Effect Of Re-acceleration Research Articles

Effects of re-acceleration and source grammage on secondary cosmic rays spectra

ABSTRACT The ratio between secondary and primary cosmic ray (CR) particles is the main source of information about CR propagation in the Galaxy. Primary CRs are thought to be accelerated mainly in supernova remnant shocks and then released in the interstellar medium. Here, they produce secondary particles by occasional collisions with interstellar matter. As a result, the ratio between the fluxes of secondary and primary particles carries information about the amount of matter CRs have encountered during their journey from their sources to the Earth. Recent measurements by AMS-02 revealed an unexpected behaviour of two main secondary-to-primary ratios, the Boron-to-Carbon ratio and the antiproton-to-proton ratio. In this work, we discuss how such anomalies may reflect the action of two phenomena that are usually overlooked, namely the fact that some fraction of secondary particles can be produced within the acceleration region, and the non-negligible probability that secondary particles encounter an accelerator (and are re-accelerated) during propagation. Both effects must be taken into account in order to correctly extract information about CR transport from secondary-to-primary ratios.

Effect of turbulent reacceleration on electrons produced by dark matter annihilation in the Coma cluster

ABSTRACT In this paper, we study the effect of reacceleration provided by turbulences on electrons produced by dark matter (DM) annihilation in the Coma cluster. We use a simplified phenomenological model to describe the effect of the turbulences, and explore a limited subset of three possible DM models for neutralino particles with different mass and annihilation channel. We find that, for values of the annihilation cross-section of the order of the upper limits found with Fermi–LAT measurements in astrophysical objects, and for conservative values of the boosting factor due to DM substructures, the reacceleration due to turbulences can enhance the radio emission produced by DM-originated electrons up to the level of the observed flux of the radio halo in Coma, for moderate reacceleration intensity in relatively short times. Therefore, we conclude that, even if it is not possible to distinguish between the fits obtained in this paper because of the scattering present in the radio flux data, the electrons produced by DM annihilation can be possible seed electrons for the reacceleration, as well as secondary electrons of hadronic origin. A possible discriminant between these two classes of models is the flux produced in the gamma ray band, which in the case of DM-originated electrons should be more than two orders of magnitude smaller than the present Fermi–LAT upper limits, whereas in the hadronic case the expected gamma ray flux should be close to the value of present upper limits.

Anomaly in the cosmic-ray energy spectrum at GeV–TeV energies

Recent measurements of cosmic rays by various experiments have found that the energy spectrum of cosmic rays is harder in the TeV region than at GeV energies. The origin of the spectral hardening is not clearly understood. In this paper, we discuss the possibility that the spectral hardening might be due to the effect of re-acceleration of cosmic rays by weak shocks associated with old supernova remnants in the Galaxy.

Electron and proton acceleration efficiency by merger shocks in galaxy clusters

Radio relics in galaxy clusters are associated with powerful shocks that (re) accelerate relativistic electrons. It is widely believed that the acceleration proceeds via diffusive shock acceleration. In the framework of thermal leakage, the ratio of the energy in relativistic electrons to the energy in relativistic protons should be smaller than K-e/p similar to 10(-2). The relativistic protons interact with the thermal gas to produce gamma-rays in hadronic interactions. Combining observations of radio relics with upper limits from gamma-ray observatories can constrain the ratio K-e/p. In this work, we selected 10 galaxy clusters that contain double radio relics, and derive new upper limits from the stacking of gamma-ray observations by Fermi. We modelled the propagation of shocks using a semi-analytical model, where we assumed a simple geometry for shocks and that cosmic ray protons are trapped in the intracluster medium. Our analysis shows that diffusive shock acceleration has difficulties in matching simultaneously the observed radio emission and the constraints imposed by Fermi, unless the magnetic field in relics is unrealistically large (>> 10 mu G). In all investigated cases (also including realistic variations of our basic model and the effect of re-acceleration), the mean emission of the sample is of the order of the stacking limit by Fermi, or larger. These findings put tension on the commonly adopted model for the powering of radio relics, and imply that the relative acceleration efficiency of electrons and protons is at odds with predictions of diffusive shock acceleration, requiring K-e/p >= 10 - 10(-2).

GeV-TeV cosmic-ray spectral anomaly as due to reacceleration by weak shocks in the Galaxy

Recent cosmic-ray measurements have found an anomaly in the cosmic-ray energy spectrum at GeV-TeV energies. Although the origin of the anomaly is not clearly understood, suggested explanations include effect of cosmic-ray source spectrum, propagation effects, and the effect of nearby sources. In this paper, we propose that the spectral anomaly might be an effect of re-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration by strong supernova remnant shock waves, cosmic rays undergo diffusive propagation through the Galaxy. During the propagation, cosmic rays may again encounter expanding supernova remnant shock waves, and get re-accelerated. As the probability of encountering old supernova remnants is expected to be larger than the younger ones due to their bigger sizes, re-acceleration is expected to be mainly due to weaker shocks. Since weaker shocks generate a softer particle spectrum, the resulting re-accelerated component will have a spectrum steeper than the initial cosmic-ray source spectrum produced by strong shocks. For a reasonable set of model parameters, it is shown that such re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, explaining the observed GeV-TeV spectral anomaly.

K-capture cosmic ray secondaries and reacceleration

We have investigated the effect of reacceleration on interstellar flux of K-capture secondaries V-49 and Cr-51. Several isotopic ratios for these two isotopes are calculated using the galactic diffusion model with and without distributed reacceleration. It is found that the statistical accuracy of the ACE experiment on itself is high enough to see a signature of reacceleration. However, the uncertainties in nuclear production cross sections are probably too large to conclude that reacceleration process took place.

Stochastic reacceleration of cosmic rays in the interstellar medium

The effects of reacceleration on cosmic rays have been studied over a wide charge and energy range using a model of reacceleration by the interstellar turbulence. We take into account only inevitable stochastic reacceleration of cosmic rays by the random hydrodynamic waves, which supposedly exist in the interstellar medium and provide a means for cosmic-ray scattering and spatial diffusion in the Galaxy. Our calculations reproduce not only the B/C ratio but also the H and He data over the entire energy range where the measurements are available. However, the sub-Fe to Fe ratio is not fitted as well as the B/C ratio, and the reacceleration effect does not seem to remove the need for truncation of short path lengths, which is apparently required by the standard leaky box model. This work demonstrates that the cosmic-ray data can be represented at least as well by a reacceleration model with a simple rigidity power-law escape length, which agrees with the Kolmogorov-type spectrum of hydromagnetic turbulence, as they can by the standard leaky box model with its ad hoc escape lengths.

Effect of re-acceleration on cosmic ray components

Re-acceleration of cosmic rays in interstellar space has been studied in detail in order to examine the behaviour of the ratios of secondary to primary nuclei in cosmic radiation. It is found that modest acceleration in a confinement region, where particles escape more freely at high energies, provides a better fit to the observed data. We have studied the effect of re-acceleration on the spectral shape of proton and helium components of cosmic rays. We have examined wto different models and showed that re-acceleration provides a poor fit to the observed proton data.

Saturation Mechanism and Improvement of Conversion Efficiency of the Free-Electron Laser

Saturation mechanisms of free-electron lasers are investigated in the Compton regime. It is found that saturation occurs because of quasilinear energy spreading of the electron beam in the case of multimode excitation. The energy conversion efficiency remains low even if many modes are taken into account. To improve the conversion efficiency, the effect of reacceleration by a microwave is investigated and turns out to increase the efficiency to more than 50%.