198 research outputs found
Cosmic-Ray Induced Diffuse Emissions from the Milky Way and Local Group Galaxies
Cosmic rays fill up the entire volume of galaxies, providing an important
source of heating and ionisation of the interstellar medium, and may play a
significant role in the regulation of star formation and galactic evolution.
Diffuse emissions from radio to high-energy gamma rays (> 100 MeV) arising from
various interactions between cosmic rays and the interstellar medium,
interstellar radiation field, and magnetic field, are currently the best way to
trace the intensities and spectra of cosmic rays in the Milky Way and other
galaxies. In this contribution, I describe our recent work to model the full
spectral energy distribution of galaxies like the Milky Way from radio to
gamma-ray energies. The application to other galaxies, in particular the
Magellanic Clouds and M31 that are detected in high-energy gamma-rays by the
Fermi-LAT, is also discussed.Comment: Contribution to "The Spectral Energy Distribution of Galaxies"
Proceedings IAU Symposium No. 284, 2011, eds. R.J. Tuffs & C.C.Popescu. 4
pages with 4 figure
Inverse Compton scattering on solar photons, heliospheric modulation, and neutrino astrophysics
We study the inverse Compton scattering of solar photons by Galactic
cosmic-ray electrons. We show that the gamma-ray emission from this process is
substantial with the maximum flux in the direction of the Sun; the angular
distribution of the emission is broad. This previously-neglected foreground
should be taken into account in studies of the diffuse Galactic and
extragalactic gamma-ray emission. Furthermore, observations by GLAST can be
used to monitor the heliosphere and determine the electron spectrum as a
function of position from distances as large as Saturn's orbit to close
proximity of the Sun, thus enabling unique studies of solar modulation. This
paves the way for the determination of other Galactic cosmic-ray species,
primarily protons, near the solar surface which will lead to accurate
predictions of gamma rays from pp-interactions in the solar atmosphere. These
albedo gamma rays will be observable by GLAST, allowing the study of deep
atmospheric layers, magnetic field(s), and cosmic-ray cascade development. The
latter is necessary to calculate the neutrino flux from pp-interactions at
higher energies (>1 TeV). Although this flux is small, it is a "guaranteed
flux" in contrast to other astrophysical sources of neutrinos, and may be
detectable by km^3 neutrino telescopes of the near future, such as IceCube.
Since the solar core is opaque for very high-energy neutrinos, directly
studying the mass distribution of the solar core may thus be possible.Comment: 4 pages, 4 figures, emulateapj.cls, final version; published in ApJ
Letters, added an erratum; conclusions unchange
Dark Matter Searches with Astroparticle Data
The existence of dark matter (DM) was first noticed by Zwicky in the 1930s,
but its nature remains one of the great unsolved problems of physics. A variety
of observations indicate that it is non-baryonic and non-relativistic. One of
the preferred candidates for non-baryonic DM is a weakly interacting massive
particle (WIMP) that in most models is stable. WIMP self-annihilation can
produce cosmic rays, gamma rays, and other particles with signatures that may
be detectable. Hints of anomalous cosmic-ray spectra found by recent
experiments, such as PAMELA, have motivated interesting interpretations in
terms of DM annihilation and/or decay. However, these signatures also have
standard astrophysical interpretations, so additional evidence is needed in
order to make a case for detection of DM annihilation or decay. Searches by the
Fermi Large Area Telescope for gamma-ray signals from clumps, nearby dwarf
spheroidal galaxies, and galaxy clusters have also been performed, along with
measurements of the diffuse Galactic and extragalactic gamma-ray emission. In
addition, imaging atmospheric Cherenkov telescopes like HESS, MAGIC, and
VERITAS have reported on searches for gamma-ray emission from dwarf galaxies.
In this review, we examine the status of searches for particle DM by these
instruments and discuss the interpretations and resulting DM limits.Comment: Solicited review article to appear in Annual Reviews of Astronomy and
Astrophysics. 52 pages, 10 figures (higher resolution figures will appear in
the journal article
The Gamma-ray Albedo of the Moon
We use the GEANT4 Monte Carlo framework to calculate the gamma-ray albedo of
the Moon due to interactions of cosmic ray (CR) nuclei with moon rock. Our
calculation of the albedo spectrum agrees with the EGRET data. We show that the
spectrum of gamma rays from the Moon is very steep with an effective cutoff
around 3-4 GeV (600 MeV for the inner part of the Moon disk) and exhibits a
narrow pion-decay line at 67.5 MeV, perhaps unique in astrophysics. Apart from
other astrophysical sources, the albedo spectrum of the Moon is well
understood, including its absolute normalisation; this makes it a useful
"standard candle" for gamma-ray telescopes. The steep albedo spectrum also
provides a unique opportunity for energy calibration of gamma-ray telescopes,
such as the forthcoming Gamma Ray Large Area Space Telescope (GLAST). Since the
albedo flux depends on the incident CR spectrum which changes over the solar
cycle, it is possible to monitor the CR spectrum using the albedo gamma-ray
flux. Simultaneous measurements of CR proton and helium spectra by the Payload
for Antimatter-Matter Exploration and Light-nuclei Astrophysics (PAMELA), and
observations of the albedo gamma rays by the GLAST Large Area Telescope (LAT),
can be used to test the model predictions and will enable the LAT to monitor
the CR spectrum near the Earth beyond the lifetime of the PAMELA.Comment: 6 pages, 4 figures, emulateapj.cls; to appear in the Astrophysical
Journa
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
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