25 research outputs found
Fine structure in the gamma-ray sky
The EGRET results for gamma-ray intensities in and near the Galactic Plane
have been analysed in some detail. Attention has been concentrated on energies
above 1 GeV and the individual intensities in a longitude bin have
been determined and compared with the large scale mean found from a nine-degree
polynomial fit.
Comparison has been made of the observed standard deviation for the ratio of
these intensities with that expected from variants of our model. The basic
model adopts cosmic ray origin from supernova remnants, the particles then
diffusing through the Galaxy with our usual 'anomalous diffusion'. The variants
involve the clustering of SN, a frequency distribution for supernova explosion
energies, and 'normal', rather than 'anomalous' diffusion.
It is found that for supernovae of unique energy, and our usual anomalous
diffusion, clustering is necessary, particularly in the Inner Galaxy. An
alternative, and preferred, situation is to adopt the model with a frequency
distribution of supernova energies. The results for the Outer Galaxy are such
that no clustering is required.Comment: 10 pages, 4 figures, 1 table, accepted for publication in J.Phys.G:
Nucl.Part.Phy
Diffuse continuum gamma rays from the Galaxy
A new study of the diffuse Galactic gamma-ray continuum radiation is
presented, using a cosmic-ray propagation model which includes nucleons,
antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of
the inverse Compton (IC) scattering includes the effect of anisotropic
scattering in the Galactic interstellar radiation field (ISRF) and a new
evaluation of the ISRF itself. Models based on locally measured electron and
nucleon spectra and synchrotron constraints are consistent with gamma-ray
measurements in the 30-500 MeV range, but outside this range excesses are
apparent. A harder nucleon spectrum is considered but fitting to gamma rays
causes it to violate limits from positrons and antiprotons. A harder
interstellar electron spectrum allows the gamma-ray spectrum to be fitted above
1 GeV as well, and this can be further improved when combined with a modified
nucleon spectrum which still respects the limits imposed by antiprotons and
positrons. A large electron/IC halo is proposed which reproduces well the
high-latitude variation of gamma-ray emission. The halo contribution of
Galactic emission to the high-latitude gamma-ray intensity is large, with
implications for the study of the diffuse extragalactic component and
signatures of dark matter. The constraints provided by the radio synchrotron
spectral index do not allow all of the <30 MeV gamma-ray emission to be
explained in terms of a steep electron spectrum unless this takes the form of a
sharp upturn below 200 MeV. This leads us to prefer a source population as the
origin of the excess low-energy gamma rays.Comment: Final version accepted for publication in The Astrophysical Journal
(vol. 537, July 10, 2000 issue); Many Updates; 20 pages including 49
ps-figures, uses emulateapj.sty. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
A Measurement of the Spatial Distribution of Diffuse TeV Gamma Ray Emission from the Galactic Plane with Milagro
Diffuse -ray emission produced by the interaction of cosmic-ray
particles with matter and radiation in the Galaxy can be used to probe the
distribution of cosmic rays and their sources in different regions of the
Galaxy. With its large field of view and long observation time, the Milagro
Gamma Ray Observatory is an ideal instrument for surveying large regions of the
Northern Hemisphere sky and for detecting diffuse -ray emission at very
high energies. Here, the spatial distribution and the flux of the diffuse
-ray emission in the TeV energy range with a median energy of 15 TeV
for Galactic longitudes between 30 and 110 and between
136 and 216 and for Galactic latitudes between -10 and
10 are determined. The measured fluxes are consistent with predictions
of the GALPROP model everywhere except for the Cygnus region
(). For the Cygnus region, the flux is twice the
predicted value. This excess can be explained by the presence of active cosmic
ray sources accelerating hadrons which interact with the local dense
interstellar medium and produce gamma rays through pion decay.Comment: 15 pages, 3 figures, accepted by Ap
Positrons from particle dark-matter annihilation in the Galactic halo: propagation Green's functions
We have made a calculation of the propagation of positrons from dark-matter
particle annihilation in the Galactic halo in different models of the dark
matter halo distribution using our 3D code, and present fits to our numerical
propagation Green's functions. We show that the Green's functions are not very
sensitive to the dark matter distribution for the same local dark matter energy
density. We compare our predictions with computed cosmic ray positron spectra
(``background'') for the ``conventional'' CR nucleon spectrum which matches the
local measurements, and a modified spectrum which respects the limits imposed
by measurements of diffuse Galactic gamma-rays, antiprotons, and positrons. We
conclude that significant detection of a dark matter signal requires favourable
conditions and precise measurements unless the dark matter is clumpy which
would produce a stronger signal. Although our conclusion qualitatively agrees
with that of previous authors, it is based on a more realistic model of
particle propagation and thus reduces the scope for future speculations.
Reliable background evaluation requires new accurate positron measurements and
further developments in modelling production and propagation of cosmic ray
species in the Galaxy.Comment: 8 pages, 6 ps-figures, 3 tables, uses revtex. Accepted for
publication in Physical Review D. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Production and propagation of cosmic-ray positrons and electrons
We have made a new calculation of the cosmic-ray secondary positron spectrum
using a diffusive halo model for Galactic cosmic-ray propagation. The code
computes self-consistently the spectra of primary and secondary nucleons,
primary electrons, and secondary positrons and electrons. The models are first
adjusted to agree with the observed cosmic-ray Boron/Carbon ratio, and the
interstellar proton and Helium spectra are then computed; these spectra are
used to obtain the source function for the secondary positrons/electrons which
are finally propagated with the same model parameters. The primary electron
spectrum is evaluated, again using the same model. Fragmentation and energy
losses are computed using realistic distributions for the interstellar gas and
radiation fields, and diffusive reacceleration is also incorporated. Our study
includes a critical re-evaluation of the secondary decay calculation for
positrons.
The predicted positron fraction is in good agreement with the measurements up
to 10 GeV, beyond which the observed flux is higher than that calculated. Since
the positron fraction is now accurately measured in the 1-10 GeV range our
primary electron spectrum should be a good estimate of the true interstellar
spectrum in this range, of interest for gamma ray and solar modulation studies.
We further show that a harder interstellar nucleon spectrum, similar to that
suggested to explain EGRET diffuse Galactic gamma ray observations above 1 GeV,
can reproduce the positron observations above 10 GeV without requiring a
primary positron component.Comment: 25 pages including 8 figures and 1 table, latex, aaspp4.sty. To be
published in ApJ 1998, v.493 (February 1 issue). Details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
GLAST: Understanding the High Energy Gamma-Ray Sky
We discuss the ability of the GLAST Large Area Telescope (LAT) to identify,
resolve, and study the high energy gamma-ray sky. Compared to previous
instruments the telescope will have greatly improved sensitivity and ability to
localize gamma-ray point sources. The ability to resolve the location and
identity of EGRET unidentified sources is described. We summarize the current
knowledge of the high energy gamma-ray sky and discuss the astrophysics of
known and some prospective classes of gamma-ray emitters. In addition, we also
describe the potential of GLAST to resolve old puzzles and to discover new
classes of sources.Comment: To appear in Cosmic Gamma Ray Sources, Kluwer ASSL Series, Edited by
K.S. Cheng and G.E. Romer
Diffuse Gamma Rays: Galactic and Extragalactic Diffuse Emission
"Diffuse" gamma rays consist of several components: truly diffuse emission
from the interstellar medium, the extragalactic background, whose origin is not
firmly established yet, and the contribution from unresolved and faint Galactic
point sources. One approach to unravel these components is to study the diffuse
emission from the interstellar medium, which traces the interactions of high
energy particles with interstellar gas and radiation fields. Because of its
origin such emission is potentially able to reveal much about the sources and
propagation of cosmic rays. The extragalactic background, if reliably
determined, can be used in cosmological and blazar studies. Studying the
derived "average" spectrum of faint Galactic sources may be able to give a clue
to the nature of the emitting objects.Comment: 32 pages, 28 figures, kapproc.cls. Chapter to the book "Cosmic
Gamma-Ray Sources," to be published by Kluwer ASSL Series, Edited by K. S.
Cheng and G. E. Romero. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm