Particle Acceleration In Shocks Research Articles

The impact of kinetic effects on the properties of relativistic electron–positron shocks

We assess the impact of non-thermally shock-accelerated particles on the magnetohydrodynamic (MHD) jump conditions of relativistic shocks. The adiabatic constant is calculated directly from first-principles particle-in-cell simulation data, enabling a semi-kinetic approach to improve the standard fluid model and allowing for an identification of the key parameters that define the shock structure. We find that the evolving upstream parameters have a stronger impact than the corrections due to non-thermal particles. We find that the decrease in the upstream bulk speed result in deviations from the standard MHD model up to 10%. Furthermore, we obtain a quantitative definition of the shock transition region from our analysis. For Weibel-mediated shocks the inclusion of a magnetic field in the MHD conservation equations is addressed for the first time.

Maximum synchrotron frequency for shock-accelerated particles

Abstract It is widely believed that the maximum energy of synchrotron photons when electrons are accelerated in shocks via the Fermi process is about 50 MeV (in plasma comoving frame). We show that under certain conditions, which are expected to be realized in relativistic shocks of gamma-ray bursts, synchrotron photons of energy much larger than 50 MeV (comoving frame) can be produced. The requirement is that magnetic field should decay downstream of the shock front on a length-scale that is small compared with the distance travelled by the highest energy electrons before they lose half their energy; photons of energy much larger than 50 MeV are produced close to the shock front, whereas the highest Lorentz factor that electrons can attain is controlled by the much weaker field that occupies most of the volume of the shocked plasma.

Diffuse Galactic Gamma Rays from Shock-Accelerated Cosmic Rays

A shock-accelerated particle flux is proportional to p(-s), where p is the particle momentum, follows from simple theoretical considerations of cosmic-ray acceleration at nonrelativistic shocks followed by rigidity-dependent escape into the Galactic halo. A flux of shock-accelerated cosmic-ray protons with s≈2.8 provides an adequate fit to the Fermi Large Area Telescope γ-ray emission spectra of high-latitude and molecular cloud gas when uncertainties in nuclear production models are considered. A break in the spectrum of cosmic-ray protons claimed by Neronov, Semikoz, and Taylor [Phys. Rev. Lett. 108, 051105 (2012)] when fitting the γ-ray spectra of high-latitude molecular clouds is a consequence of using a cosmic-ray proton flux described by a power law in kinetic energy.

NON-STANDARD ENERGY SPECTRA OF SHOCK-ACCELERATED SOLAR PARTICLES

We consider a numerical model for the shock acceleration of energetic ions in the magnetic environment of the solar corona. The model is motivated by observations of the deka-to-hecto-MeV proton energy spectra, ion and electron timing, and abundances in the beginning of major solar energetic particle (SEP) events, prior to the event's main phase associated with coronal mass ejection (CME) driven shock in the solar wind. Inasmuch as the obliquity of the CME-liftoff-associated shocks in solar corona and hence the seed-particle supply for the shock acceleration are essentially time dependent, a steady state energy spectrum of accelerated protons near the shock could not be attained. Energy spectrum of the SEP emission depends on the spatial and energy distribution of seed particles for the coronal shock acceleration, on the shock wave history, and on the location and scenario of the energetic particle escape into the interplanetary medium. We use a numerical model of the shock acceleration on a semicircular magnetic field line to learn a significance of different effects. If the shock geometry in a particular magnetic tube changes from nearly parallel to perpendicular, the resulting SEP spectrum in most distant sections of the tube, e.g., at the top of a transequatorial loop, resembles a wide beam, which is very different from the standard power-law spectrum that would be expected in a steady state. Possible escape of the shock-accelerated particles from more than one coronal location, stochastic re-acceleration, and the magnetic tube expansion can make the SEP spectra even more complicated.

MODELING HIGH-ENERGY LIGHT CURVES OF THE PSR B1259–63/LS 2883 BINARY BASED ON 3D SPH SIMULATIONS

Temporal changes of X-ray to very-high-energy gamma-ray emissions from the pulsar-Be star binary PSR B1259-63/LS 2883 are studied based on 3-D SPH simulations of pulsar wind interaction with Be-disk and wind. We focus on the periastron passage of the binary and calculate the variation of the synchrotron and inverse-Compton emissions using the simulated shock geometry and pressure distribution of the pulsar wind. The characteristic double-peaked X-ray light curve from observations is reproduced by our simulation under a dense Be disk condition (base density ~10^{-9} g cm^{-3}). We interpret the pre- and post-periastron peaks as being due to a significant increase in the conversion efficiency from pulsar spin down power to the shock-accelerated particle energy at orbital phases when the pulsar crosses the disk before periastron passage, and when the pulsar wind creates a cavity in the disk gas after periastron passage, respectively. On the contrary, in the model TeV light curve, which also shows a double peak feature, the first peak appears around the periastron phase. The possible effects of cooling processes on the TeV light curve are briefly discussed.

Studying astrophysical collisionless shocks with counterstreaming plasmas from high power lasers

Collisions of high Mach number flows occur frequently in astrophysics, and the resulting shock waves are responsible for the properties of many astrophysical phenomena, such as supernova remnants, Gamma Ray Bursts and jets from Active Galactic Nuclei. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are “collisionless”, since they form due to plasma instabilities and self-generated magnetic fields. Laboratory experiments at the laser facilities can achieve the conditions necessary for the formation of collisionless shocks, and will provide a unique avenue for studying the nonlinear physics of collisionless shock waves. We are performing a series of experiments at the Omega and Omega-EP lasers, in Rochester, NY, with the goal of generating collisionless shock conditions by the collision of two high-speed plasma flows resulting from laser ablation of solid targets using ∼10 16 W/cm 2 laser irradiation. The experiments will aim to answer several questions of relevance to collisionless shock physics: the importance of the electromagnetic filamentation (Weibel) instabilities in shock formation, the self-generation of magnetic fields in shocks, the influence of external magnetic fields on shock formation, and the signatures of particle acceleration in shocks. Our first experiments using Thomson scattering diagnostics studied the plasma state from a single foil and from double foils whose flows collide “head-on”. Our data showed that the flow velocity and electron density were 10 8 cm/s and 10 19 cm −3, respectively, where the Coulomb mean free path is much larger than the size of the interaction region. Simulations of our experimental conditions show that weak Weibel mediated current filamentation and magnetic field generation were likely starting to occur. This paper presents the results from these first Omega experiments.

Heavy-ion acceleration and self-generated waves in coronal shocks

Context: Acceleration in coronal mass ejection driven shocks is currently considered the primary source of large solar energetic particle events. Aims: The solar wind, which feeds shock-accelerated particles, includes numerous ion populations, which offer much insight into acceleration processes. We present first simulations of shock-accelerated minor ions, in order to explore trapping dynamics and acceleration timescales in detail. Methods: We have simulated diffusive shock acceleration of minor ions (3He2+, 4He2+, 16O6+ and 56Fe14+) and protons using a Monte Carlo method, where self-generated Alfv\'enic turbulence allows for repeated shock crossings and acceleration to high energies. Results: We present the effect of minor ions on wave generation, especially at low wavenumbers, and show that it is significant. We find that maximum ion energy is determined by the competing effects of particle escape due to focusing in an expanding flux tube and trapping due to the amplified turbulence. We show the dependence of cut-off energy on the particle charge to mass ratio to be approximately (Q/A)^1.5. Conclusions: We suggest that understanding the acceleration of minor ions at coronal shocks requires simulations which allow us to explore trapping dynamics and acceleration timescales in detail, including evolution of the turbulent trapping boundary. We conclude that steady-state models do not adequately describe the acceleration of heavy ions in coronal shocks.

Numerical modeling of relativistic particle shock acceleration

The shock acceleration mechanism is invoked to explain non-thermal cosmic rays in Supernova Remnants, Active Galactic Nuclei and Gamma Ray Bursts jets. Espe- cially, the importance of relativistic shock acceleration in ex- tragalactic sources is a recurring theme raising a significant interest in the research community. We will briefly overview the shock acceleration mechanism and we will address the properties of non-relativistic and relativistic shocks, particu- larly focusing on relativistic simulation studies. 1 Outline It is accepted that Super Novae Remnants (SNRs) are plau- sible environments for the acceleration of cosmic-ray parti- cles up to energies of about 10 17 eV, while for the higher en- ergies, Active Galactic Nuclei (AGN) and possibly Gamma Ray Bursts (GRBs) are favorable candidates. It is believed that the source of cosmic rays is plasma colliding at su- personic speeds, where shock waves form along with other instabilities, competing for the dissipation and acceleration mechanisms. It is until now not fully understood to which extent the astrophysical bulk flows are due to leptonic flow (electrons and positrons) or baryonic flow (electrons and ions), and by which exact mechanisms this bulk flow energy can be con- verted into cosmic ray radiation, reaching energies of TeV and beyond. Evidence in form of power-law spectra of the observed cosmic ray radiation over wide energy intervals, favors the Fermi shock acceleration mechanism (i.e. first or- der Fermi acceleration), namely diffusive shock acceleration mechanism, which raises a significant interest in the research community. In the diffusive shock acceleration mechanism

Survey of High-Beta Superthermal Plasma Events in the Interplanetary Medium as Observed by Ulysses Throughout its Mission Lifetime (1990–2009)

The ratio of the plasma pressure to the magnetic field pressure (or the ratio of their energy densities)-plasma parameter “beta” (β)-is critical in determining the dynamics of the interaction of the solar wind with planetary magnetospheres and has important implications to the propagation of energetic particles and to the structure of the shocks. In the interplanetary space, the value of “beta” is usually in the range of 0.1-1.0, and the contribution of the superthermal particles to the plasma pressure is generally assumed negligible. However, the analysis of energetic particles and magnetic field measurements by the Ulysses spacecraft shows that, in a series of events, the energy density contained in the superthermal tail of the particle distribution is comparable to or even higher than the one of the magnetic field, creating conditions of high-beta plasma. In this paper, we extensively survey and analyze measurements of the energy density ratio (parameter ) of the energetic particles (20 keV-5 MeV) to the magnetic fields by Ulysses, for its entire trajectory (1990-2009), in order to find occurrences of high-beta (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ep</sub> >; 1) superthermal plasma conditions. As expected, periods of dominant magnetic energy were observed most of the time, for the above mentioned particle energy range. Nevertheless, a number of 484 distinct periods with a total duration of 406.02 days were identified when the energy density carried by the energetic ions overwhelmingly dominated that of the magnetic field. These interplanetary high-beta (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ep</sub> >; 1) events are characterized by a very high parameter (up to ~1733), a great total duration (of 406.02 days), and a large percentage (11.3%) of the total mission lifetime (in presence of energetic particle events). Furthermore, these events were detected within well-identified regions corresponding mainly to the following: 1) the vicinity of shock fronts; 2) the vicinity of large transient disturbances associated with shock waves; 3) filamentary magnetic structures; and 4) the vicinity of the Jovian magnetospheric environment. These events were also associated with energetic particle intensity enhancements due to the following: 1) reacceleration at interplanetary corotating interaction regions (CIRs), solar flare (blast), transient, and/or CME-driven shocks; 2) unusually large magnetic field depressions; 3) Jupiter's bow shock particle acceleration; and 4) leakage from Jupiter's magnetosphere.

The upstream magnetic field of collisionless GRB shocks: constraint by Fermi-LAT observations

Long-lived > 100 MeV emission has been a common feature of mostFermi-LAT detected gamma-ray bursts (GRBs), e.g., detected up to ∼ 103s in long GRBs 080916C and 090902B and ∼ 102s inshort GRB 090510. This emission is consistent with being producedby synchrotron emission of electrons accelerated to high energy bythe relativistic collisionless shock propagating into the weaklymagnetized medium. Here we show that this high-energy afterglowemission constrains the preshock magnetic field to satisfy100n09/8mG < Bu < 102n03/8mG, where n0 is thepreshock density in unit of 1 cm−3, more stringent thanthe previous constraint by X-ray afterglow observations on dayscale. This suggests that the preshock magnetic field is stronglyamplified, most likely by the streaming of high energy shockaccelerated particles.

Specular refection at a non-stationary shock: A simple model

Analytic treatments of a particle encountering a collisionless shock have commonly been based on the assumption that the shock surface is quasi-planar with length scales larger than the particle gyroradius. Within this framework, the particle distribution function width is supposed to be conserved in any shock reflection process. It is well known, however, that the thermal energy associated with backstreaming ions upstream of Earth's bow shock is significantly larger than the incident solar wind thermal energy. In a previous study, we found that non-thermal features of ions reflected quasi-adiabatically can be accounted for by considering the effect of small, normally distributed fluctuations of the shock normal over short temporal or spatial scales. The strong dependence of the particle acceleration on shock geometry leads to an increase in the temperature and to a non-thermal tail. Here, we conduct a similar analysis to investigate the effects of small, normally distributed fluctuations in the shock normal direction for specularly reflected ions. This later mechanism is considered of first importance in the dissipation process occurring at quasi-perpendicular shocks. We have derived the probability distribution functions f ( v ∥ ) and f ( v ⊥ ) of ions issued from a specular reflection of incident solar wind in the presence of normal direction fluctuations. These distributions deviate weakly from a Maxwellian, in agreement with the observations. In particular, a qualitative agreement with the ion thermal energy is obtained for fluctuations of the normal orientation in the 5–8° range about the nominal direction. Also, we have found that the shock θ B n has a weak effect on the shape of the distribution. While, not a strong determinant of the reflected distribution characteristics, the dynamical shock structure at ion scales cannot be ignored when accounting for the shock-accelerated particle thermal energy.

Why should the latitude of the observer be considered when modeling gradual proton events? An insight using the concept of cobpoint

The shape of flux profiles of gradual solar energetic particle (SEP) events depends on several not well-understood factors, such as the strength of the associated shock, the relative position of the observer in space with respect to the traveling shock, the existence of a background seed particle population, the interplanetary conditions for particle transport, as well as the particle energy. Here, we focus on two of these factors: the influence of the shock strength and the relative position of the observer. We performed a 3D simulation of the propagation of a coronal/interplanetary CME-driven shock in the framework of ideal MHD modeling. We analyze the passage of this shock by nine spacecraft located at ∼0.4 AU (Mercury’s orbit) and at different longitudes and latitudes. We study the evolution of the plasma conditions in the shock front region magnetically connected to each spacecraft, that is the region of the shock front scanned by the “cobpoint” ( Heras et al., 1995), as the shock propagates away from the Sun. Particularly, we discuss the influence of the latitude of the observer on the injection rate of shock-accelerated particles and, hence, on the resulting proton flux profiles to be detected by each spacecraft.

WAVE TRANSMISSION AND HARD PARTICLE SPECTRA FROM PARALLEL SHOCKS

I review recent results concerning the effects that relativistic parallel shocks can have on the particle-scattering turbulence, and how these effects, in turn, can change the shock's particle acceleration properties. I discuss the possibility of increased compression ratio due to this kind of turbulence transmission, and consider the effects of additional compression on the energy spectrum of the accelerated particles. Emphasis is put on the possibility of producing spectral indices that are significantly harder than what is usually expected from the first-order Fermi mechanism, but required by some observations. Finally, I discuss the physical requirements — and their plausibility in real astronomical objects — of this mechanism to have notable effects.

Superdiffusive transport of electrons accelerated at corotating interaction regions

Energetic particles in the solar wind are accelerated at interplanetary shock waves associated with corotating interaction regions (CIRs) and propagate in the presence of a spectrum of magnetic turbulence, which causes pitch angle scattering upstream of the shock. Besides normal diffusive transport, we study the possibility of superdiffusive transport characterized by Levy random walks. We consider the relevant propagator, which is a power law with slope 2 &lt; μ &lt; 3, instead of being a Gaussian, as for standard diffusion. We show that in the case of superdiffusive transport, the time profile of particles accelerated at a travelling planar shock is a power law with slope γ = μ−2, rather than an exponential decay as expected in the case of normal diffusion. We consider a data set of interplanetary shocks in the solar wind, the CIR‐associated forward and reverse shocks detected by Ulysses between January 1992 and May 1993. We find that the time profiles of energetic electrons, with energies between 42 and 290 keV, correspond to power laws, with slopes γ≃ 0.60–0.98, implying superdiffusive transport. These results show that the propagation of energetic electrons in the turbulent environment of the solar wind is intermediate between ballistic (or scatter‐free) and diffusive, in agreement with the results of recent numerical simulations. Our results promise to have application in models of shock particle acceleration and represent a new tool for the interpretation of the observed particle profiles throughout the heliosphere.

Particle shock acceleration in astrophysics

The Fermi (diffusive) particle acceleration in astrophysical shocks is reviewed and evaluated. We discuss their properties and we present Monte Carlo simulations studying the shocks’ efficiency in accelerating particles (i.e. protons) up to very high energies with an application to astrophysical regions such as Supernovae, Active Galactic Nuclei hot spots and Gamma Ray Bursts. We find that the efficiency of the acceleration mechanism at shocks, varies in regard to the inclination of the magnetic field and the shock normal (e.g. sub-luminal shocks, super-luminal shocks), with consequences to the contribution of the very high energy particles to the observed cosmic ray spectrum.

Stochastic Acceleration in the Western Hot Spot of Pictor A

Chandra's high-resolution observations of radio galaxies require a revisit of the relevant electron acceleration processes. Although the diffusive shock particle acceleration model may explain spectra of spatially unresolved sources, it encounters difficulties in explaining the structure and spectral properties of recently discovered Chandra X-ray features in several low-power radio sources. We argue that these observations strongly suggest stochastic electron acceleration by magnetized turbulence, and show that the simplest stochastic particle acceleration model with energy-independent acceleration and escape timescales can overcome most of these difficulties. We use the bright core of the western hot spot of Pictor A as an example to demonstrate the model characteristics, which may be tested with high-energy observations.

Accretion vs. colliding wind models for the gamma-ray binary LS I +61 303: an assessment

Context. LS I +61 303 is a puzzling Be/X-ray binary with variable gamma-ray emission up to TeV energies. The nature of the compact object and the origin of the high-energy emission are unclear. One family of models invokes particle acceleration in shocks from the collision between the B-star wind and a relativistic pulsar wind, whereas another centers on a relativistic jet powered by accretion from the Be star decretion disc onto a black hole. Recent high-resolution radio observations showing a putative “cometary tail” pointing away from the Be star near periastron have been cited as support for the pulsar-wind model. Aims. We wish to carry out a quantitative assessment of these competing models. Methods. We apply a “Smoothed Particle Hydrodynamics” (SPH) code in 3D dynamical simulations for both the pulsar-windinteraction and accretion-jet models. The former yields a dynamical description of the shape of the wind-wind interaction surface. The latter provides a dynamical estimation of the accretion rate under a variety of conditions, and how this varies with orbital phase. Results. The results allow critical evaluation of how the two distinct models confront the data in various wavebands. When one accounts for the 3D dynamical wind interaction under realistic constraints for the relative strength of the B-star and pulsar winds, the resulting form of the interaction front does not match the putative “cometary tail” claimed from radio observations. On the other hand, dynamical simulations of the accretion-jet model indicate that the orbital phase variation of accretion power includes a secondary broad peak well away from periastron, thus providing a plausible way to explain the observed TeV gamma ray emission toward apastron. Conclusions. Contrary to previous claims, the colliding-wind model is not clearly established for LS I +61 303, whereas the accretionjet model can reproduce many key characteristics, such as required energy budget, lightcurve, and spectrum of the observed TeV gamma-ray emission.

Evidence for Nonlinear Diffusive Shock Acceleration of Cosmic Rays in the 2006 Outburst of the Recurrent Nova RS Ophiuchi

Spectroscopic observations of the 2006 outburst of the recurrent nova RS Ophiuchi at both infrared (IR) and X-ray wavelengths have shown that the blast wave has decelerated at a higher rate than predicted by the standard test-particle adiabatic shock-wave model. Here we show that the observed evolution of the nova remnant can be explained by the diffusive shock acceleration of particles at the blast wave and the subsequent escape of the highest energy ions from the shock region. Nonlinear particle acceleration can also account for the difference of shock velocities deduced from the IR and X-ray data. The maximum energy that accelerated electrons and protons can have achieved in few days after outburst is found to be as high as a few TeV. Using the semi-analytic model of nonlinear diffusive shock acceleration developed by Berezhko & Ellison, we show that the postshock temperature of the shocked gas measured with RXTE/PCA and Swift/XRT imply a relatively moderate acceleration efficiency.

Cluster Merger Shock Constraints on Particle Acceleration and Nonthermal Pressure in the Intracluster Medium

X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well as the shock acceleration of particles and their escape, all affect shock jump conditions in distinct ways. Therefore, these processes can be constrained using X-ray surface brightness and temperature maps of merger shock fronts. Here we use these observations to place constraints on particle acceleration efficiency in intermediate Mach number (M ~ 2-3) shocks and explore the potential to constrain the contribution of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in merger shocks discovered in the galaxy clusters A520 (M ~ 2) and 1E 0657-56 (M ~ 3) using a multifluid model comprised of a thermal plasma, a nonthermal plasma, and a magnetic field. Based on the published X-ray spectroscopic data alone, we find that the fractional contribution of cosmic rays accelerated in these shocks is lower than about 10% of the shock downstream pressure. Current observations do not constrain the fractional contribution of nonthermal components to the pressure of the undisturbed shock upstream. Future X-ray observations, however, have the potential to either detect particle acceleration in these shocks through its effect on the shock dynamics, or to place a lower limit on the nonthermal pressure contributions in the undisturbed ICM. We briefly discuss implications for models of particle acceleration in collisionless shocks and the estimates of galaxy cluster masses derived from X-ray and Sunyaev-Zel'dovich effect observations.

The Suprathermal Ion Telescope (SIT) For the IMPACT/SEP Investigation

The Solar-Terrestrial Relations Observatory (STEREO) mission addresses critical problems of the physics of explosive disturbances in the solar corona, and their propagation and interactions in the interplanetary medium between the Sun and Earth. The In-Situ-Measurements of Particles and CME Transients (IMPACT) investigation observes the consequences of these disturbances and other transients at 1 AU. The generation of energetic particles is a fundamentally important feature of shock-associated Coronal Mass Ejections (CMEs) and other transients in the interplanetary medium. Multiple sensors within the IMPACT suite measure the particle population from energies just above the solar wind up to hundreds of MeV/nucleon. This paper describes a portion of the IMPACT Solar Energetic Particles (SEP) package, the Suprathermal Ion Telescope (SIT) which identifies the heavy ion composition from the suprathermal through the energetic particle range (∼few 10 s of keV/nucleon to several MeV/nucleon). SIT will trace and identify processes that energize low energy ions, and characterize their transport in the interplanetary medium. SIT is a time-of-flight mass spectrometer with high sensitivity designed to derive detailed multi-species particle spectra with a cadence of 60 s, thereby enabling detailed studies of shock-accelerated and other energetic particle populations observed at 1 AU.