arXiv subjects

Etienne Parizot

Publications and source records attributed to Etienne Parizot.

34 records · Page 2Linked to original sources

arXiv2014

Cosmic Ray Origin: Lessons from Ultra-High-Energy Cosmic Rays and the Galactic/Extragalactic Transition

We examine the question of the origin of the Galactic cosmic-rays (GCRs) in the light of the data available at the highest energy end of the spectrum. We argue that the data of the Pierre Auger Observatory and of the KASCADE-Grande experiment suggest that the transition between the Galactic and the extragalactic components takes place at the energy of the ankle in the all-particle cosmic-ray spectrum, and at an energy of the order of $10^{17}$ eV for protons. Such a high energy for Galactic protons appears difficult to reconcile with the general view that GCRs are accelerated by the standard diffusive shock acceleration process at the forward shock of individual supernova remnants (SNRs). We also review various difficulties of the standard SNR-GCR connection, related to the evolution of the light element abundances and to significant isotopic anomalies. We point out that most of the power injected by the supernovae in the Galaxy is actually released inside superbubbles, which may thus play an important role in the origin of cosmic-rays, and could solve some persistent problems of the standard SNR-GCR scenario in a rather natural way.

astro-ph.HE↗

arXiv2014

UHECR acceleration at GRB internal shocks

We study the acceleration of CR protons and nuclei at GRB internal shocks. Physical quantities and their time evolution are estimated using the internal shock modeling implemented by Daigne & Mochkovitch 1998. We consider different hypotheses about the way the energy dissipated at internal shocks is shared between accelerated CR, e- and B field. We model CR acceleration at mildly relativistic shocks, including all the significant energy loss processes. We calculate CR and neutrino release from single GRBs, assuming that nuclei heavier than protons are present in the relativistic wind. Protons can only reach maximum energies of ~ 10^19.5 eV, while intermediate and heavy nuclei are able to reach values of ~ 10^20 eV and above. The spectra of nuclei escaping from the acceleration site are found to be very hard while the combined spectrum of protons and neutrons is much softer. We calculate the diffuse UHECR flux expected on Earth using the GRB luminosity function from Wanderman & Piran 2010. Only the models assuming that the prompt emission represent a very small fraction of the energy dissipated at internal shocks, and that most of this dissipated energy is communicated to accelerated CR, are able to reproduce the magnitude of the UHECR flux observed. For these models, the observed shape of the UHECR spectrum can be well reproduced and the evolution of the composition is compatible with the trend suggested by Auger. We discuss implications of the softer proton component for the GCR to EGCR transition in the light of the recent composition analyses (KASCADE-Grande experiment). The associated secondary particle diffuse fluxes do not upset any current observational limit. Diffuse neutrino flux from GRB sources should however be detected with the lifetime of neutrino observatories.

astro-ph.HE↗

arXiv2014

Anisotropy expectations for ultra-high-energy cosmic rays with future high statistics experiments

UHECRs have attracted a lot of attention due to their challengingly high energies and their potential value to constrain physical processes and astrophysical parameters in the most energetic sources of the universe. Current detectors have failed to detect significant anisotropies which had been expected to allow source identification. Some indications about the UHECR composition, which may become heavier at the highest energies, has even put into question the possibility that such a goal could be achieved soon. We investigate the potential value of a new-generation detector, with 10 times larger exposure, to overcome the current situation and make significant progress in the detection of anisotropies and thus in the study of UHECRs. We take as an example the expected performances of the JEM-EUSO, assuming a uniform full-sky coverage with a total exposure of 300,000 km2 sr yr. We simulate realistic UHECR sky maps for a wide range of possible astrophysical scenarios allowed by the current constraints, taking into account the energy losses and photo-dissociation of the UHECRs, as well as their deflections by magnetic fields. These sky maps, built for the expected statistics of JEM-EUSO as well as for the current Auger statistics, as a reference, are analyzed from the point of view of their intrinsic anisotropies, using the two-point correlation function. A statistical study of the resulting anisotropies is performed for each astrophysical scenario, varying the UHECR source composition and spectrum as well as the source density. 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. Our results show that a gain of one order of magnitude in exposure would make a significant difference compared to the existing detectors.

astro-ph.HE↗

arXiv2011

Extended clustering analyses to constrain the deflection angular scale and source density of the ultra-high-energy cosmic rays

The search of a clustering signal in the arrival directions of ultra-high-energy cosmic rays (UHECRs) is a standard method to assess the level of anisotropy of the data sets under investigation. Here, we first show how to quantify the sensitivity of a UHECR detector to the detection of anisotropy, and then propose a new method that pushes forward the study of the two-point auto-correlation function, enabling one to put astrophysically meaningful constraints on both the effective UHECR source density and the angular deflections that these charged particles suffer while they propagate through the galactic and intergalactic magnetic fields. We apply the method to simulated data sets obtained under various astrophysical conditions, and show how the input model parameters can be estimated through our analysis, introducing the notion of "clustering similarity" (between data sets), to which we give a precise statistical meaning. We also study how the constraining power of the method is influenced by the size of the data set under investigation, the minimum energy of the UHECRs to which it is applied, and a prior assumption about the underlying source distribution. We also show that this method is particularly adapted to data sets consisting of a few tens to a few hundreds of events, which corresponds to the current and near-future observational situation in the field of UHECRs.

astro-ph.HE↗

arXiv2011

Enhancing the Relative Fe-to-Proton Abundance in Ultra-High-Energy Cosmic Rays

We study a generic class of models for ultra-high energy cosmic ray (UHECR) phenomenology, in which the sources accelerate protons and nuclei with a power-law spectrum having the same index, but with different values for the maximum proton energies, distributed according to a power-law. We show that, for energies sufficiently lower than the maximum proton energy, such models are equivalent to single-type source models, with a larger effective power law index and a heavier composition at the source. We calculate the resulting enhancement of the abundance of nuclei, and find typical values of a factor 2-10 for Fe nuclei. At the highest energies, the heavy nuclei enhancement ratios become larger, and the granularity of the sources must also be taken into account. We conclude that the effect of a distribution of maximum energies among sources must be considered in order to understand both the energy spectrum and the composition of UHECRs, as measured on Earth.

astro-ph.HE↗

arXiv2011

Cosmic rays in galactic and extragalactic magnetic fields

We briefly review sources of cosmic rays, their composition and spectra as well as their propagation in the galactic and extragalactic magnetic fields, both regular and fluctuating. A special attention is paid to the recent results of the X-ray and gamma-ray observations that shed light on the origin of the galactic cosmic rays and the challenging results of Pierre Auger Observatory on the ultra high energy cosmic rays. The perspectives of both high energy astrophysics and cosmic-ray astronomy to identify the sources of ultra high energy cosmic rays, the mechanisms of particle acceleration, to measure the intergalactic radiation fields and to reveal the structure of magnetic fields of very different scales are outlined.

astro-ph.HE↗

arXiv2005

Indirect manifestation of GRBs in the Galaxy: Compton trails and positrons

Two independent indirect manifestations of Galactic gamma-ray bursts (GRBs) are considered, whose importance for gamma-ray astronomy will depend on a still poorly known -- though astrophysically important -- parameter, namely the GRB repetition timescale in a Galaxy like ours. Both phenomena are expected to lead to observational constrains about this timescale. The first one relates to the Compton scattering of the GRB gamma-rays in the disk of the Galaxy, and the second one to the long-term annihilation pattern of positrons produced just ahead of the ultra-relativistic plasma, by photo-pair production.

astro-ph↗

arXiv2004

GZK horizon and magnetic fields

We discuss some aspects of the propagation of high-energy cosmic rays (CRs) in turbulent magnetic fields, and propose a formula for the diffusion coefficient based on accurate simulations in a wide energy range. We discuss the transition between ballistic and diffusive regimes and the magnetic horizon of CR sources, showing how magnetic fields of a few nG could modify the shape of the GZK feature. Such fields would roughly be in equipartition with the extragalactic CRs, and could build through the resonant growth of waves within the lifetime of the universe.

astro-ph↗

arXiv2004

The GZK cutoff behind a Magnetized Universe

We study the effect of random extra-galactic magnetic fields on the propagation of protons of energy larger than $10^{19}$ eV. We show that for reasonable field values (in the 100 nG range) the transition between diffusive and ballistic regimes occurs in the same energy range as the GZK cutoff (a few $10^{19}$ eV). The usual interpretation of the flux reduction above the GZK energy in terms of a sudden reduction of the visible horizon is modified. Moreover, since the size of the diffusion sphere of a continuous source of cosmic rays is of the order of 10 Mpc, the local structure of the Universe and therefore of potential local astrophysical sources plays a dominant role in the expected spectrum. Under reasonable assumptions on the sources configurations the expected GZK cutoff is reduced.

astro-ph↗

arXiv2000

Superbubbles and the Galactic evolution of Li, Be and B

From a recent re-analysis of the available data, Be and B Galactic evolution appears to show evidence for a two-slope behaviour with respect to O. The inferred Be/O abundance ratio in halo stars is constant at very low metallicity (primary behaviour) and increases proportionally to O/H at high metallicity (secondary behaviour). We show that this can be explained in the framework of one single model, the `superbubble model', in which Li, Be and B are produced by spallation reactions induced by energetic particles accelerated out of a mixture of supernova (SN) ejecta and ambient interstellar medium inside superbubbles (SBs). All the qualitative and quantitative constraints, including the energetics, the value of the transition metallicity and the 6Li/9Be isotopic ratio, are satisfied provided that the energetic particles have a spectrum flattened at low energy (in E^-1) and that the proportion of the SN ejecta inside SBs is of the order of a few percent. This lends support to Bykov's acceleration mechanism inside SBs and to the SB dynamical evolution model of Mac Low & McCray (1988).

astro-ph↗

arXiv2000

The superbubble model for Li, Be and B production and Galactic evolution

We show that the available constraints relating to $^{6}$LiBeB Galactic evolution can be accounted for by the so-called superbubble model, according to which particles are efficiently accelerated inside superbubbles out of a mixture of supernova ejecta and ambient interstellar medium. The corresponding energy spectrum is required to be flat at low energy (in E^-1 below 500 MeV/n, say), as expected from Bykov's acceleration mechanism. The only free parameter is also found to have the value expected from standard SB dynamical evolution models. Our model predicts a slope 1 (primary) and a slope 2 (secondary) behaviour at respectively low and high metallicity, with all intermediate slopes achieved in the transition region, between 10^-2 and 10^-1 Z_sol.

astro-ph↗

arXiv1999

Time dependent models for Li, Be and B production in the early Galaxy

We calculate the light element production induced by the explosion of an isolated supernova in the ISM. We use a time-dependent model and consider energetic particles accelerated at the forward (process~1) and reverse (process~2) shocks. Both processes are primary, but are shown to underproduce Be and B. The reasons for this failure are analyzed and used to propose a possible alternative, based on the acceleration of particles inside superbubbles.

astro-ph↗

arXiv1999

Time dependent models for the interaction of energetic particles in the ISM

We present a general model allowing one to calculate the distribution function of energetic particles in the interstellar medium, and hence any relevant nuclear reaction rate, for any given time-dependent injection function, as well as in situations when the propagation coefficients themselves are non stationary. We review and provide physical interpretation of general formal solutions of a propagation equation taking into account energy losses, nuclear destruction and escape. Both one-zone and extended models are investigated. Our main goal is to provide standard and general tools for subsequent use in nuclear astrophysics, notably to calculate the gamma-ray emission and spallation rates associated with energetic events and specific acceleration mechanisms. Considering the stationary limit of the model, we show that only time-dependent calculations can probe the density dependence of the physical processes under study, while steady-state models can \emph{in principle} only give information about density-independent processes.

astro-ph↗

arXiv1999

Spallative Nucleosynthesis in Supernova Remnants. II. Time-dependent numerical results

We calculate the spallative production of light elements associated with the explosion of an isolated supernova in the interstellar medium, using a time-dependent model taking into account the dilution of the ejected enriched material and the adiabatic energy losses. We first derive the injection function of energetic particles (EPs) accelerated at both the forward and the reverse shock, as a function of time. Then we calculate the Be yields obtained in both cases and compare them to the value implied by the observational data for metal-poor stars in the halo of our Galaxy, using both O and Fe data. We find that none of the processes investigated here can account for the amount of Be found in these stars, which confirms the analytical results of Parizot and Drury (1999). We finally analyze the consequences of these results for Galactic chemical evolution, and suggest that a model involving superbubbles might alleviate the energetics problem in a quite natural way.

astro-ph↗

arXiv1999

Gamma-Ray Line Emission from Superbubbles

We present an evolutionary model for gamma-ray line emission from superbubbles based on shock acceleration of metal-rich stellar ejecta. Application to the Orion OB1 association shows that gamma-ray lines at the detection threshold of the SPI spectrometer aboard of INTEGRAL are expected, making this region an interesting target for studies of the interaction of supernova shocks with the interstellar medium.

astro-ph↗

arXiv1998

Spallative Nucleosynthesis in Supernova Remnants. I. Analytical Estimates

Spallative nucleosynthesis is thought to be the only process capable of producing significant amount of Beryllium (Be) in the universe. Therefore, both energetic particles (EPs) and nuclei to be spalled (most efficiently C, N and O nuclei in this case) are required, which indicates that supernovae (SNe) may be directly involved in the synthesis of the Be nuclei observed in the halo stars of the Galaxy. We apply current knowledge relating to supernova remnant (SNR) evolution and particle shock acceleration to calculate the total Be yield associated with a SN explosion in the interstellar medium, focusing on the first stages of Galactic chemical evolution (i.e. when metallicity Z < 0.01 Z_odot) We show that dynamical aspects must be taken into account carefully, and present analytical calculations of the spallation reactions induced by the EPs accelerated at both the forward and the reverse shocks following the SN explosion. Our results show that the production of Be in the early Galaxy is still poorly understood, and probably implies either selective acceleration processes (greatly favouring CNO acceleration), reconsideration of the observational data (notably the O vs. Fe correlation), or even new energy sources.

astro-ph↗