Cosmic Ray Origin and Propagation Model

The Astronomy and Astrophysics Review, 2013

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.

Romanian Astronomical Journal, 2004

Popescu et al. (2004) gave a model for the observed cosmic rays between 5 * 10^15 and 3 * 10^18 eV. Their source is presumed to be the supernova of stars that explode in their winds. The observed cosmic rays abundance at the source are affected by spallation in the supernova shell, by the difference in ionization degree (being one or two times ionized) at the injection in the supernova shock, the stars with initial masses 15MSun ≤ M ≤ 30MSun having a different contribution to them than the stars with 30MSun ≤ M ≤ 50MSun, this being 2:1 for the elements with Z ≥ 6. Still, the abundances after these corrections are different by a factor , where is the atomic number for the element i. This paper is dedicated to the explanation of this factor and its physical meanings by considering that, prior to the shock injection, the wind particles are radiative accelerated.

2001

Nuclear Physics B - Proceedings Supplements, 2013

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.

Publications of the Astronomy Department of the Eötvös University (PADEU), 2005

In the article "Cosmic Rays VIII" of Popescu et al. (2005) we gave an model for the observed cosmic rays between 5 • 10^15 and 3 • 10^18 eV. Their surse is presumed to be supernova of stars that explode in their winds. The observed cosmic abundance at the source are affected by spallation in the supernova shell, by the difference in ionization degree (being one two times ionized) at the injection in the supernova shock, the stars with initial masses 15M⊙ ≤ M ≤ 30M⊙ having a different contribution to them than the stars with 30M⊙ ≤ M ≤ 50M⊙ and this is 2:1 for the elements with Z ≥ 6. Still, the abundances after these corrections are different by a factor Z_{i} /Z_{He} where Z_{i} is the atomic number for the element i and Z_{He} the one for He. This paper is dedicated to the explanation of this factor and its physical meanings.

In the common model supernova shock-acceleration of cosmic rays there are two open questions: 1. where does the high energy cosmic rays below the knee (10 4 − 10 6 Gev) come from, and 2. are cosmic ray accelerated only at their origin or contineuosly during their residence in the Galaxy. We show that 10 1 5 eV light nuclei are probably accelerted by associations of supernovae. The ratio of the spectra of secondary to primary cosmic rays would be affected by repeated (or secondary) acceleration in the ISM during their propagation in the galaxy. The observed secondary and primary CR spectra are used to constrain the amount of such secondary acceleration by supernova remnants (SNR). Two cases are considered: weak shocks (1 < M < 2) of old, dispersed remnants, and strong shocks (M > 3) of relatively young remnants. It is shown that weak shocks produce more secondary acceleration than what is permitted in the framework of the standard leaky box (SLB) model, making it inconsistent with dispersed acceleration that should be produced by SNR. If the SLB is modified to allow a moderate amount of RA by week shocks, the RA produced by old SNRs agrees with the rate reqired to fit the secondary-to primaray cosmic-ray data, making a self consistent picture. Significant secondary acceleration by strong shocks of young SNRs should lead to flattening of the secondary-to primaray ratio at high energies, near 1TeV/nucleon.

Advances in Space Research, 2005

In a recent paper , it has been shown that the flux of secondary cosmic ray (CR) antiprotons appears to be contradictory to measurements of secondary to primary nuclei ratios in cosmic rays when calculated in the same Galactic propagation model. The contradiction appears as a value of the diffusion coefficient necessary to match the secondary ratiosp/p and B/C. In particlular, it was shown that the reacceleration models designed to match secondary to primary nuclei ratios produce too few antiprotons. It is, however, clear that some reacceleration is unavoidable in the turbulent interstellar medium. Here we discuss an idea of how to improve reacceleration model by allowing for the damping of interstellar turbulences on the small scale by cosmic rays, mostly protons. This would lead to increase in the mean free path lenghts at low energies, the well-known phenomena empirically discovered in the Leaky Box models, thus producing less secondary nuclei. Antiprotons will remain almost non-affected due to their high energy threshold of production cross section.

2005

The aspects of Cosmic Ray (CR) origin are reviewed. Recent observational evidence on the spatial patterns of non-thermal X-ray radiation from Supernovae Remnants (SNR) supports long-awaited expectations of proton and nuclei acceleration up to PeV energies. We add new arguments based on the experimental data from surface arrays measuring Extensive Air Showers (EAS) and on data from solar accelerators available now from space-borne X-ray and gamma-ray spectrometers. Energy spectra of primary nuclei with atomic number from = 1 to = 26 can provide useful information on the validity of models of cosmic ray acceleration. By estimating the threshold energy of the onset of the suppression of the different nuclei flux, the so-called spectral 'knee' energy, we can directly check the hypothesis of rigidity-dependent acceleration of the hadrons in SNR sites. Unfortunately, information from the EAS experiments does not provide enough clues for such 'spectroscopy' of the 'knee region'. Nonetheless, by grouping the primary nuclei in two or three broad mass groups (light, intermediate and heavy) we can obtain useful information on energy spectra of the primaries. Recently, using multidimensional classification methods on MAKET-ANI experimental data, we categorized the 'all-particle' spectra into two distinct primary mass groups. From the spectra analysis, we come to the conclusion that the SNR-based particle acceleration model is valid and presents evidence that there exists a nearby source of cosmic rays, which provides a significant portion of the CR flux.

Monthly Notices of the Royal Astronomical Society, 2013

Galactic cosmic rays are believed to be accelerated at supernova remnant shocks. Though very popular and robust, this conjecture still needs a conclusive proof. The strongest support to this idea is probably the fact that supernova remnants are observed in gamma-rays, which are indeed expected as the result of the hadronic interactions between the cosmic rays accelerated at the shock and the ambient gas. However, also leptonic processes can, in most cases, explain the observed gamma-ray emission. This implies that the detections in gamma rays do not necessarily mean that supernova remnants accelerate cosmic ray protons. To overcome this degeneracy, the multi-wavelength emission (from radio to gamma rays) from individual supernova remnants has been studied and in a few cases it has been possible to ascribe the gamma-ray emission to one of the two processes (hadronic or leptonic). Here we adopt a different approach and, instead of a case-by-case study we aim for a population study and we compute the number of supernova remnants which are expected to be seen in TeV gamma rays above a given flux under the assumption that these objects indeed are the sources of cosmic rays. The predictions found here match well with current observational results, thus providing a novel consistency check for the supernova remnant paradigm for the origin of galactic cosmic rays. Moreover, hints are presented for the fact that particle spectra significantly steeper than E −2 are produced at supernova remnants. Finally, we expect that several of the supernova remnants detected by H.E.S.S. in the survey of the galactic plane should exhibit a gamma-ray emission dominated by hadronic processes (i.e. neutral pion decay). The fraction of the detected remnants for which the leptonic emission dominates over the hadronic one depends on the assumed values of the physical parameters (especially the magnetic field strength at the shock) and can be as high as roughly a half.

2019

The direct measurements of cosmic rays (CRs), after correction for the propagation effects in the interstellar medium, indicate that their source spectra should be significantly steeper than the canonical E^-2 spectrum predicted by the standard Diffusive Shock Acceleration (DSA) mechanism. The DSA has long been held responsible for the production of galactic CRs in supernova remnant (SNR) shocks. The γ-ray &quot;probes&quot; of the acceleration spectra of CRs on-the-spot, inside of the SNRs, lead to the same conclusion. We show that the steep acceleration spectrum can be attributed to the combination of (i) spherical expansion, (ii) tilting of the magnetic field along the shock surface and (iii) shock deceleration. Because of (i) and (ii), the DSA is efficient only on two &quot;polar caps&quot; of a spherical shock where the local magnetic field is within ≃45^∘ to its normal. The shock-produced spectrum observed edge-on steepens with the particle energy because the number of freshly...

Journal of Cosmology and Astroparticle Physics, 2012

In this paper we investigate the effects of stochasticity in the spatial and temporal distribution of supernova remnants on the anisotropy of cosmic rays observed at Earth. The calculations are carried out for different choices of the diffusion coefficient D(E) experienced by cosmic rays during propagation in the Galaxy. The propagation and spallation of nuclei (with charge 1 ≤ Z ≤ 26) are taken into account. At high energies (E > 1 TeV) we assume that D(E) ∝ (E/Z) δ , with δ = 1/3 and δ = 0.6 being the reference scenarios. The large scale distribution of supernova remnants in the Galaxy is modeled following the distribution of pulsars with and without accounting for the spiral structure of the Galaxy. Our calculations allow us to determine the contribution to anisotropy resulting from both the large scale distribution of SNRs in the Galaxy and the random distribution of the nearest remnants. The naive expectation that the anisotropy amplitude scales as δ A ∝ D(E) is shown to be a wild oversimplification of reality which does not reflect in the predicted anisotropy for any realistic distribution of the sources. The fluctuations in the anisotropy pattern are dominated by nearby sources, so that predicting or explaining the observed anisotropy amplitude and phase becomes close to impossible. Nevertheless, the results of our calculations, when compared to the data, allow us to draw interesting conclusions in terms of the propagation scenario to be preferred both in terms of the energy dependence of the diffusion coefficient and of the size of the halo. We find that the very weak energy dependence of the anisotropy amplitude below 10 5 GeV, as observed by numerous experiments, as well as the rise at higher energies, can best be explained if the diffusion coefficient is D(E) ∝ E 1/3. Faster diffusion, for instance with δ = 0.6, leads in general to an exceedingly large anisotropy amplitude. The spiral structure introduces interesting trends in the energy dependence of the anisotropy pattern, which qualitatively reflect the trend seen in the data. The inhomogeneous spatial distribution of the sources in the Galactic disc induces a large scale anisotropy which is not sensitive to the stochastic nature of nearby SNRs: we find that this additional contribution to δ A becomes more important for large values of the size of the halo, H. The two terms are comparable in size for H ∼ 2 kpc which corresponds to the scale height of the gradient of the spatial distribution of SNRs in the Galaxy. The dependence on energy of δ A (E) is close to monotonic when the large-scale, regular term dominates, and does not seem to reflect the observed anisotropy amplitude. Both contributions to the total anisotropy are illustrated and discussed with the help of semi-analytical results. Contents 1 Introduction 1 2 Summary of the Green functions formalism 3 3 Anisotropy and its fluctuations 6 4 CR anisotropy for realistic distributions of SNRs 9 4.1 Anisotropy for the cylindrical model 10 4.2 CR anisotropy in the spiral model 14 5 The dependence of the anisotropy amplitude on the size of the halo H 15 6 Conclusions 18

Astronomy & Astrophysics, 2017

The acceleration times of the highest-energy particles, which emit gamma-rays in young and middle-age supernova remnants (SNRs), are comparable with SNR age. If the number of particles starting acceleration was varying during early times after the supernova explosion then this variation should be reflected in the shape of the gamma-ray spectrum. We use the solution of the non-stationary equation for particle acceleration in order to analyse this effect. As a test case, we apply our method to describe gamma-rays from IC 443. As a proxy of the IC 443 parent supernova we consider SN1987A. First, we infer the time dependence of injection efficiency from evolution of the radio spectral index in SN1987A. Then, we use the inferred injection behaviour to fit the gamma-ray spectrum of IC 443. We show that the break in the proton spectrum needed to explain the gamma-ray emission is a natural consequence of the early variation of the cosmic ray injection, and that the very-high-energy gamma rays originate from particles which began acceleration during the first months after the supernova explosion. We conclude that the shape of the gamma-ray spectrum observed today in SNRs critically depends on the time variation of the cosmic ray injection process in the immediate post-explosion phases. With the same model, we also estimate the future possibility of detecting gamma-rays from SN 1987A.

The Advanced Composition Explorer Mission, 1998

The composition of Galactic Cosmic Ray Sources (GCRS) shows the following features: (i) an enhancement of the refractory elements relative to the volatile ones, and (ii) an enhancement of the heavier volatile elements relative to the lighter ones; this mass dependence should reflect a mass-to-charge (A/Q) dependence of the acceleration efficiency; among the refractory elements, there is only a very weak enhancement of heavier species, or none at all. We consider it fortuitous that the GCRS composition resembles that of the solar corona, which is biased according to first ionization potential. In a companion paper by Ellison et al. (1998, this issue), this GCRS composition is interpreted in terms of a supernova shock wave acceleration of interstellar and/or circumstellar (e.g., 22 Ne-rich Wolf-Rayet wind) gas phase and especially dust grain material. These two papers summarize and complement the content of two papers that recently appeared in Astrophys.

Journal of Cosmology and Astroparticle Physics, 2012

A rapidly growing amount of evidences, mostly coming from the recent gammaray observations of Galactic supernova remnants (SNRs), is seriously challenging our understanding of how particles are accelerated at fast shocks. The cosmic-ray (CR) spectra required to account for the observed phenomenology are in fact as steep as E −2.2 -E −2.4 , i.e., steeper than the test-particle prediction of first-order Fermi acceleration, and significantly steeper than what expected in a more refined non-linear theory of diffusive shock acceleration. By accounting for the dynamical back-reaction of the non-thermal particles, such a theory in fact predicts that the more efficient the particle acceleration, the flatter the CR spectrum. In this work we put forward a self-consistent scenario in which the account for the magnetic field amplification induced by CR streaming produces the conditions for reversing such a trend, allowing -at the same time -for rather steep spectra and CR acceleration efficiencies (about 20%) consistent with the hypothesis that SNRs are the sources of Galactic CRs. In particular, we quantitatively work out the details of instantaneous and cumulative CR spectra during the evolution of a typical SNR, also stressing the implications of the observed levels of magnetization on both the expected maximum energy and the predicted CR acceleration efficiency. The latter naturally turns out to saturate around 10-30%, almost independently of the fraction of particles injected into the acceleration process as long as this fraction is larger than about 10 −4 .

2019

Cosmic ray (CR) sources leave signatures in the isotopic abundances of CRs. Current models of Galactic CRs that consider supernovae (SNe) shocks as the main sites of particle acceleration cannot satisfactorily explain the higher ^22Ne/^20Ne ratio in CRs compared to the interstellar medium. Although stellar winds from massive stars have been invoked, their contribution relative to SNe ejecta has been taken as a free parameter. Here we present a theoretical calculation of the relative contributions of wind termination shocks (WTSs) and SNe shocks in superbubbles, based on the hydrodynamics of winds in clusters, the standard stellar mass function, and stellar evolution theory. We find that the contribution of WTSs towards the total CR production is at least 25%, which rises to ≳ 50% for young (≲ 10 Myr) clusters, and explains the observed ^22 Ne/^20 Ne ratio. We argue that since the progenitors of apparently isolated supernovae remnants (SNRs) are born in massive star clusters, both WT...