External Shock Model for Gamma-Ray Bursts during the Prompt Phase

The hard X-ray and gamma-ray phenomenology of gamma-ray bursts (GRBs) can be explained by an external shock model where a single relativistic blast wave interacts with the surrounding medium. Besides reproducing the generic spectral behavior of GRB profiles, the external shock model provides quantitative fits to the peak flux distribution, the > 1 s t_{50} duration distribution, and the distribution of the peaks E_pk of the vF_v spectra of GRBs measured with BATSE. The apparent paradox between a relativistic beaming scenario and the empirical finding that E_pk values are preferentially measured within the triggering range of a GRB detector is resolved by this model when blast wave physics and detector triggering criteria are taken into account. Some surprising implications follow, namely that the fireball event rate is ~1 per 10^4 years per Milky Way galaxy for unbeamed sources, and proportionally more if fireball outflows are collimated. This is ~ 3 orders of magnitude larger than normally quoted. Most of the clean and dirty fireball transients are undetected due to telescope sensitivity and design limitations. Strongly variable GRB time histories with good radiative efficiencies are possible because of the strongly enhanced emissions when a blast wave interacts with density inhomogeneities located nearly along the line-of-sight to the observer. Arguments against short timescale variability in an external shock model are answered, and difficulties in an internal shock/colliding shell model are mentioned.Comment: 10 pages, 9 figures; invited review for 5th Huntsville Gamma-Ray Burst Workshop, ed. by M. Kippen, R. Mallozzi, and G. Fishma

Impact of Fermi-LAT and AMS-02 results on cosmic-ray astrophysics

This article reviews a few topics relevant to Galactic cosmic-ray astrophysics, focusing on the recent AMS-02 data release and Fermi Large Area Telescope data on the diffuse Galactic gamma-ray emissivity. Calculations are made of the diffuse cosmic-ray induced p+p --> pi^0 --> 2 gamma spectra, normalized to the AMS-02 cosmic-ray proton spectrum at ~ 10 - 100 GV, with and without a hardening in the cosmic-ray proton spectrum at rigidities R >~ 300 GV. A single power-law momentum "shock" spectrum for the local interstellar medium cosmic-ray proton spectrum cannot be ruled out from the gamma-ray emissivity data alone without considering the additional contribution of electron bremsstrahlung. Metallicity corrections are discussed, and a maximal range of nuclear enhancement factors from 1.52 to 1.92 is estimated.Origins of the 300 GV cosmic-ray proton and alpha-particle hardening are discussed.Comment: In SuGAR2015, Searching for the sources of Galactic Cosmic Rays, University of Geneva, 21-23 January 2015; eds. E. Prandini & S. Toscano; 6 pages, 3 figure

First Light on GRBs with Fermi

Fermi LAT (Large Area Telescope) and GBM (Gamma ray Burst Monitor) observations of GRBs are briefly reviewed, keeping in mind EGRET expectations. Using gamma\gamma constraints on outflow Lorentz factors, leptonic models are pitted against hadronic models, and found to be energetically favored. Interpretation of the Fermi data on GRBs helps establish whether GRBs accelerate cosmic rays, including those reaching $\approx 10^{20}$ eV.Comment: 9 pages, 2 figures, in Deciphering the Ancient Universe with GRBs, Kyoto, Japan, April 19-23, 201