791 research outputs found
How to use molecular clouds to study the propagation of cosmic rays in the Galaxy
Observations of molecular clouds in the gamma ray domain provide us with a
tool to study the distribution of cosmic rays in the Galaxy. This is because
cosmic rays can penetrate molecular clouds, undergo hadronic interactions in
the dense gas, and produce neutral pions that in turn decay into gamma rays.
The detection of this radiation allows us to estimate the spectrum and
intensity of cosmic rays at the cloud's position. Remarkably, this fact can be
used to constrain the cosmic ray diffusion coefficient at specific locations in
the Galaxy.Comment: Invited talk, to appear on the proceedings of ICATPP Conference on
Cosmic Rays for Particle and Astroparticle Physics, Villa Olmo, Como 7-8
October 201
The gamma ray background from large scale structure formation
Hierarchical clustering of dark matter halos is thought to describe well the
large scale structure of the universe. The baryonic component of the halos is
shock heated to the virial temperature while a small fraction of the energy
flux through the shocks may be energized through the first order Fermi process
to relativistic energy per particle. It has been proposed that the electrons
accelerated in this way may upscatter the photons of the universal microwave
background to gamma ray energies and indeed generate a diffuse background of
gamma rays that compares well to the observations. In this paper we calculate
the spectra of the particles accelerated at the merger shocks and re-evaluate
the contribution of structure formation to the extragalactic diffuse gamma ray
background (EDGRB), concluding that this contribution adds up to at most 10% of
the observed EDGRB.Comment: 19 pages, 4 figures. A few references and some comments added.
Version in press in Astropart. Phy
Anisotropic CR diffusion and gamma-ray production close to supernova remnants, with an application to W28
Cosmic rays that escape their acceleration site interact with the ambient
medium and produce gamma rays as the result of inelastic proton-proton
collisions. The detection of such diffuse emission may reveal the presence of
an accelerator of cosmic rays, and also constrain the cosmic ray diffusion
coefficient in its vicinity. Preliminary results in this direction have been
obtained in the last years from studies of the gamma-ray emission from
molecular clouds located in the vicinity of supernova remnants, which are the
prime candidate for cosmic ray production. Hints have been found for a
significant suppression of the diffusion coefficient with respect to the
average one in the Galaxy. However, most of these studies rely on the
assumption of isotropic diffusion, which may not be very well justified. Here,
we extend this study to the case in which cosmic rays that escape an
accelerator diffuse preferentially along the magnetic field lines. As a first
approximation, we further assume that particles are strongly magnetized and
that their transport perpendicular to the magnetic field is mainly due to the
wandering of the field lines. The resulting spatial distribution of runaway
cosmic rays around the accelerator is, in this case, strongly anisotropic. An
application of the model to the case of the supernova remnant W28 demonstrates
how the estimates of the diffusion coefficient from gamma-ray observations
strongly depend on the assumptions made on the isotropy (or anisotropy) of
diffusion. For higher levels of anisotropy of the diffusion, larger values of
the diffusion coefficient are found to provide a good fit to data. Thus,
detailed models for the propagation of cosmic rays are needed in order to
interpret in a correct way the gamma-ray observations.Comment: 10 pages, 5 figures, submitted to MNRA
Cosmic ray penetration in diffuse clouds
Cosmic rays are a fundamental source of ionization for molecular and diffuse
clouds, influencing their chemical, thermal, and dynamical evolution. The
amount of cosmic rays inside a cloud also determines the -ray flux
produced by hadronic collisions between cosmic rays and cloud material. We
study the spectrum of cosmic rays inside and outside of a diffuse cloud, by
solving the stationary transport equation for cosmic rays including diffusion,
advection and energy losses due to ionization of neutral hydrogen atoms. We
found that the cosmic ray spectrum inside a diffuse cloud differs from the one
in the interstellar medium for energies smaller than MeV,
irrespective of the model details. Below , the spectrum is harder
(softer) than that in the interstellar medium if the latter is a power law
with larger (smaller) than .Comment: 5 pages, 4 figures. Published in MNRAS Letters. Minor changes to
match the published versio
CRIME - cosmic ray interactions in molecular environments
Molecular clouds act as targets for cosmic rays (CR), revealing their
presence through either gamma-ray emission due to proton-proton interactions,
and/or through the ionization level in the cloud, produced by the CR flux. The
ionization rate is a unique tool, to some extent complementary to the gamma-ray
emission, in that it allows to constrain the CR spectrum especially for
energies below the pion production rate ( MeV). Here we study the
effect of ionization on clouds due to both CR protons and electrons,
using the fully relativistic ionization cross sections, which is important to
correctly account for the contribution due to relativistic CRs. The
contribution to ionization due to secondary electrons is also included
self-consistently. The whole calculation has been implemented into a numerical
code which is publicly accessible through a web-interface. The code also
include the calculation of gamma-ray emission once the CR spectrumComment: 8 pages 2 figures, The 34th International Cosmic Ray Conferenc
Escaping the accelerator; how, when and in what numbers do cosmic rays get out of supernova remnants?
The escape of charged particles accelerated by diffusive shock acceleration
from supernova remnants is shown to be a more complex process than normally
appreciated. Using a box model it is shown that the high-energy end of the
spectrum can exhibit spectral breaks even with no formal escape as a result of
geometrical dilution and changing time-scales. It is pointed out that the bulk
of the cosmic ray particles at lower energies must be produced and released in
the late stages of the remnant's evolution whereas the high energy particles
are produced early on; this may explain recent observations of slight
compositional variations with energy. Escape resulting from ion-neutral
friction in dense and partially ionized media is discussed briefly and some
comments made on the use of so-called "free escape boundary conditions".
Finally estimates are made of the total production spectrum integrated over the
life of the remnant.Comment: To appear in MNRA
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