Astrophys.J.661:879-891,2007 If highly efficient, cosmic ray production can have a significant effect on
the X-ray emission from SNRs as well as their dynamical evolution. Using
hydrodynamical ...simulations including diffusive shock acceleration, we produce
spectra for both the thermal and nonthermal forward shock emission. For a given
ambient density and explosion energy, we find that the position of the forward
shock at a given age is a strong function of the acceleration efficiency,
providing a signature of cosmic-ray production. Using an approximate treatment
for the ionization state of the plasma, we investigate the effects of slow vs.
rapid heating of the postshock electrons on the ratio of thermal to nonthermal
X-ray emission at the forward shock. We also investigate the effects of
magnetic field strength on the observed spectrum for efficient cosmic-ray
acceleration. The primary effect of a large field is a considerable flattening
of the nonthermal spectrum in the soft X-ray band. Spectral index measurements
from X-ray observations may thus be indicators of the postshock magnetic field
strength. The predicted gamma-ray flux from inverse-Compton (IC) scattering and
neutral pion decay is strongly affected by the ambient conditions and, for the
particular parameters used in our examples, the IC emission at E ~ 1 TeV
exceeds that from pion decay, although at both lower and higher energies this
trend is reversed for cases of high ambient density. More importantly, high
magnetic fields produce a steepening of the electron spectrum over a wide
energy range which may make it more difficult to differentiate between IC and
pion-decay emission solely by spectral shape.
Mon.Not.Roy.Astron.Soc.350:1174,2004 There is a growing evidence that extended radio halos are most likely
generated by electrons reaccelerated via some kind of turbulence generated in
the cluster ...volume during major mergers. It is well known that Alfv\'en waves
channel most of their energy flux in the acceleration of relativistic
particles. Much work has been done recently to study this phenomenon and its
consequences for the explanation of the observed non-thermal phenomena in
clusters of galaxies. We investigate here the problem of particle-wave
interactions in the most general situation in which relativistic electrons,
thermal protons and relativistic protons exist within the cluster volume. The
interaction of all these components with the waves, as well as the turbulent
cascading and damping processes of Alfv\'en waves, are treated in a fully
time-dependent way. This allows us to calculate the spectra of electrons,
protons and waves at any fixed time. The {\it Lighthill} mechanism is invoked
to couple the fluid turbulence, supposedly injected during cluster mergers, to
MHD turbulence. We find that present observations of non-thermal radiation from
clusters of galaxies are well described within this approach, provided the
fraction of relativistic hadrons in the intracluster medium (ICM) is smaller
than 5-10 %.
Gamma rays from molecular clouds Gabici, Stefano; Aharonian, Felix A.; Blasi, Pasquale
The Multi-Messenger Approach to High-Energy Gamma-Ray Sources
Book Chapter
It is believed that the observed diffuse gamma-ray emission from the galactic plane is the result of interactions between cosmic rays and the interstellar gas. Such emission can be amplified if ...cosmic rays penetrate into dense molecular clouds. The propagation of cosmic rays inside a molecular cloud has been studied assuming an arbitrary energy and space dependent diffusion coefficient. If the diffusion coefficient inside the cloud is significantly smaller compared to the average one derived for the galactic disk, the observed gamma-ray spectrum appears harder than the cosmic ray spectrum, mainly due to the slower penetration of the low energy particles towards the core of the cloud. This may produce a great variety of gamma-ray spectra.
There is a growing evidence that extended radio halos are most likely generated by electrons reaccelerated via some kind of turbulence generated in the cluster volume during major mergers. It is well ...known that Alfvén waves channel most of their energy flux in the acceleration of relativistic particles. Much work has been done recently to study this phenomenon and its consequences for the explanation of the observed non-thermal phenomena in clusters of galaxies. We investigate here the problem of particle-wave interactions in the most general situation in which relativistic electrons, thermal protons and relativistic protons exist within the cluster volume. The interaction of all these components with the waves, as well as the turbulent cascading and damping processes of Alfvén waves, are treated in a fully time-dependent way. This allows us to calculate the spectra of electrons, protons and waves at any fixed time. The {\it Lighthill} mechanism is invoked to couple the fluid turbulence, supposedly injected during cluster mergers, to MHD turbulence. We find that present observations of non-thermal radiation from clusters of galaxies are well described within this approach, provided the fraction of relativistic hadrons in the intracluster medium (ICM) is smaller than 5-10 %.
If highly efficient, cosmic ray production can have a significant effect on the X-ray emission from SNRs as well as their dynamical evolution. Using hydrodynamical simulations including diffusive ...shock acceleration, we produce spectra for both the thermal and nonthermal forward shock emission. For a given ambient density and explosion energy, we find that the position of the forward shock at a given age is a strong function of the acceleration efficiency, providing a signature of cosmic-ray production. Using an approximate treatment for the ionization state of the plasma, we investigate the effects of slow vs. rapid heating of the postshock electrons on the ratio of thermal to nonthermal X-ray emission at the forward shock. We also investigate the effects of magnetic field strength on the observed spectrum for efficient cosmic-ray acceleration. The primary effect of a large field is a considerable flattening of the nonthermal spectrum in the soft X-ray band. Spectral index measurements from X-ray observations may thus be indicators of the postshock magnetic field strength. The predicted gamma-ray flux from inverse-Compton (IC) scattering and neutral pion decay is strongly affected by the ambient conditions and, for the particular parameters used in our examples, the IC emission at E ~ 1 TeV exceeds that from pion decay, although at both lower and higher energies this trend is reversed for cases of high ambient density. More importantly, high magnetic fields produce a steepening of the electron spectrum over a wide energy range which may make it more difficult to differentiate between IC and pion-decay emission solely by spectral shape.
