwhile, an ultra-relativistic electron/positron pair wind from the pulsar sweeps up its ambient dense interstellar medium, giving rise to a non-relativistic pulsar wind nebula (PWN). We show that the synchrotron radio emission from such a PWN is bright enough to account for the recently-discovered persistent radio source associated with the repeating FRB 121102 in reasonable ranges of the model parameters. In addition, our PWN scenario is consistent with the non-evolution of the dispersion measure inferred from all the repeating bursts observed in four years.Comment: 6 pages, 1 figure, ApJ Letters in pres

Gamma-Ray Burst Afterglows from Realistic Fireballs

A GRB afterglow has been commonly thought to be due to continuous deceleration of a postburst fireball. Many analytical models have made simplifications for deceleration dynamics of the fireball and its radiation property, although they are successful at explaining the overall features of the observed afterglows. We here propose a model for a GRB afterglow in which the evolution of a postburst fireball is in an intermediate case between the adiabatic and highly radiative expansion. In our model, the afterglow is both due to the contribution of the adiabatic electrons behind the external blastwave of the fireball and due to the contribution of the radiative electrons. In addition, this model can describe evolution of the fireball from the extremely relativistic phase to the non-relativistic phase. Our calculations show that the fireball will go to the adiabatic expansion phase after about a day if the accelerated electrons are assumed to occupy the total internal energy. In all cases considered, the fireball will go to the mildly relativistic phase about $10^4$ seconds later, and to the non-relativistic phase after several days. These results imply that the relativistic adiabatic model cannot describe the deceleration dynamics of the several-days-later fireball. The comparison of the calculated light curves with the observed results at late times may imply the presence of impulsive events or energy injection with much longer durations.Comment: 18 pages, 10 figures, plain latex file, submitted to Ap