arXiv subjects

Gwenael Giacinti

Publications and source records attributed to Gwenael Giacinti.

At least 37 records · Page 2Linked to original sources

arXiv2023

Prospects for ultra-high-energy particle acceleration at relativistic shocks

We study the acceleration of charged particles by ultra-relativistic shocks using test-particle Monte-Carlo simulations. Two field configurations are considered: (i) shocks with uniform upstream magnetic field in the plane of the shock, and (ii) shocks in which the upstream magnetic field has a cylindrical geometry. Particles are assumed to diffuse in angle due to frequent non-resonant scattering on small-scale fields. The steady-state distribution of particles' Lorentz factors is shown to approximately satisfy $dN/dγ\propto γ^{-2.2}$ provided the particle motion is scattering dominated on at least one side of the shock. For scattering dominated transport, the acceleration rate scales as $t_{\rm acc}\propto t^{1/2}$, though recovers Bohm scaling $t_{\rm acc}\propto t$ if particles become magnetised on one side of the shock. For uniform field configurations, a limiting energy is reached when particles are magnetised on both sides of the shock. For the cylindrical field configuration, this limit does not apply, and particles of one sign of charge will experience a curvature drift that redirects particles upstream. For the non-resonant scattering model considered, these particles preferentially escape only when they reach the confinement limit determined by the finite system size, and the distribution approaches the escapeless limit $dN/dγ\propto γ^{-1}$. The cylindrical field configuration resembles that expected for jets launched by the Blandford $\&$ Znajek mechanism, the luminous jets of AGN and GRBs thus provide favourable sites for the production of ultra-high energy cosmic rays.

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arXiv2022

Using TeV Cosmic Rays to probe the Heliosphere's Boundary with the Local Interstellar Medium

The heliosphere is the magnetic structure formed by the Sun's atmosphere extending into the local interstellar medium (ISM). The boundary separating the heliosphere from the ISM is a still largely unexplored region of space. Even though both Voyager spacecraft entered the local ISM and are delivering data, they are two points piercing a vast region of space at specific times. The heliospheric boundary regulates the penetration of MeV- GeV galactic cosmic rays (CR) in the inner heliosphere. Interstellar keV neutral atoms are crucial to the outer heliosphere since they can penetrate unperturbed and transfer energy to the solar wind. Missions such as NASA's IBEX and Cassini are designed to detect neutral atoms and monitor charge exchange processes at the heliospheric boundary. The heliosphere does not modulate the TeV CR intensity, but it does influence their arrival direction distribution. Ground-based CR observatories have provided accurate maps of CR anisotropy as a function of energy in the last couple of decades. Combining observations to produce all-sky coverage makes it possible to investigate the heliosphere's impact on TeV CR particles. We can numerically calculate the pristine TeV CR distribution in the local ISM with state-of-the-art heliosphere models. Only by subtracting the heliospheric influence is it possible to use TeV CR observations to infer propagation properties and the characteristics of magnetic turbulence in the ISM. Numerical calculations of CR particle trajectories through heliospheric models provide a complementary tool to probe the boundary region properties. A program boosting heliospheric modeling with emphasis on the boundary region and promoting combined CR experimental data analyses from multiple experiments benefits CR astrophysics and provides additional data and tools to explore the interaction between the heliosphere and the local ISM.

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arXiv2022

The implications of TeV detected GRB afterglows for acceleration at relativistic shocks

Motivated by the detection of very high energy gamma-rays deep in the afterglow emission of a gamma-ray burst, we revisit predictions of the maximum energy to which electrons can be accelerated at a relativistic blast wave. Acceleration at the weakly-magnetized forward shock of a blast-wave can be limited either by the rapid damping of turbulence generated behind the shock, by the effect of a large-scale ambient magnetic field, or by radiation losses. Within the confines of a standard, single zone, synchrotron-self-Compton (SSC) model, we show that observations of GRB190829A rule out a rapid damping of the downstream turbulence. Furthermore, simultaneous fits to the X-ray and TeV gamma-ray emission of this object are not possible unless the limit on acceleration imposed by the ambient magnetic field is comparable or weaker than that imposed by radiation losses. This requires the dominant length scale of the turbulence behind the shock to be larger than that implied by particle-in-cell simulations. However, even then, Klein-Nishina effects prevent production of the hard VHE gamma-ray spectrum suggested by observations. Thus, TeV observations of GRB afterglows, though still very sparse, are already in tension with the SSC emission scenario.

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arXiv2021

Formation of giant plasmoids at the pulsar wind termination shock: A possible origin of the inner-ring knots in the Crab Nebula

Nearby pulsar wind nebulae exhibit complex morphological features: jets, torus, arcs and knots. These structures are well captured and understood in the scope of global magnetohydrodynamic models. However, the origin of knots in the inner radius of the Crab Nebula remains elusive. In this work, we investigate the dynamics of the shock front and downstream flow with a special emphasis on the reconnecting equatorial current sheet. We examine whether giant plasmoids produced in the reconnection process could be good candidates for the knots. To this end, we perform large semi-global three-dimensional particle-in-cell simulations in a spherical geometry. The hierarchical merging plasmoid model is used to extrapolate numerical results to pulsar wind nebula scales. The shocked material collapses into the midplane, forming and feeding a large-scale, but thin, ring-like current layer. The sheet breaks up into a dynamical chain of merging plasmoids reminiscent of three-dimensional reconnection. Plasmoids grow to a macroscopic size. The final number of plasmoids predicted is solely governed by the inverse of the dimensionless reconnection rate. The formation of giant plasmoids is a robust feature of pulsar wind termination shocks. They provide a natural explanation for the inner-ring knots in the Crab Nebula, provided that the nebula is highly magnetized.

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arXiv2021

Direct Numerical Simulations of Cosmic-ray Acceleration at Dense Circumstellar Medium: Magnetic Field Amplification by Bell Instability and Maximum Energy

Galactic cosmic rays are believed to be accelerated at supernova remnants. However, whether supernova remnants can be Pevatrons is still very unclear. In this work we argue that PeV cosmic rays can be accelerated during the early phase of a supernova blast wave expansion in dense red supergiant winds. We solve in spherical geometry a system combining a diffusive-convection equation which treats cosmic-ray dynamics coupled to magnetohydrodynamics to follow gas dynamics. The fast shock expanding in a dense ionized wind is able to trigger the fast non-resonant streaming instability over day timescales, and energizes cosmic-rays even under the effect of p-p losses. We find that such environments make the blast wave a Pevatron, although the maximum energy depends on various parameters such as the injection rate and mass-loss rate of the winds. Multi-PeV energies can be reached if the progenitor mass loss rates are of the order of $10^{-3}$ Msun yr$^{-1}$. It has been recently invoked that, prior to the explosion, hydrogen rich massive stars can produce enhanced mass loss rates. These enhanced rates would then favor the production of a Pevatron phase in early times after the shock breakout.

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arXiv2020

A global model of particle acceleration at pulsar wind termination shocks

Pulsar wind nebulae are efficient particle accelerators, and yet the processes at work remain elusive. Self-generated, microturbulence is too weak in relativistic magnetized shocks to accelerate particles over a wide energy range, suggesting that the global dynamics of the nebula may be involved in the acceleration process instead. In this work, we study the role played by the large-scale anisotropy of the transverse magnetic field profile on the shock dynamics. We performed large two-dimensional particle-in-cell simulations for a wide range of upstream plasma magnetizations. A large-scale velocity shear and current sheets form in the equatorial regions and at the poles, where they drive strong plasma turbulence via Kelvin-Helmholtz vortices and kinks. The mixing of current sheets in the downstream flow leads to efficient nonthermal particle acceleration. The power-law spectrum hardens with increasing magnetization, akin to those found in relativistic reconnection and kinetic turbulence studies. The high end of the spectrum is composed of particles surfing on the wake produced by elongated spearhead-shaped cavities forming at the shock front and piercing through the upstream flow. These particles are efficiently accelerated via the shear-flow acceleration mechanism near the Bohm limit. Magnetized relativistic shocks are very efficient particle accelerators. Capturing the global dynamics of the downstream flow is crucial to understanding them, and therefore local plane parallel studies may not be appropriate for pulsar wind nebulae and possibly other astrophysical relativistic magnetized shocks. A natural outcome of such shocks is a variable and Doppler-boosted synchrotron emission at the high end of the spectrum originating from the shock-front cavities, reminiscent of the mysterious Crab Nebula gamma-ray flares.

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arXiv2019

Electron Acceleration in the Crab Nebula

We study electron and positron acceleration at the termination shock of a striped pulsar wind. Drift motion along the shock surface keeps either electrons or positrons -but not both, close to the equatorial plane of the pulsar, where they are accelerated by the first-order Fermi process. Their energy spectrum is a power law, and both the X-ray flux and photon index of the Crab Nebula, as measured by NuSTAR, can be reproduced for sufficiently large downstream turbulence levels. The implication that one sign of charge is preferentially accelerated in pulsar wind nebulae is potentially important for the interpretation of the positron fraction in cosmic-rays.

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arXiv2019

Inductive acceleration of ions in Poynting-flux dominated outflows

Two-fluid (electron-positron) plasma modelling has shown that inductive acceleration can convert Poynting flux directly into bulk kinetic energy in the relativistic flows driven by rotating magnetized neutron stars and black holes. Here, we generalize this approach by adding an ion fluid. Solutions are presented in which all particles are accelerated as the flow expands, with comparable power channeled into each of the plasma components. In an ion-dominated flow, each species reaches the limiting rigidity, according to Hillas' criterion, in a distance significantly shorter than in a lepton-dominated flow. These solutions support the hypothesis that newly born magnetars and pulsars are potential sources of ultra-high energy cosmic rays. The competing process of Poynting flux dissipation by magnetic reconnection is shown to be ineffective in low-density flows in which the conventionally defined electron multiplicity satisfies $κ_{\rm e}\lesssim 10^5\left(4πL_{38}/Ω\right)^{1/4} /\textrm{Max}\left(η_{\rm ion}^{1/2},1\right)$, where $L_{38}\times 10^{38}\textrm{erg s}^{-1}$ is the power carried by the flow in a solid angle $Ω$, and $η_{\rm ion}$ is the ratio of the ion to lepton power at launch.

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arXiv2019

Numerical Simulations of Cosmic-Ray Acceleration at Core-Collapse Supernovae

Core-collapse supernovae exploding in dense winds are favorable sites for cosmic-ray (CR) acceleration to very high energies. We present our CR-radiation-hydrodynamics simulations of the explosion of a red supergiant. We study the evolution of the shock wave during the first day following core collapse, and estimate the time at which CR acceleration can start. We then calculate the maximum CR energy at the forward shock as a function of time, and show that it may already exceed 100 TeV only a few hours after shock breakout from the surface of the star.

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arXiv2018

Constraints on the properties of the turbulent magnetic field around Geminga using HAWC measurements

We place constraints on the properties of the interstellar turbulence that surrounds Geminga pulsar, using the recent measurements from the HAWC Observatory in this region. We propagate very-high-energy electrons in realizations of 3D isotropic Kolmogorov or Kraichnan turbulence, calculate their gamma-ray emission, and compare with HAWC measurements. We show that the measurements can be well fitted for both models of the turbulence and for reasonable values of its strength, $B_{\rm rms}$, and coherence length, $L_{\rm c}$. Our best fits are obtained for $B_{\rm rms} \simeq 3 μ$G and $L_{\rm c} \simeq 1$ pc. Furthermore, the absence of strong asymmetries in the observed emission favours $L_{\rm c} \leq 5$ pc.

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arXiv2018

TeV-PeV Cosmic-Ray Anisotropy and Local Interstellar Turbulence

We calculate the shape of the large-scale anisotropy of TeV-PeV cosmic-rays (CR) in different models of the interstellar turbulence. In general, the large-scale CR anisotropy (CRA) is not a dipole, and its shape can be used as a new probe of the turbulence. The 400 TeV and 2 PeV data sets of IceTop can be fitted with Goldreich-Sridhar turbulence and a broad resonance function, but other possibilities are not excluded. We then present our first numerical calculations of the CRA down to 3 TeV energies in 3D isotropic Kolmogorov turbulence. At these low energies, the large-scale CRA aligns well with the direction of local magnetic field lines around the observer. In this type of turbulence, the CR intensity is flat in a broad region perpendicular to field lines. Even though the CRA is quite gyrotropic, we show that the local configuration of the turbulence around the observer does result in the appearance of weak, "non-gyrotropic" small-scale anisotropies, which contain information on the local turbulence level.

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arXiv2018

Acceleration of X-ray Emitting Electrons in the Crab Nebula

We study particle acceleration at the termination shock of a striped pulsar wind by integrating trajectories in a prescribed model of the magnetic field and flow pattern. Drift motion on the shock surface maintains either electrons or positrons on "Speiser" orbits in a ring-shaped region close to the equatorial plane of the pulsar, enabling them to be accelerated to very high energy by the first-order Fermi mechanism. A power-law spectrum results: $dN_{\rm e}/dγ\proptoγ^{α_{\rm e}}$, where $α_{\rm e}$ lies in the range $-1.8$ to $-2.4$ and depends on the downstream turbulence level. For sufficiently strong turbulence, we find $α_{\rm e} \simeq -2.2$, and both the photon index and the flux of $1-100$ keV X-rays from the Crab Nebula, as measured by NuSTAR, can be reproduced. The particle spectrum hardens to $α_{\rm e} \simeq -1.8$ at lower turbulence levels, which may explain the hard photon index observed by the Chandra X-ray Observatory in the central regions of the Nebula.

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arXiv2018

Core collapse supernovae as Cosmic Ray sources

Core collapse supernovae (CCSNe) produce fast shocks which pervade the dense circum-stellar medium (CSM) of the stellar progenitor. Cosmic rays (CRs) if accelerated at these shocks can induce the growth of electromagnetic fluctuations in the foreshock medium. In this study, using a self-similar description of the shock evolution, we calculate the growth timescales of CR-driven instabilities. We select a sample of nearby core collapse radio supernova of type II and Ib/Ic. From radio data we infer the parameters which enter in the calculation of the instability growth times. We find that extended IIb SNe shocks can trigger fast intra day instabilities, strong magnetic field amplification and CR acceleration. In particular, the non-resonant streaming instability can contribute to about 50\% of the magnetic field intensity deduced from radio data. This results in the acceleration of CRs in the range 1-10 PeV within a few days after the shock breakout. In order to produce strong magnetic field amplification and CR acceleration a fast shocks pervading a dense CSM is necessary. In that aspect IIn supernovæ~are also good candidates. But a detailed modeling of the blast wave dynamics coupled with particle acceleration is mandatory for this class of object before providing any firm conclusions. Finally, we find that the trans-relativistic object SN 2009bb even if it produces more modest magnetic field amplification can accelerate CRs up to 2-3 PeV within 20 days after the outburst.

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arXiv2017

Constraining the properties of the magnetic turbulence in the Geminga region using HAWC $γ$-ray data

Observations of extended gamma-ray emission around Galactic cosmic-ray (CR) sources can be used as novel probes of interstellar magnetic fields. Using very-high-energy gamma-ray data from the HAWC Observatory, we place constraints on the properties of the magnetic turbulence within $\approx 25$ pc from Geminga. We inject and propagate individual CR electrons in 3D realizations of turbulent magnetic fields, calculate the resulting gamma-ray emission, and compare with HAWC measurements of this region. We find that HAWC data is compatible with expectations for Kolmogorov or Kraichnan turbulence, and can be well fitted for reasonable coherence lengths and strengths of the turbulence, despite implying a CR diffusion coefficient significantly smaller than those suggested by Galactic CR propagation codes. The best fit is found for a coherence length $L_{\rm c} \approx 1$ pc and a magnetic field strength $B_{\rm rms} \approx 3 μ$G, and the preferred value for $L_{\rm c}$ increases with $B_{\rm rms}$. Moreover, the apparent lack of strong asymmetries in the observed emission allows us to constrain the coherence length to $L_{\rm c} \lesssim 5$ pc in this region.

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arXiv2017

Inductive spikes and gamma-ray flares from the Crab Nebula

The ~400 MeV flaring emission from the Crab Nebula is naturally explained as the result of an abrupt reduction in the mass-loading of the pulsar wind. Very few particles are then available to carry the current required to maintain wave activity, causing them to achieve high Lorentz factors. When they penetrate the Nebula, a tightly beamed, high luminosity burst of hard gamma-rays results, with characteristics similar to the observed flares. This mechanism may operate in other powerful pulsars, such as J0537-6910 (PWN N 157B), B0540-69, B1957+20 and J0205+6449 (3C 58).

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arXiv2017

Galactic Cosmic-Rays in a Breeze

We study a scenario in which the Fermi bubbles are formed through a Galactocentric outflow of gas and pre-accelerated cosmic-rays (CR). We take into account CR energy losses due to proton-proton interactions with the gas present in the bubbles, and calculate the associated gamma-ray emission. We find that CRs diffusing and advecting within a breeze outflow result in an approximately flat surface brightness profile of the gamma-ray emission, as observed by Fermi satellite. Finally, we apply similar outflow profiles to larger Galactocentric radii, and investigate their effects on the CR spectrum and boron-to-carbon ratio. Hardenings can appear in the spectrum, even in cases with equal CR diffusion coefficients in the disk and halo.

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arXiv2017

Inductive spikes in the Crab Nebula - a theory of gamma-ray flares

We show that the mysterious, rapidly variable emission at ~400 MeV observed from the Crab Nebula by the AGILE and Fermi experiments could be the result of a sudden drop in the mass-loading of the pulsar wind. The current required to maintain wave activity in the wind is then carried by very few particles of high Lorentz factor. On impacting the Nebula, these particles produce a tightly beamed, high luminosity burst of hard gamma-rays, similar to those observed. This implies (i) the emission is synchrotron radiation in the toroidal field of the Nebula, and, therefore, linearly polarized and (ii) this mechanism potentially contributes to the gamma-ray emission from other powerful pulsars, such as the Magellanic Cloud objects J0537-6910 and B0540-69.

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arXiv2017

Large-Scale Cosmic-Ray Anisotropy as a Probe of Interstellar Turbulence

We calculate the large-scale cosmic-ray (CR) anisotropies predicted for a range of Goldreich-Sridhar (GS) and isotropic models of interstellar turbulence, and compare them with IceTop data. In general, the predicted CR anisotropy is not a pure dipole; the cold spots reported at 400 TeV and 2 PeV are consistent with a GS model that contains a smooth deficit of parallel-propagating waves and a broad resonance function, though some other possibilities cannot, as yet, be ruled out. In particular, isotropic fast magnetosonic wave turbulence can match the observations at high energy, but cannot accommodate an energy dependence in the shape of the CR anisotropy. Our findings suggest that improved data on the large-scale CR anisotropy could provide a valuable probe of the properties - notably the power-spectrum - of the interstellar turbulence within a few tens of parsecs from Earth.

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