1,162 research outputs found
Cosmic Rays from Gamma Ray Bursts in the Galaxy
The rate of terrestrial irradiation events by galactic gamma-ray bursts
(GRBs) is estimated using recent standard-energy results. We assume that GRBs
accelerate high-energy cosmic rays, and present results of three-dimensional
simulations of cosmic rays moving in the Galactic magnetic field and diffusing
through pitch-angle scattering. An on-axis GRB extinction event begins with a
powerful prompt gamma-ray and neutron pulse, followed by a longer-lived phase
from cosmic-ray protons and neutron-decay protons that diffuse towards Earth.
Our results force a reinterpretation of reported ~ 10^{18} eV cosmic-ray
anisotropies and offer a rigorous test of the model where high-energy cosmic
rays originate from GRBs, which will soon be tested with the Auger Observatory.Comment: 9 pages, 4 figures, ApJ Letters, in press. Clarified limit of
test-particle approximation, prediction that Auger will not confirm SUGAR
source. (Data may not appear onscreen at low magnification.) Simulations at
http://heseweb.nrl.navy.mil/gamma/~dermer/invest/sim/index.ht
Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039
Recent HESS observations show that microquasars in high-mass systems are
sources of VHE gamma-rays. A leptonic jet model for microquasar gamma-ray
emission is developed. Using the head-on approximation for the Compton cross
section and taking into account angular effects from the star's orbital motion,
we derive expressions to calculate the spectrum of gamma rays when nonthermal
jet electrons Compton-scatter photons of the stellar radiation field.
Calculations are presented for power-law distributions of nonthermal electrons
that are assumed to be isotropically distributed in the comoving jet frame, and
applied to -ray observations of LS 5039. We conclude that (1) the TeV
emission measured with HESS cannot result only from Compton-scattered stellar
radiation (CSSR), but could be synchrotron self-Compton (SSC) emission or a
combination of CSSR and SSC; (2) fitting both the HESS data and the EGRET data
associated with LS 5039 requires a very improbable leptonic model with a very
hard electron spectrum. Because the gamma rays would be variable in a leptonic
jet model, the data sets are unlikely to be representative of a simultaneously
measured gamma-ray spectrum. We therefore attribute EGRET gamma rays primarily
to CSSR emission, and HESS gamma rays to SSC emission. Detection of periodic
modulation of the TeV emission from LS 5039 would favor a leptonic SSC or
cascade hadron origin of the emission in the inner jet, whereas stochastic
variability alone would support a more extended leptonic model. The puzzle of
the EGRET gamma rays from LS 5039 will be quickly solved with GLAST. (Abridged)Comment: 17 pages, 11 figures, ApJ, in press, June 1, 2006, corrected eq.
Blazar synchrotron emission of instantaneously power-law injected electrons under linear synchrotron, non-linear SSC, and combined synchrotron-SSC cooling
The broadband SEDs of blazars show two distinct components which in leptonic
models are associated with synchrotron and SSC emission of highly relativistic
electrons. In some sources the SSC component dominates the synchrotron peak by
one or more orders of magnitude implying that the electrons mainly cool by
inverse Compton collisions with their self-made synchrotron photons. Therefore,
the linear synchrotron loss of electrons, which is normally invoked in emission
models, has to be replaced by a nonlinear loss rate depending on an energy
integral of the electron distribution. This modified electron cooling changes
significantly the emerging radiation spectra. It is the purpose of this work to
apply this new cooling scenario to relativistic power-law distributed
electrons, which are injected instantaneously into the jet. We will first solve
the differential equation of the volume-averaged differential number density of
the electrons, and then discuss their temporal evolution. Since any non-linear
cooling will turn into linear cooling after some time, we also calculated the
electron number density for a combined cooling scenario consisting of both the
linear and non-linear cooling. For all cases, we will also calculate
analytically the emerging optically thin synchrotron fluence spectrum which
will be compared to a numerical solution. For small normalized frequencies f <
1 the fluence spectra show constant spectral indices. We find for linear
cooling a_SYN = 1/2, and for non-linear cooling a_SSC = 3/2. In the combined
cooling scenario we obtain for the small injection parameter b_1 = 1/2, and for
the large injection parameter b_2 = 3/2, which becomes b_1 = 1/2 for very small
frequencies, again. This is the same behaviour as for monoenergetically
injected electrons.Comment: 24 pages, 25 figures, submitted to A&
The obscured gamma-ray and UHECR universe
Auger results on clustering of > 60 EeV ultra-high energy cosmic ray (UHECR)
ions and the interpretation of the gamma-ray spectra of TeV blazars are
connected by effects from the extragalactic background light (EBL). The EBL
acts as an obscuring medium for gamma rays and a reprocessing medium for UHECR
ions and protons, causing the GZK cutoff. The study of the physics underlying
the coincidence between the GZK energy and the clustering energy of UHECR ions
favors a composition of > 60 EeV UHECRs in CNO group nucleons. This has
interesting implications for the sources of UHECRs. We also comment on the
Auger analysis.Comment: 11 pages, 10 figures, in the International Conference on Topics in
Astroparticle and Underground Physics (TAUP) 2007, Sendai, Japan, September
11-15, 200
Best-Bet Astrophysical Neutrino Sources
Likely astrophysical sources of detectable high-energy (>> TeV) neutrinos are
considered. Based on gamma-ray emission properties, the most probable sources
of neutrinos are argued to be GRBs, blazars, microquasars, and supernova
remnants. Diffuse neutrino sources are also briefly considered.Comment: 6 pages, 2 figures, in Proc. of TeV-Particle Astrophysics II,
Madison, WI, 28-31 Aug, 200
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