1,204 research outputs found
A New Determination Of The Diffuse Galactic and Extragalactic Gamma-Ray Emission
The GALPROP model for cosmic-ray propagation is able to make explicit
predictions for the distribution of galactic diffuse gamma-rays. We compare
different propagation models with gamma-ray spectra measured by EGRET for
various regions of the sky. This allows sensitive tests of alternative
explanations for the apparent excess emission observed at GeV gamma-rays. We
find that a population of hard-spectrum gamma-ray sources cannot be solely
responsible for the excess since it also appears at high latitudes; on the
other hand a hard cosmic-ray electron spectrum cannot explain the gamma-ray
excess in the inner Galaxy. By normalizing the cosmic ray spectra within
reasonable bounds under preservation of their shape we are able to obtain our
best prediction of the Galactic component of diffuse gamma rays, and show that
away from the Galactic plane it gives an accurate prediction of the observed
gamma-ray intensities. On this basis we reevaluate the extragalactic gamma-ray
background. We find that for some energies previous work underestimated the
Galactic contribution and hence overestimated the background. The new EGRB
spectrum shows a positive curvature similar to that expected for models of the
extragalactic gamma-ray emission based on contributions from unresolved
blazars.Comment: 6 pages, 3 figures, 1 tabl
Propagation in 3D spiral-arm cosmic-ray source distribution models and secondary particle production using PICARD
We study the impact of possible spiral-arm distributions of Galactic
cosmic-ray sources on the flux of various cosmic-ray nuclei throughout our
Galaxy. We investigate model cosmic-ray spectra at the nominal position of the
sun and at different positions within the Galaxy. The modelling is performed
using the recently introduced numerical cosmic ray propagation code
\textsc{Picard}. Assuming non-axisymmetric cosmic ray source distributions
yields new insights on the behaviour of primary versus secondary nuclei.
We find that primary cosmic rays are more strongly confined to the vicinity
of the sources, while the distribution of secondary cosmic rays is much more
homogeneous compared to the primaries. This leads to stronger spatial variation
in secondary to primary ratios when compared to axisymmetric source
distribution models. A good fit to the cosmic-ray data at Earth can be
accomplished in different spiral-arm models, although leading to decisively
different spatial distributions of the cosmic-ray flux. This results in very
different cosmic ray anisotropies, where even a good fit to the data becomes
possible. Consequently, we advocate directions to seek best fit propagation
parameters that take into account the higher complexity introduced by the
spiral-arm structure on the cosmic-ray distribution. We specifically
investigate whether the flux at Earth is representative for a large fraction of
the Galaxy. The variance among possible spiral-arm models allows us to quantify
the spatial variation of the cosmic-ray flux within the Galaxy in presence of
non-axisymmetric source distributions.Comment: 38 pages, 16 figures, accepted for publication in Astroparticle
Physic
Inverse-Compton emission from halos around stars
Inverse Compton scattering by relativistic electrons produces a major
component of the diffuse emission from the Galaxy. The photon fields involved
are the cosmic microwave background and the interstellar radiation field from
stars and dust. Calculations of the inverse Compton distribution have usually
assumed a smooth ISRF, but in fact a large part of the Galactic luminosity
comes from the most luminous stars which are rare. Therefore we expect the
ISRF, and hence the inverse Compton emission, to be clumpy. We also show that
some of the most luminous stars may be visible to GLAST. In this paper we give
an update on our previous work including examples of the intensity distribution
around stars, and the predicted spectrum of Cygnus OB2.Comment: 4 pages, 5 figures, Proceedings of the 30th ICRC 2007 Caption of
figure 5 update
Developing the Galactic diffuse emission model for the GLAST Large Area Telescope
Diffuse emission is produced in energetic cosmic ray (CR) interactions,
mainly protons and electrons, with the interstellar gas and radiation field and
contains the information about particle spectra in distant regions of the
Galaxy. It may also contain information about exotic processes such as dark
matter annihilation, black hole evaporation etc. A model of the diffuse
emission is important for determination of the source positions and spectra.
Calculation of the Galactic diffuse continuum gamma-ray emission requires a
model for CR propagation as the first step. Such a model is based on theory of
particle transport in the interstellar medium as well as on many kinds of data
provided by different experiments in Astrophysics and Particle and Nuclear
Physics. Such data include: secondary particle and isotopic production cross
sections, total interaction nuclear cross sections and lifetimes of radioactive
species, gas mass calibrations and gas distribution in the Galaxy (H_2, H I, H
II), interstellar radiation field, CR source distribution and particle spectra
at the sources, magnetic field, energy losses, gamma-ray and synchrotron
production mechanisms, and many other issues. We are continuously improving the
GALPROP model and the code to keep up with a flow of new data. Improvement in
any field may affect the Galactic diffuse continuum gamma-ray emission model
used as a background model by the GLAST LAT instrument. Here we report about
the latest improvements of the GALPROP and the diffuse emission model.Comment: 2 pages, 2 figures; to appear in the Proc. of the First Int. GLAST
Symp. (Stanford, Feb. 5-8, 2007), eds. S.Ritz, P.F.Michelson, and C.Meegan,
AIP Conf. Pro
Diffuse emission measurement with INTEGRAL/SPI as indirect probe of cosmic-ray electrons and positrons
Significant advances have been made in the understanding of the diffuse
Galactic hard X-ray continuum emission using data from the INTEGRAL
observatory. The diffuse hard power-law component seen with the INTEGRAL/SPI
spectrometer has been identified with inverse-Compton emission from
relativistic (GeV) electrons on the cosmic microwave background and Galactic
interstellar radiation field. In the present analysis, SPI data from 2003 to
2009, with a total exposure time of ~ 10^8 s, are used to derive the Galactic
ridge hard X-ray spatial distribution and spectrum between 20 keV and 2.4 MeV.
Both are consistent with predictions from the GALPROP code. The good agreement
between measured and predicted emission from keV to GeV energies suggests that
the correct production mechanisms have been identified. We discuss the
potential of the SPI data to provide an indirect probe of the interstellar
cosmic-ray electron distribution, in particular for energies below a few GeV.Comment: 39 pages, 11 figures. Accepted for publication in The Astrophysical
Journa
Inverse Compton Emission from Galactic Supernova Remnants: Effect of the Interstellar Radiation Field
The evidence for particle acceleration in supernova shells comes from
electrons whose synchrotron emission is observed in radio and X-rays. Recent
observations by the HESS instrument reveal that supernova remnants also emit
TeV gamma-rays; long awaited experimental evidence that supernova remnants can
accelerate cosmic rays up to the ``knee'' energies. Still, uncertainty exists
whether these gamma-rays are produced by electrons via inverse Compton
scattering or by protons via neutral pion decay. The multi-wavelength spectra
of supernova remnants can be fitted with both mechanisms, although a preference
is often given to neutral pion decay due to the spectral shape at very high
energies. A recent study of the interstellar radiation field indicates that its
energy density, especially in the inner Galaxy, is higher than previously
thought. In this paper we evaluate the effect of the interstellar radiation
field on the inverse Compton emission of electrons accelerated in a supernova
remnant located at different distances from the Galactic Centre. We show that
contribution of optical and infra-red photons to the inverse Compton emission
may exceed the contribution of cosmic microwave background and in some cases
broaden the resulted gamma-ray spectrum. Additionally, we show that if a
supernova remnant is located close to the Galactic Centre its gamma-ray
spectrum will exhibit a ``universal'' cutoff at very high energies due to the
Klein-Nishina effect and not due to the cut-off of the electron spectrum. As an
example, we apply our calculations to the supernova remnants RX J1713.7-3946
and G0.9+0.1 recently observed by HESS.Comment: 4 pages, 4 figures. Uses emulateapj.cls. Accepted by ApJ
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