Noemie Globus

Propagation of high-energy cosmic rays in extragalactic turbulent magnetic fields: resulting energy spectrum and composition

We extend previous studies of mixed-composition extragalactic cosmic-ray source models, investigating the influence of a nonnegligible 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.

A complete model of the cosmic ray spectrum and composition across the Galactic to extragalactic transition

We present a complete phenomenological model accounting for the evolution of the cosmic-ray spectrum and composition with energy, based on the available data over the entire spectrum. We show that there is no need to postulate any additional component, other than one single Galactic component depending on rigidity alone, and one extragalactic component, whose characteristics are similar to those derived from a study of particle acceleration at mildly relativistic shocks in a GRB environment (Globus et al., 2015). In particular, we show that the resulting cosmic ray spectrum and composition satisfy the various constraints derived from the current data in the Galactic/extragalactic transition region, notably from the measurements of KASCADE Grande and Auger. Finally, we derive some generic features that a working phenomenological scenario may exhibit to give a global account of the cosmic ray data with a minimum number of free parameters.

UHECR acceleration at GRB internal shocks

We study the acceleration of cosmic-ray protons and nuclei at GRB internal shocks. Physical quantities (magnetic fields, baryon and photon densities, shock velocity) and their time evolution, relevant to cosmic-ray acceleration and energy losses, are estimated using the internal shock modeling implemented by Daigne & Mochkovitch (1998). Within this framework, we consider di erent hypotheses about the way the energy dissipated at internal shocks is shared between accelerated cosmic-rays, electrons and the magnetic field. We model cosmicray acceleration at mildly relativistic shocks, using numerical tools inspired by the work of Niemiec & Ostrowski (2004), including all the significant energy loss processes that might limit cosmic-ray acceleration at GRB internal shocks. We calculate cosmic-ray and neutrino release from single GRBs, for various prompt emission luminosities, assuming that nuclei heavier than protons are present in the relativistic wind at the beginning of the internal shock phase. We find that protons can only reach maximum energies of the order 10 19:5 eV in the most favorable cases, while intermediate and heavy nuclei are able to reach higher values of the order of 10 20 eV and above. The spectra of particles escaping from the acceleration site are found to be very hard for the di erent nuclear species. In addition a significant and much softer neutron component is present in the cases of intermediate and high luminosity GRBs due to the photodisintegration of accelerated nuclei during the early stages of the shock propagation. As a result, the combined spectrum of protons and neutrons from single GRBs is found to be much softer than those of the other nuclear species. We calculate the di use UHECR flux expected on Earth by convoluting the cosmic-ray output from single GRBs of various luminosities by the GRB luminosity function derived by Wanderman & Piran (2010). We show that only the models assuming that (i) the prompt emission represent only a very small fraction of the energy dissipated at internal shocks (especially for low and intermediate luminosity bursts), and that (ii) most of this dissipated energy is communicated to accelerated cosmic-rays, are able to reproduce the magnitude of the UHECR flux observed on Earth. For these models, the observed shape of the UHECR spectrum can be well reproduced above the ankle and the evolution of the composition is compatible with the trend suggested by Auger data. We discuss the implications of the softer proton component (consequence of the neutron emission in the sources) for the phenomenology of the transition from Galactic to extragalactic cosmic-ray, in the light of the recent composition analyses from the KASCADE-Grande experiment. Finally, we find that the associated secondary particle di use fluxes do not upset any current observational limit or measurement. Di use neutrino flux from GRB sources of the order of those we calculated should however be detected with the lifetime of neutrino observatories such as IceCube or KM3Net.

Jet formation in GRBs: a semi-analytic model of MHD flow in Kerr geometry with realistic plasma injection

We construct a semi-analytic model for magnetohydrodynamic (MHD) flows in Kerr geometry that incorporates energy loading via neutrino annihilation on magnetic field lines threading the horizon. We compute the structure of the double-flow established in the magnetisphere for a wide range of energy injection rates and identify the different operation regimes. At low injection rates, the outflow is powered by the spinning black hole via the Blandford-Znajek mechanism, whereas at high injection rates, it is driven by the pressure of the plasma deposited on magnetic field lines. In the intermediate regime, both processes contribute to the outflow formation. The parameter that quantifies the load is the ratio of the net power injected below the stagnation radius and the maximum power that can be extracted magnetically from the black hole.

ULTRA-RELATIVISTIC, NEUTRINO-DRIVEN FLOWS IN GAMMA-RAY BURSTS: A DOUBLE TRANSONIC FLOW SOLUTION IN A SCHWARZSCHILD SPACETIME

The structure of a hydrodynamic, double transonic flow driven by neutrino annihilation in the polar region of a Schwarzschild black hole is computed for different energy deposition profiles. The requirement that both, the inflow into the black hole and the outflow to infinity pass smoothly through their sonic points fixes the stagnation radius and stagnation pressure. The asymptotic power of the outflow is shown to be the integral of the energy deposition rate above the stagnation radius. The outflow production efficiency depends on the energy deposition profile, and is generally higher for shallower profiles. Using recent calculations of the neutrino annihilation rate, we estimate that over 50 percents of the total energy deposited above the horizon can emerge in the form of a relativistic outflow at infinity. The continuous creation of plasma during the expansion of the outflow leads to generation of a large specific entropy. This has important implications for the prompt photospheric emission in GRBs.

Loaded magnetohydrodynamic flows in Kerr spacetime

The effect of mass and energy loading on the efficiency at which energy can be extracted magnetically from a Kerr black hole is explored, using a semi-analytic, ideal MHD model that incorporates plasma injection on magnetic field lines. We find a critical load below which the specific energy of the plasma inflowing into the black hole is negative, and above which it is positive, and identify two types of flows with distinct properties; at sub-critical loads a magnetic outflow is launched from the ergosphere, owing to extraction of the black hole spin energy, as originally proposed by Blandford and Znajek. At super-critical loads the structure of the flow depends on the details of the injection process. In cases where the injected plasma is relativistically hot, a pressure driven, double trans-magnetosonic flow is launched from a stagnation point located outside the ergosphere, between the inner and outer light cylinders. Some fraction of the energy deposited in the magnetosphere is then absorbed by the black hole and the rest emerges at infinity in the form of a relativistic outflow. When the injected plasma is cold an outflow may not form at all. We discuss the implications of our results to gamma ray bursts and active galactic nuclei.

Probing the Extragalactic Cosmic-Ray Origin with Gamma-Ray and Neutrino Backgrounds

GeV-TeV gamma-ray and PeV-EeV neutrino backgrounds provide a unique window on the nature of the ultra-high-energy cosmic-rays (UHECRs). We discuss the implications of the recent Fermi-LAT data regarding the extragalactic gamma-ray background (EGB) and related estimates of the contribution of point sources as well as IceCube neutrino data on the origin of the UHECRs. We calculate the diffuse flux of cosmogenic

Magnetic collimation of meridional-self-similar general relativistic MHD flows

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Probing the Extragalactic Cosmic Rays origin with gamma-ray and neutrino backgrounds

-rays and neutrinos produced during the UHECRs propagation and derive constraints on the possible cosmological evolution of UHECR sources. In particular, we show that the mixed-composition scenario which is in agreement with both (i) Auger measurements of the energy spectrum and composition up to the highest energies and (ii) the ankle-like feature in the light component detected by KASCADE-Grande, is compatible with both the Fermi-LAT measurements and with current IceCube limits.

Can we reconcile the TA excess and hotspot with Auger observations

We present a model for the spine of relativistic MHD outflows in the Kerr geometry. Meridional self-similarity is invoked to derive semi-analytical solutions close to the polar axis. The study of the energy conservation along a particular field line gives a simple criterion for the collimation of jets. Such parameter have already been derived in the classical case by Sauty et al. 1999 and also extended to the Schwarzschild metric by Meliani et al. 2006. We generalize the same study to the Kerr metric. We show that the rotation of the black hole increases the magnetic self-confinement.