1,192 research outputs found
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
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
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 eV.Comment: 9 pages, 2 figures, in Deciphering the Ancient Universe with GRBs,
Kyoto, Japan, April 19-23, 201
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