UHECR acceleration at GRB internal shocks Globus, N; Allard, D; Mochkovitch, R ...
Monthly Notices of the Royal Astronomical Society,
07/2015, Letnik:
451, Številka:
1
Journal Article
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Recent results from the Pierre Auger Observatory suggest that nuclei heavier than protons might be present in significant amounts among ultrahigh-energy cosmic rays (UHECRs). It is therefore ...interesting to investigate the acceleration both protons and nuclei in relativistic jets. We calculate the acceleration of a mixed composition of cosmic rays at Gamma-ray burst (GRB) internal shocks, taking into account the relevant energy loss processes. 3D trajectories during the relativistic Fermi cycles are simulated following previous works by Niemiec & Ostrowski. We use the internal shock model of Daigne & Mochkovitch to derive the evolution of the relevant physical quantities (magnetic fields, baryon and photon densities, shock velocity). We consider different phenomenological hypotheses about the sharing of the dissipated energy between accelerated cosmic rays, electrons and the magnetic field. For various choices of the parameters, we calculate the spectrum of cosmic rays escaping from the GRB environment as well as secondary particles produced either during the acceleration or extragalactic propagation of UHECRs. Only models where (i) the prompt emission represents only a small fraction of the energy dissipated at internal shocks and (ii) most of this dissipated energy is communicated to cosmic rays, are able to reproduce the magnitude of the UHECR flux observed on Earth. For these models, however, the observed shape of the UHECR spectrum can be well reproduced above the ankle, with an evolution of the composition compatible with the trend suggested by Auger, and associated diffuse fluxes of secondary particles which do not violate current observational limits.
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 multiwavelength 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 HESS 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.
We extend previous studies of mixed-composition extragalactic cosmic-ray source models, investigating the influence of a non-negligible extragalactic magnetic field on the propagated cosmic-ray ...spectrum and composition. We study the transport of charged particles in turbulent fields and the transition from a ballistic to a diffusive propagation regime. We introduce a method allowing a fast integration of the particle trajectories, which allows us to calculate extragalactic cosmic-ray spectra in the general case, without using either the diffusive or the rectilinear approximation. We find that the main features of the mixed-composition models – regarding the interpretation of the ankle and the non-monotonous evolution of the average cosmic-ray mass – remain essentially unchanged as long as the magnetic field intensity does not exceed a few nG.
Constraints on the diffusion and acceleration parameters in five young supernova remnants (SNRs) are derived from the observed thickness of their X-ray rims, as limited by the synchrotron losses of ...the highest energy electrons, assuming uniform and isotropic turbulence. From a joint study of the electrons diffusion and advection in the downstream medium of the shock, it is shown that the magnetic field must be amplified up to values between 250 and 500 μG in the case of Cas A, Kepler, and Tycho, or ${\sim} 100\,\mu$G in the case of SN 1006 and G347.3-0.5. The diffusion coefficient at the highest electron energy can also be derived from the data, by relating the X-ray energy cutoff to the acceleration timescale. Values typically between 1 and 10 times the Bohm diffusion coefficient are found to be required. We also find interesting constraints on the energy dependence of the diffusion coefficient, by requiring that the diffusion coefficient at the maximum proton energy be not smaller than the Bohm value in the amplified field. This favours diffusion regime between the Kraichnan and the Bohm regime, and rejects turbulence spectrum indices larger than ${\simeq} 3/2$. Finally, the maximum energy of the accelerated particles is found to lay between 1013 and $5\times 10^{13}$ eV for electrons, and around $Z\times 8\times 10^{14}$ eV at most for nuclei (or ∼2.5 times less if a Bohm diffusion regime is assumed), roughly independently of the compression ratio assumed at the shock. Even by taking advantage of the uncertainties on the measured parameters, it appears very difficult for the considered SNRs in their current stage of evolution to produce protons up to the knee of the cosmic-ray spectrum, at ${\sim} 3\times 10^{15}$ eV, and essentially impossible to accelerate Fe nuclei up to either the ankle at ${\sim} 3\times 10^{18}$ eV or the second knee at ${\sim} 5\times 10^{17}$ eV.
Abstract
Mini-EUSO is a telescope observing the Earth in the ultraviolet band from the International Space Station. It is a part of the JEM-EUSO program, paving the way to future larger missions, ...such as K-EUSO and POEMMA, devoted primarily to the observation of ultrahigh-energy cosmic rays from space. Mini-EUSO is capable of observing extensive air showers generated by ultrahigh-energy cosmic rays with an energy above 10
21
eV and to detect artificial showers generated with lasers from the ground. Other main scientific objectives of the mission are the search for nuclearites and strange quark matter, the study of atmospheric phenomena such as transient luminous events, meteors, and meteoroids, the observation of sea bioluminescence and of artificial satellites and man-made space debris. Mini-EUSO will map the nighttime Earth in the UV range (290–430 nm), with a spatial resolution of about 6.3 km and a temporal resolution of 2.5
μ
s, through a nadir-facing UV-transparent window in the Russian Zvezda module. The instrument, launched on 2019 August 22, from the Baikonur Cosmodrome, is based on an optical system employing two Fresnel lenses and a focal surface composed of 36 multianode photomultiplier tubes, 64 channels each, for a total of 2304 channels with single-photon counting sensitivity and an overall field of view of 44°. Mini-EUSO also contains two ancillary cameras to complement measurements in the near-infrared and visible ranges. In this paper, we describe the detector and present the various phenomena observed in the first months of operations.
We study the phenomenology of cosmic-rays (CRs) at the galactic/extragalactic transition, focusing on two opposite models for the composition of the extragalactic (EG) component. Model A assumes a ...mixed source composition, with nuclear abundances similar to that of the low-energy CRs, while model B assumes that EG sources accelerate only protons. We study the limits within which both scenarios can reproduce the observed high-energy CR spectrum and composition. The ankle in model A is interpreted as the GCR/EGCR transition, while in model B it is the pair-production dip. Model A has a source spectrum ∝
E
−
x
with
x
∼
2.2–2.3, while model B requires
x
∼
2.6–2.7. We compare the predictions of both models with the available data on the energy evolution of the high-energy CR composition using the two main composition-related observables:
X
max and 〈ln
A〉. We conclude that model A is currently favoured. Uncertainties are discussed and distinctive features of the two models are identified, which should allow one to distinguish between the models in the near future when more precise measurements are available with higher statistics experiments.
Abstract
Supernova remnants are often presented as the most probable sources of Galactic cosmic rays. This idea is supported by the accumulation of evidence that particle acceleration is happening at ...supernova remnant shocks. Observations in the TeV range have especially contributed to increase the understanding of the mechanisms, but many aspects of the particle acceleration at supernova remnant shocks are still debated. The Cherenkov telescope array is expected to lead to the detection of many new supernova remnants in the TeV and multi-TeV range. In addition to the individual study of each, the study of these objects as a population can help constrain the parameters describing the acceleration of particles and increase our understanding of the mechanisms involved.
The Greisen-Zatsepin-Kuzmin (GZK) effect, i.e. the interaction of ultra-high-energy cosmic ray (UHECR) protons and nuclei with the intergalactic photon background, results in a drastic reduction of ...the number of sources contributing to the observed flux above ~60 EeV. We study quantitatively the source statistics as a function of energy for a range of models compatible with the current data, varying source composition, injection spectrum, source density, and luminosity distribution. We also explore various realizations of the source distribution. We find that, in typical cases, the brightest source in the sky contributes more than one-fifth of the total flux above 80 EeV and about one-third of the total flux at 100 EeV. We show that typically between two and five sources contribute more than half of the UHECR flux at 100 EeV. With such low source numbers, the isolation of the few brightest sources in the sky may be possible for experiments collecting sufficient statistics at the highest energies, even in the event of relatively large particle deflections.
Context. Ultra-high-energy cosmic rays (UHECRs) have attracted a lot of attention in astroparticle physics and high-energy astrophysics, due to their challengingly high energies, and to their ability ...to constrain the physical processes and astrophysical parameters in the most energetic sources of the universe. Despite their very large acceptance, current detectors have failed to detect significant anisotropies in their arrival directions, which had been expected to lead to the long-sought identification of their sources. Some indications about the composition of the UHECRs, which may become heavier at the highest energies, have even called into question the possibility that such a goal could be achieved in the foreseeable future. Aims. We investigate the potential value of a new-generation detector, with an exposure increased by one order of magnitude, to overcome the current situation and make notable progress in detecting anisotropies and thus in the study of UHECRs. We take as an example the expected performances of the JEM-EUSO detector, assuming a uniform full-sky coverage with a total exposure of 300 000 km2 sr yr. Methods. We simulated realistic UHECR sky maps for a wide range of possible astrophysical scenarios allowed by the current constraints, taking the energy losses and photo-dissociation of the UHE protons and nuclei into account, as well as their deflections by intervening magnetic fields. These sky maps, built for both the expected statistics of JEM-EUSO and the current Pierre Auger Observatory statistics, as a reference, were analysed from the point of view of their intrinsic anisotropies, using the two-point correlation function. A statistical study of the resulting anisotropies was performed for each astrophysical scenario, varying the UHECR source composition and spectrum and the source density and exploring a set of five hundred independent realizations for each choice of a parameter set. Results. We find that significant anisotropies are expected to be detected by a next-generation UHECR detector, for essentially all the astrophysical scenarios studied, and give precise, quantitative meaning to this statement. Conclusions. Our results show that a gain of one order of magnitude in the total exposure of UHECR detectors would make a significant difference compared to the existing experiments, and would allow considerable progress in the study of these mysterious particles and their sources.
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