While from the energetic point of view supernova remnants are viable sources of Galactic cosmic r... more While from the energetic point of view supernova remnants are viable sources of Galactic cosmic rays (CRs), the issue of whether they can accelerate protons up to a few PeV remains unsolved. Here we discuss particle acceleration at the forward shock of supernovae, and discuss the possibility that the current of escaping particles may excite a non-resonant instability that in turn leads to the formation of resonant modes that confine particles close to the shock, thereby increasing the maximum energy. This mechanism is at work throughout the expansion of the supernova explosion, from the ejecta dominated (ED) phase to the Sedov-Taylor (ST) phase. The transition from one stage to the other reflects in a break in the spectrum of injected particles. Because of their higher explosion rate, we focus our work on type II SNe expanding in the slow, dense wind, produced by the red supergiant progenitor stars. When the explosion occurs in such winds, the transition between the ED and the ST phase is likely to take place within a few tens of years. The highest energies are reached at even earlier times, when, however, a small fraction of the mass of ejecta has been processed. As a result, the spectrum of accelerated particles shows a break in the slope, at an energy that is the maximum energy (EM ) achieved at the beginning of the ST phase. Above this characteristic energy, the spectrum becomes steeper but remains a power law rather than developing an exponential cutoff. An exponential cut is eventually present at much higher energies but it does not have a phenomenological relevance. We show that for parameters typical of type II supernovae, EM for protons can easily reach values in the PeV range, confirming that type II SNRs are the best candidate sources for CRs at the knee.
There is compelling evidence that at least some clusters of galaxies are powerful sources of non-... more There is compelling evidence that at least some clusters of galaxies are powerful sources of non-thermal radiation. In all cases where this radiation has been detected, a general trend is that high energy densities of cosmic rays and correspondingly low values of the average magnetic field are required in order to have a self-consistent picture of the multiwavelength observations. Mergers of clusters of galaxies might provide large enough cosmic ray injection rates and at the same time provide the mechanism for the heating of the intracluster medium to the observed temperatures. In this paper we analyze critically all the components that play a role in the non-thermal emission of clusters during a merger, with special attention to mergers occurred in the past of the cluster. We outline the consequences of this model for high energy gamma ray observations and for Faraday rotation measurements of the intracluster magnetic field.
We discuss the small scale anisotropy signal predicted for present and future UHECR experiments s... more We discuss the small scale anisotropy signal predicted for present and future UHECR experiments such as Auger, Telescope Array, Auger North and EUSO. We relax the unrealistic, but commonly used assumption that the sources are all equal and we concentrate our attention on how the expected signal depends on possible distributions of the properties of the sources such as a
Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is sup... more Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is super-alfvenic, thereby producing the conditions for their own diffusion. We present the results of a self-consistent solution of the transport equation where diffusion occurs because of the self-generated turbulence together with a preexisting turbulence injected, for instance, by supernova explosions and cascading to smaller scales. All chemicals are included in our calculations, so that we are able to show the secondary to primary ratios in addition to the spectra of the individual elements. All predictions appear to be in good agreement with observations.
The Multi-Messenger Approach to High-Energy Gamma-Ray Sources, 2007
It is believed that the observed diffuse gamma ray emission from the galactic plane is the result... more 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.
Proceedings of the International Astronomical Union, 2006
We review some recent progresses in semi-analytic kinetic approaches to the problem of non linear... more We review some recent progresses in semi-analytic kinetic approaches to the problem of non linear particle acceleration at non relativistic shock waves.
Pulsars, or more generally rotation powered neutron stars, are excellent factories of antimatter ... more Pulsars, or more generally rotation powered neutron stars, are excellent factories of antimatter in the Galaxy, in the form of pairs of electrons and positrons. Electrons are initially extracted from the surface of the star by the intense rotation induced electric fields and later transformed into electron-positron pairs through electromagnetic cascading. Observations of Pulsar Wind Nebulae (PWNe) show that cascades in the pulsar magnetosphere must ensure pair multiplicities of order 10 4 − 10 5 . These pairs finally end up as part of the relativistic magnetized wind emanating from the pulsar. The wind is slowed down, from its highly relativistic bulk motion, at a termination shock, which represents the reverse shock due to its interaction with the surrounding ejecta of the progenitor supernova. At the (relativistic) termination shock, acceleration of the pairs occurs, as part of the dissipation process, so that the cold wind is transformed into a plasma of relativistic non-thermal particles, plus a potential thermal component, which however has never been observed. As long as the pulsar wind is embedded in the supernova remnant these pairs are forced to escavate a bubble and lose energy adiabatically (because of the expansion) and radiatively (because of magnetic and radiation fields). We discuss here the observational constraints on the energy and number content of such pairs and discuss the scenarios that may allow for the pairs to escape in the interstellar medium and possibly contribute to the positron excess that has recently been detected by the PAMELA satellite. Special attention is dedicated to the case of nebulae surrounding high speed pulsars, observationally classified as Pulsar Bow Shock Nebulae. The pairs produced in these objects may be effectively carried out of the Supernova Remnant and released in the Interstellar Medium. As a result, Bow Shock Pulsar Wind Nebulae might be the main contributors to the positron excess in the Galaxy.
We show that the complex shape of the cosmic ray (CR) spectrum, as recently measured by PAMELA an... more We show that the complex shape of the cosmic ray (CR) spectrum, as recently measured by PAMELA and inferred from Fermi-LAT γ-ray observations of molecular clouds in the Gould belt, can be naturally understood in terms of basic plasma astrophysics phenomena. A break from a harder to a softer spectrum at rigidity R is approximately equal to 10 GV follows from a transition from transport dominated by advection of particles with Alfvén waves to a regime where diffusion in the turbulence generated by the same CRs is dominant. A second break at R is approximately equal to 200 GV happens when the diffusive propagation is no longer determined by the self-generated turbulence, but rather by the cascading of externally generated turbulence (for instance due to supernova bubbles) from large spatial scales to smaller scales where CRs can resonate. Implications of this scenario for the cosmic ray spectrum, grammage, and anisotropy are discussed.
Nonthermal radiation is observed from clusters of galaxies in the radio, hard X-rays, and possibl... more Nonthermal radiation is observed from clusters of galaxies in the radio, hard X-rays, and possibly in the soft X-ray/UV bands. While it is known that radiative processes related to nonthermal electrons are responsible for this radiation, the sites and nature of particle acceleration are not known. We investigate here the acceleration of protons and electrons in the shocks originated during mergers of clusters of galaxies, where the Fermi acceleration may work. We propose a semi-analytical model to evaluate the Mach number of the shocks generated during clusters mergers and we use this procedure to determine the spectrum of the accelerated particles for each one of the shocks produced during the merger history of a cluster. We follow the proton component, accumulated over cosmological time scales, and the short lived electron component. We conclude that efficient particle acceleration, resulting in nonthermal spectra that compare to observations, occurs mainly in minor mergers, namely mergers between clusters with very different masses. Major mergers, often invoked to be sites for the production of extended radio halos, are found to have on average too weak shocks and are unlikely to result in appreciable nonthermal activity.
According to high resolution cold dark matter (CDM) simulations, large virialized halos are forme... more According to high resolution cold dark matter (CDM) simulations, large virialized halos are formed through the constant merging of smaller halos formed at earlier times. In particular, the halo of our Galaxy may have hundreds of dark matter clumps. The annihilation of dark matter particles such as the neutralino in these clumps generates γ-ray fluxes that can potentially be detected by future experiments such as GLAST. We find that, depending on the parameters of the
Efficient cosmic-ray production can have a significant effect on the evolution and X-ray emission... more Efficient cosmic-ray production can have a significant effect on the evolution and X-ray emission from SNRs. Using hydrodynamic simulations coupled to diffusive shock acceleration, we produce thermal and nonthermal forwardshock photon spectra. 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 versus 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 flattening of the nonthermal spectrum in the soft X-ray band. X-ray spectral index measurements 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 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. Subject headingg s: cosmic rays -MHD -supernova remnants -turbulence 1 We do not consider emission from heated dust or from a compact object in this paper.
Clusters of galaxies are revealing themselves as powerful sources of non thermal radiation in a w... more Clusters of galaxies are revealing themselves as powerful sources of non thermal radiation in a wide range of wavelengths. In order to account for these multifrequency observations equipartition of cosmic rays (CRs) with the thermal gas in clusters of galaxies is often invoked. This condition might suggest a dynamical role played by cosmic rays in the virialization of these large scale structures and is now testable through gamma ray observations. We show here, in the specific case of the Coma and Virgo clusters, for which upper limits on the gamma ray emission exist, that equipartition implies gamma ray fluxes that are close or even in excess of the EGRET limit, depending on the adopted model of CR injection. We use this bound to limit the validity of the equipartition condition. We also show that, contrary to what claimed in previous calculations, the equipartition assumption implies gamma ray fluxes in the TeV range which can be detectable even by currently operating gamma ray observatories if the injection cosmic ray spectrum is flatter than E −2.4 .
The probability that a particle, crossing the shock along a given direction, be reflected backwar... more The probability that a particle, crossing the shock along a given direction, be reflected backwards along another direction, was shown to be the key element in determining the spectrum of non-thermal particles accelerated via the Fermi mechanism around a plane-parallel shock in the test-particle limit. Here an explicit equation for this probability distribution is given, for both the upstream and downstream sections. Though analytically intractable, this equation is solved numerically, allowing the determination of the spectrum in full generality, without limitation to shock speed or scattering properties. A number of cases is then computed, making contact with previous numerical work, in all regimes: Newtonian, trans-relativistic, and fully relativistic.
We calculate the distribution of electrons in clusters of galaxies, resulting from thermalization... more We calculate the distribution of electrons in clusters of galaxies, resulting from thermalization processes in the presence of stochastic acceleration due to plasma waves. We show that the electron distribution can deviate from a Maxwell-Boltzmann distribution, due to the effect of the stochastic energy gain, provided waves can be sustained against damping. The non-thermal tail of electrons can generate as bremsstrahlung emission a flux of hard X-rays compatible with the ones recently detected in some clusters of galaxies.
... cosmological magnetic field very well. In fact, Oren & Wolfe (1995) show how sensitive th... more ... cosmological magnetic field very well. In fact, Oren & Wolfe (1995) show how sensitive the redshift dependence of the variance is to different binning procedures and how the observed distribution is non-Gaussian. The trends in the ...
One century ago Viktor Hess carried out several balloon flights that led him to conclude that the... more One century ago Viktor Hess carried out several balloon flights that led him to conclude that the penetrating radiation responsible for the discharge of electroscopes was of extraterrestrial origin. One century from the discovery of this phenomenon seems to be a good time to stop and think about what we have understood about Cosmic Rays. The aim of this review is to illustrate the ideas that have been and are being explored in order to account for the observable quantities related to cosmic rays and to summarize the numerous new pieces of observation that are becoming available. In fact, despite the possible impression that development in this field is somewhat slow, the rate of new discoveries in the last decade or so has been impressive, and mainly driven by beautiful pieces of observation. At the same time scientists in this field have been able to propose new, fascinating ways to investigate particle acceleration inside the sources, making use of multifrequency observations that range from the radio, to the optical, to X-rays and gamma rays. These ideas can now be confronted with data.
The decay of very heavy metastable relics of the Early Universe can produce ultrahigh energy cosm... more The decay of very heavy metastable relics of the Early Universe can produce ultrahigh energy cosmic rays (UHECRs) in the halo of our own Galaxy. On distance scales of the order of the halo size, energy losses are negligible-no Greisen-Zatsepin-Kuzmin cutoff is expected. In this letter we show that, as a consequence of the hierarchical build up of the halo, this scenario predicts the existence of small scale anisotropies in the arrival directions of UHECRs. We also suggest some consequences of this scenario which will be testable with upcoming experiments, as Auger.
We discuss the signatures of the transition from galactic to extragalactic cosmic rays in differe... more We discuss the signatures of the transition from galactic to extragalactic cosmic rays in different scenarios, giving most attention to the dip scenario. The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons in the energy range 1 × 10 18 − 4 × 10 19 eV, which is caused by electron-positron pair production on the cosmic microwave background (CMB) radiation. The dip scenario provides a simple physical description of the transition from galactic to extragalactic cosmic rays. Here we summarize the signatures of the pair production dip model for the transition, most notably the spectrum, the anisotropy and the chemical composition. The main focus of our work is however on the description of the features that arise in the elongation rate and in the distribution of the depths of shower maximum Xmax in the dip scenario. We find that the curve for Xmax(E) shows a sharp increase with energy, which reflects a sharp transition from an iron dominated flux at low energies to a proton dominated flux at E ∼ 10 18 eV. We also discuss in detail the shape of the Xmax distributions for cosmic rays of given energy and demonstrate that this represents a powerful tool to discriminate between the dip scenario and other possible models of the transition. PACS numbers: 12.60.Jv, 95.35.+d, 98.35.Gi
The spatial clustering of galaxies in galaxy clusters implies that the background of infrared (IR... more The spatial clustering of galaxies in galaxy clusters implies that the background of infrared (IR) light in the intracluster medium (ICM) may exceed the universal background. Cosmic rays injected within the ICM propagate diffusively and at low enough energies are trapped there for cosmological times. The photopion production interactions of cosmic rays with the IR photons are responsible for the generation of neutrinos whose detection may shed some light on the origin and propagation of high energy cosmic rays in the universe. Here we discuss our calculations of the flux of neutrinos from single clusters as well as the contribution of photopion production in clusters of galaxies to the diffuse neutrino background.
A recently proposed model explains the rise in energy of the positron fraction measured by the PA... more A recently proposed model explains the rise in energy of the positron fraction measured by the PAMELA satellite in terms of hadronic production of positrons in aged supernova remnants, and acceleration therein. Here we present a preliminary calculation of the antiproton flux produced by the same mechanism. While the model is consistent with present data, a rise of the antiproton to proton ratio is predicted at high energy, which strikingly distinguishes this scenario from other astrophysical explanations of the positron fraction (such as pulsars). We briefly discuss important implications for dark matter searches via antimatter.
While from the energetic point of view supernova remnants are viable sources of Galactic cosmic r... more While from the energetic point of view supernova remnants are viable sources of Galactic cosmic rays (CRs), the issue of whether they can accelerate protons up to a few PeV remains unsolved. Here we discuss particle acceleration at the forward shock of supernovae, and discuss the possibility that the current of escaping particles may excite a non-resonant instability that in turn leads to the formation of resonant modes that confine particles close to the shock, thereby increasing the maximum energy. This mechanism is at work throughout the expansion of the supernova explosion, from the ejecta dominated (ED) phase to the Sedov-Taylor (ST) phase. The transition from one stage to the other reflects in a break in the spectrum of injected particles. Because of their higher explosion rate, we focus our work on type II SNe expanding in the slow, dense wind, produced by the red supergiant progenitor stars. When the explosion occurs in such winds, the transition between the ED and the ST phase is likely to take place within a few tens of years. The highest energies are reached at even earlier times, when, however, a small fraction of the mass of ejecta has been processed. As a result, the spectrum of accelerated particles shows a break in the slope, at an energy that is the maximum energy (EM ) achieved at the beginning of the ST phase. Above this characteristic energy, the spectrum becomes steeper but remains a power law rather than developing an exponential cutoff. An exponential cut is eventually present at much higher energies but it does not have a phenomenological relevance. We show that for parameters typical of type II supernovae, EM for protons can easily reach values in the PeV range, confirming that type II SNRs are the best candidate sources for CRs at the knee.
There is compelling evidence that at least some clusters of galaxies are powerful sources of non-... more There is compelling evidence that at least some clusters of galaxies are powerful sources of non-thermal radiation. In all cases where this radiation has been detected, a general trend is that high energy densities of cosmic rays and correspondingly low values of the average magnetic field are required in order to have a self-consistent picture of the multiwavelength observations. Mergers of clusters of galaxies might provide large enough cosmic ray injection rates and at the same time provide the mechanism for the heating of the intracluster medium to the observed temperatures. In this paper we analyze critically all the components that play a role in the non-thermal emission of clusters during a merger, with special attention to mergers occurred in the past of the cluster. We outline the consequences of this model for high energy gamma ray observations and for Faraday rotation measurements of the intracluster magnetic field.
We discuss the small scale anisotropy signal predicted for present and future UHECR experiments s... more We discuss the small scale anisotropy signal predicted for present and future UHECR experiments such as Auger, Telescope Array, Auger North and EUSO. We relax the unrealistic, but commonly used assumption that the sources are all equal and we concentrate our attention on how the expected signal depends on possible distributions of the properties of the sources such as a
Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is sup... more Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is super-alfvenic, thereby producing the conditions for their own diffusion. We present the results of a self-consistent solution of the transport equation where diffusion occurs because of the self-generated turbulence together with a preexisting turbulence injected, for instance, by supernova explosions and cascading to smaller scales. All chemicals are included in our calculations, so that we are able to show the secondary to primary ratios in addition to the spectra of the individual elements. All predictions appear to be in good agreement with observations.
The Multi-Messenger Approach to High-Energy Gamma-Ray Sources, 2007
It is believed that the observed diffuse gamma ray emission from the galactic plane is the result... more 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.
Proceedings of the International Astronomical Union, 2006
We review some recent progresses in semi-analytic kinetic approaches to the problem of non linear... more We review some recent progresses in semi-analytic kinetic approaches to the problem of non linear particle acceleration at non relativistic shock waves.
Pulsars, or more generally rotation powered neutron stars, are excellent factories of antimatter ... more Pulsars, or more generally rotation powered neutron stars, are excellent factories of antimatter in the Galaxy, in the form of pairs of electrons and positrons. Electrons are initially extracted from the surface of the star by the intense rotation induced electric fields and later transformed into electron-positron pairs through electromagnetic cascading. Observations of Pulsar Wind Nebulae (PWNe) show that cascades in the pulsar magnetosphere must ensure pair multiplicities of order 10 4 − 10 5 . These pairs finally end up as part of the relativistic magnetized wind emanating from the pulsar. The wind is slowed down, from its highly relativistic bulk motion, at a termination shock, which represents the reverse shock due to its interaction with the surrounding ejecta of the progenitor supernova. At the (relativistic) termination shock, acceleration of the pairs occurs, as part of the dissipation process, so that the cold wind is transformed into a plasma of relativistic non-thermal particles, plus a potential thermal component, which however has never been observed. As long as the pulsar wind is embedded in the supernova remnant these pairs are forced to escavate a bubble and lose energy adiabatically (because of the expansion) and radiatively (because of magnetic and radiation fields). We discuss here the observational constraints on the energy and number content of such pairs and discuss the scenarios that may allow for the pairs to escape in the interstellar medium and possibly contribute to the positron excess that has recently been detected by the PAMELA satellite. Special attention is dedicated to the case of nebulae surrounding high speed pulsars, observationally classified as Pulsar Bow Shock Nebulae. The pairs produced in these objects may be effectively carried out of the Supernova Remnant and released in the Interstellar Medium. As a result, Bow Shock Pulsar Wind Nebulae might be the main contributors to the positron excess in the Galaxy.
We show that the complex shape of the cosmic ray (CR) spectrum, as recently measured by PAMELA an... more We show that the complex shape of the cosmic ray (CR) spectrum, as recently measured by PAMELA and inferred from Fermi-LAT γ-ray observations of molecular clouds in the Gould belt, can be naturally understood in terms of basic plasma astrophysics phenomena. A break from a harder to a softer spectrum at rigidity R is approximately equal to 10 GV follows from a transition from transport dominated by advection of particles with Alfvén waves to a regime where diffusion in the turbulence generated by the same CRs is dominant. A second break at R is approximately equal to 200 GV happens when the diffusive propagation is no longer determined by the self-generated turbulence, but rather by the cascading of externally generated turbulence (for instance due to supernova bubbles) from large spatial scales to smaller scales where CRs can resonate. Implications of this scenario for the cosmic ray spectrum, grammage, and anisotropy are discussed.
Nonthermal radiation is observed from clusters of galaxies in the radio, hard X-rays, and possibl... more Nonthermal radiation is observed from clusters of galaxies in the radio, hard X-rays, and possibly in the soft X-ray/UV bands. While it is known that radiative processes related to nonthermal electrons are responsible for this radiation, the sites and nature of particle acceleration are not known. We investigate here the acceleration of protons and electrons in the shocks originated during mergers of clusters of galaxies, where the Fermi acceleration may work. We propose a semi-analytical model to evaluate the Mach number of the shocks generated during clusters mergers and we use this procedure to determine the spectrum of the accelerated particles for each one of the shocks produced during the merger history of a cluster. We follow the proton component, accumulated over cosmological time scales, and the short lived electron component. We conclude that efficient particle acceleration, resulting in nonthermal spectra that compare to observations, occurs mainly in minor mergers, namely mergers between clusters with very different masses. Major mergers, often invoked to be sites for the production of extended radio halos, are found to have on average too weak shocks and are unlikely to result in appreciable nonthermal activity.
According to high resolution cold dark matter (CDM) simulations, large virialized halos are forme... more According to high resolution cold dark matter (CDM) simulations, large virialized halos are formed through the constant merging of smaller halos formed at earlier times. In particular, the halo of our Galaxy may have hundreds of dark matter clumps. The annihilation of dark matter particles such as the neutralino in these clumps generates γ-ray fluxes that can potentially be detected by future experiments such as GLAST. We find that, depending on the parameters of the
Efficient cosmic-ray production can have a significant effect on the evolution and X-ray emission... more Efficient cosmic-ray production can have a significant effect on the evolution and X-ray emission from SNRs. Using hydrodynamic simulations coupled to diffusive shock acceleration, we produce thermal and nonthermal forwardshock photon spectra. 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 versus 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 flattening of the nonthermal spectrum in the soft X-ray band. X-ray spectral index measurements 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 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. Subject headingg s: cosmic rays -MHD -supernova remnants -turbulence 1 We do not consider emission from heated dust or from a compact object in this paper.
Clusters of galaxies are revealing themselves as powerful sources of non thermal radiation in a w... more Clusters of galaxies are revealing themselves as powerful sources of non thermal radiation in a wide range of wavelengths. In order to account for these multifrequency observations equipartition of cosmic rays (CRs) with the thermal gas in clusters of galaxies is often invoked. This condition might suggest a dynamical role played by cosmic rays in the virialization of these large scale structures and is now testable through gamma ray observations. We show here, in the specific case of the Coma and Virgo clusters, for which upper limits on the gamma ray emission exist, that equipartition implies gamma ray fluxes that are close or even in excess of the EGRET limit, depending on the adopted model of CR injection. We use this bound to limit the validity of the equipartition condition. We also show that, contrary to what claimed in previous calculations, the equipartition assumption implies gamma ray fluxes in the TeV range which can be detectable even by currently operating gamma ray observatories if the injection cosmic ray spectrum is flatter than E −2.4 .
The probability that a particle, crossing the shock along a given direction, be reflected backwar... more The probability that a particle, crossing the shock along a given direction, be reflected backwards along another direction, was shown to be the key element in determining the spectrum of non-thermal particles accelerated via the Fermi mechanism around a plane-parallel shock in the test-particle limit. Here an explicit equation for this probability distribution is given, for both the upstream and downstream sections. Though analytically intractable, this equation is solved numerically, allowing the determination of the spectrum in full generality, without limitation to shock speed or scattering properties. A number of cases is then computed, making contact with previous numerical work, in all regimes: Newtonian, trans-relativistic, and fully relativistic.
We calculate the distribution of electrons in clusters of galaxies, resulting from thermalization... more We calculate the distribution of electrons in clusters of galaxies, resulting from thermalization processes in the presence of stochastic acceleration due to plasma waves. We show that the electron distribution can deviate from a Maxwell-Boltzmann distribution, due to the effect of the stochastic energy gain, provided waves can be sustained against damping. The non-thermal tail of electrons can generate as bremsstrahlung emission a flux of hard X-rays compatible with the ones recently detected in some clusters of galaxies.
... cosmological magnetic field very well. In fact, Oren & Wolfe (1995) show how sensitive th... more ... cosmological magnetic field very well. In fact, Oren & Wolfe (1995) show how sensitive the redshift dependence of the variance is to different binning procedures and how the observed distribution is non-Gaussian. The trends in the ...
One century ago Viktor Hess carried out several balloon flights that led him to conclude that the... more One century ago Viktor Hess carried out several balloon flights that led him to conclude that the penetrating radiation responsible for the discharge of electroscopes was of extraterrestrial origin. One century from the discovery of this phenomenon seems to be a good time to stop and think about what we have understood about Cosmic Rays. The aim of this review is to illustrate the ideas that have been and are being explored in order to account for the observable quantities related to cosmic rays and to summarize the numerous new pieces of observation that are becoming available. In fact, despite the possible impression that development in this field is somewhat slow, the rate of new discoveries in the last decade or so has been impressive, and mainly driven by beautiful pieces of observation. At the same time scientists in this field have been able to propose new, fascinating ways to investigate particle acceleration inside the sources, making use of multifrequency observations that range from the radio, to the optical, to X-rays and gamma rays. These ideas can now be confronted with data.
The decay of very heavy metastable relics of the Early Universe can produce ultrahigh energy cosm... more The decay of very heavy metastable relics of the Early Universe can produce ultrahigh energy cosmic rays (UHECRs) in the halo of our own Galaxy. On distance scales of the order of the halo size, energy losses are negligible-no Greisen-Zatsepin-Kuzmin cutoff is expected. In this letter we show that, as a consequence of the hierarchical build up of the halo, this scenario predicts the existence of small scale anisotropies in the arrival directions of UHECRs. We also suggest some consequences of this scenario which will be testable with upcoming experiments, as Auger.
We discuss the signatures of the transition from galactic to extragalactic cosmic rays in differe... more We discuss the signatures of the transition from galactic to extragalactic cosmic rays in different scenarios, giving most attention to the dip scenario. The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons in the energy range 1 × 10 18 − 4 × 10 19 eV, which is caused by electron-positron pair production on the cosmic microwave background (CMB) radiation. The dip scenario provides a simple physical description of the transition from galactic to extragalactic cosmic rays. Here we summarize the signatures of the pair production dip model for the transition, most notably the spectrum, the anisotropy and the chemical composition. The main focus of our work is however on the description of the features that arise in the elongation rate and in the distribution of the depths of shower maximum Xmax in the dip scenario. We find that the curve for Xmax(E) shows a sharp increase with energy, which reflects a sharp transition from an iron dominated flux at low energies to a proton dominated flux at E ∼ 10 18 eV. We also discuss in detail the shape of the Xmax distributions for cosmic rays of given energy and demonstrate that this represents a powerful tool to discriminate between the dip scenario and other possible models of the transition. PACS numbers: 12.60.Jv, 95.35.+d, 98.35.Gi
The spatial clustering of galaxies in galaxy clusters implies that the background of infrared (IR... more The spatial clustering of galaxies in galaxy clusters implies that the background of infrared (IR) light in the intracluster medium (ICM) may exceed the universal background. Cosmic rays injected within the ICM propagate diffusively and at low enough energies are trapped there for cosmological times. The photopion production interactions of cosmic rays with the IR photons are responsible for the generation of neutrinos whose detection may shed some light on the origin and propagation of high energy cosmic rays in the universe. Here we discuss our calculations of the flux of neutrinos from single clusters as well as the contribution of photopion production in clusters of galaxies to the diffuse neutrino background.
A recently proposed model explains the rise in energy of the positron fraction measured by the PA... more A recently proposed model explains the rise in energy of the positron fraction measured by the PAMELA satellite in terms of hadronic production of positrons in aged supernova remnants, and acceleration therein. Here we present a preliminary calculation of the antiproton flux produced by the same mechanism. While the model is consistent with present data, a rise of the antiproton to proton ratio is predicted at high energy, which strikingly distinguishes this scenario from other astrophysical explanations of the positron fraction (such as pulsars). We briefly discuss important implications for dark matter searches via antimatter.
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Papers by Pasquale Blasi