Surfing and drift acceleration at high mach number quasi-perpendicular shocks
Electron acceleration in high Mach number collisionless shocks relevant to supernova remnant is discussed. By performing one- and two-dimensional particle-in-cell simulations of quasi-perpendicular shocks, we find that energetic electrons are quickly generated in the shock transition region through shock surfing and drift acceleration. The electron energization is strong enough to account for the observed injection at supernova remnant shocks.
Osservatorio Astrofisico di Arcetri
PIC simulations of relativistic transverse magnetosonic shocks
I will present the results of one-dimensional PIC simulations of magnetized ultrarelativistic shock waves in proton-electron-positron plasmas. Relativistic cyclotron instability, as the incoming particles encounter the increasing magnetic field within the shock front, provides the basic plasma heating mechanism. When the protons provide a sufficiently large fraction of the upstream flow energy density (including particle kinetic energy and Poynting flux), a substantial fraction of the shock heating goes into the formation of suprathermal power-law spectra of electrons and positrons. Cyclotron absorption by the pairs of the high harmonic ion cyclotron waves provides the nonthermal acceleration mechanism. The major new results come from simulations with mass ratio of 100 between ions and pairs, which show that electrons can be accelerated as efficiently as positrons when the proton fraction is small enough (pair plasma almost charge-symmetric). Both the acceleration efficiency and the non-thermal particles' spectra depend on the fraction of flow energy carried by the ions, suggesting that the varying power-law spectra observed in synchrotron sources powered by magnetized winds and jets might reflect the correlation of the proton-to-pair content enforced by the underlying electrodynamics of these sources' outflows.
University of Oxford & Rutherford Appleton Laboratory
Amplification of magnetic field near SNR shocks and cosmic-ray origin
The diffusive shock acceleration of cosmic rays (CR) has to be modeled kinetically in momentum and configuration space. For most aspects of the process particles can be divided into two components: (i) high energy CR which are essentially collisionless with a large Larmor radius and should be modeled kinetically and (ii) low energy thermal particles with small Larmor radius that can be modeled as a fluid. The two components are coupled through the magnetic field which is 'frozen in' to the thermal plasma and an ideal MHD model can be used, although the model must be three-dimensional because the field is generated by a non-linear instability driven by the CR. Observation and theory indicate that magnetic field can be amplified by orders of magnitude above its upstream value, thus facilitating cosmic ray acceleration to high energy. The full 3D interaction between CR and the magnetized thermal fluid under conditions relevant to supernova shocks has yet to be modeled kinetically and self-consistently. In this talk, I shall outline the basic physics which must be included in a comprehensive model. The process of magnetic field amplification is strong in dense plasma, so I shall discuss various additional processes which might occur during or following shock breakout in supernovae and their relevance to x-ray flashes or under-luminous gamma-ray bursts.
University of New Hampshire
PIC simulations of particle acceleration and heating during magnetic reconnection in pair plasmas
Magnetic reconnection in electron-positron (pair) plasmas has attracted significant attention recently, and particle-in-cell (PIC) simulations have shed new light on the underlying physics. Because the mass of the ion is the same as that of the electron, there is an exact cancellation of the Hall current in pair plasmas. Consequently, there is no scale separation between ion and electron motion in the diffusion region. Fast magnetic reconnection is realized due to the off-diagonal components in the positron and electron pressure tensors. It has been demonstrated in the non-relativistic regime that on a fast time-scale, in the absence of a guide-field, the effect of the pressure tensors is essentially equivalent to a collisionless resistivity at the X-line. We have also carried out large scale 2D simulations of a relativistic Harris sheet with no guide-field, and identified the mechanisms that energize particles in the presence of X-lines, contracting magnetic islands, and island coalescence. The most energetic particles are observed in the vicinity of X-lines, where particles are accelerated by the reconnection electric field. Within magnetic islands, particles are accelerated during a contraction process. At the two ends of magnetic islands, strong out-of plane electric fields are produced by downstream outflows. Particles are energized by these electric fields through multiple reflections. We also elucidate the heating and cooling effects associated with island coalescence. Whereas the thin current sheet where reconnection occurs is a site of particle heating, the regions surrounding the current sheet between two coalescing islands produce cooling.
University of New Hampshire
Magnetic reconnection in the heliosphere: impulsive dynamics and particle acceleration
Magnetic reconnection in the heliosphere is often time-dependent and impulsive, as in the classic instance of solar flares. A large fraction of the energy liberated during such events shows up in the energy of accelerated particles. Collisionless reconnection theory can account for significant features of these observations, but several open questions remain. Recent theoretical developments and observations that shed light on impulsive recon-nection dynamics and particle acceleration mechanisms will be reviewed.
ETSI Industriales - Universidad Castilla la Mancha
Instabilities of a relativistic electron beam in a plasma
The linear theory describing the interaction of a relativistic electron beam with a plasma is relevant to many fields of physics, and especially of astrophysics. The unstable spectrum is particularly rich as the system is subject to various instabilities such as the two-stream or the filamentation ones. In this talk, we review the progresses made from the understanding of the two-stream instability to the unified description of the whole unstable spectrum. After introducing the various unstable modes, emphasis is made on determining the fastest growing one in terms of the system parameters. The magnetized case is briefly mentioned.
University of California, Berkeley
Relativistic MHD modeling of PWN
Our understanding of Pulsar Wind Nebulae (PWN), has greatly improved in the last years thanks to unprecedented high resolution images taken from the HUBBLE, CHANDRA and XMM satellites. This has prompted a new investigation into the dynamics of the interaction of the pulsar winds with the surrounding SNR, which has let to a better understanding of the properties of these objects. On the other hand the discovery of non-thermal emission from bow shock PWN, and of systems with a complex interaction between pulsar and SNR, has led to the development of more reliable evolutionary models. I will review the standard theory of PWN, their evolution, and the current status in the modeling of their emission properties, in particular I will show that our evolutionary models are able to describe the observations, and that many aspects of the X-ray emission can now be reproduced with sufficient accuracy, to the point that we can use these nebulae to investigate fundamental issues as the properties of relativistic outflows and particle acceleration.
MPI for Solar System Rasearch
Magnetic reconnection at the Sun - observations and numerical simulation models
Magnetic reconnection is thought supposed to play an important role during the initiation of solar flares and CMEs, but also in coronal heating and solar particle acceleration. Unfortunately, current sheets, the sites of magnetic reconnection, are not directly observable at sun. Hence, numerical modeling is desired as an appropriate tool to better understand the formation, properties and behavior of current sheets and, finally, reconnection in the solar atmosphere. We present the results of numerical simulations based on observations of the photospheric magnetic field. We derive the formation of current sheets and the consequent three-dimensional magnetic reconnection in the solar corona. We compare signatures of the latter with those observed in the solar corona.
Astronomy Unit, Queen Mary London
Electron acceleration at quasi-perpendicular shocks: the effects of surface fluctuations
Shock associated acceleration of electrons is a common feature in the solar system, and of relevance to astrophysical shocks. At intermediate Mach number, typical of the solar system, the presence of reflected protons induces surface fluctuations at the shock. Results are presented of simulation studies of electron acceleration for high Mach number, quasi-perpendicular shocks. Two-dimensional self-consistent hybrid shock simulations provide the electric and magnetic fields in which test particle electrons are followed; the rationale for this model is explained. A range of different shock types, shock normal angles, and injection energies are studied. When the Mach number is low, so that there are no fluctuations along the magnetic field direction, the results agree with theory assuming magnetic moment conserving reflection (or fast Fermi acceleration), with electron energy gains of a factor only 2-3. For higher Mach numbers, with a realistic simulation configuration, the shock front has a dynamic rippled character. The corresponding electron energization is radically different: energy spectra display (1) considerably higher maximum energies than fast Fermi acceleration; (2) power-law falloff with increasing energy, for both upstream and downstream particles, with a slope decreasing as the shock normal angle approaches perpendicular; (3) sustained flux levels over a broader region of shock normal angle than for adiabatic reflection. These results indicate the need for 2D or 3D simulations for studies of electron shock acceleration.
Scuola Normale Superiore (PISA- Italy)
The dynamical effects of self-generated magnetic fields in cosmic-ray-modified shocks
Evidences of greatly amplified magnetic fields (\delta B/B \approx 100) around supernova shocks are consistent with the predictions of the non-linear theory of particle acceleration (NLT), if the field is generated upstream of the shock by cosmic ray induced streaming instability. The high acceleration efficiencies and large shock modifications predicted by NLT need, however, to be mitigated to confront observations, and this is usually assumed to be accomplished by some form of turbulent heating. We show that the pressure and the energy density of magnetic fields with the strength inferred from observations have an important dynamical role on the shock, implying, with respect to the standard predictions of NLT: 1) a shock modification substantially reduced and in good agreement with the inferred values; 2) an enhanced maximum momentum of the accelerated particles; 3) a reduction of the concavity of the cosmic ray spectra. The relative importance of this effect, which is unavoidable when the pressure in the turbulent magnetic field becomes comparable with the pressure of the thermal gas, and of the poorly known turbulent heating is assessed. More specifically, we conclude that in young supernova remnants, even if the turbulent heating may be of some importance, the magnetic feedback cannot be neglected, as is usually done in most current calculations.
Broad-band observations of AGN jet structure
The formation of a relativistic partially electromagnetic planar plasma shock
Relativistically colliding plasma is modelled by PIC simulations in one and two spatial dimensions at a high ion-to-electron mass ratio and a temperature of 100 keV, which may be representative for the jets of gamma-ray bursts. The energy of an initial quasi-parallel magnetic field is one percent of the plasma kinetic energy, which suppresses the beam filamentation instability. Energy dissipation by a growing wave pulse of mixed polarity, probably an oblique whistler wave, and different densities of the colliding plasma slabs result in a shock forming during milliseconds. The shock, which develops for a collision speed of 0.9c, accelerates electrons to ultrarelativistic speeds. A downstream region forms, in which the plasma approaches an energy equipartition between electrons, ions and the magnetic field.
University of Maryland
Magnetic reconnection: dynamics and particle acceleration
A significant fraction of the magnetic energy released during magnetic reconnection in solar flares appears as energetic electrons and protons and the observations suggest a common acceleration mechanism. How this conversion of magnetic energy takes place so efficiently has been a scientific topic of great interest. Recent developments in our understanding of reconnection have important implications for understanding energetic particle production. The onset of fast reconnection occurs only after sufficiently narrow current sheets develop. It has been suggested that the coronae of stars and accretions discs naturally evolve to a critical state close to fast reconnection onset. In flares the classical picture of the formation of a single large x-line does not seem to be viable: the narrow current layers that develop near the reconnection sites break up into secondary magnetic islands whose dynamics and size spectrum are likely to control particle acceleration. Energetic electrons are produced though the Fermi-like reflection in contracting magnetic islands rather than by parallel electric fields. Their energy gain is linked to the release of magnetic energy. The Fermi mechanism is viable only for super-Alfvenic ions so a seed mechanism for ions is required. Ion pickup in reconnection outflow exhausts is proposed as the seed mechanism. This seed model has the property that low Q/M ions gain energy faster than protons, which is consistent with observed enhancements of low Q/M ions in impulsive flares. Ion and electron acceleration in a multi-island environment remains poorly understood.
North Carolina State University
Nonlinear particle acceleration at nonrelativistic shocks
I will review the theory of diffusive shock acceleration (DSA) with an emphasis on the role of escaping particles. There is convincing evidence that supernova remnant shocks can produce cosmic rays (CRs) with high efficiency and the most likely acceleration mechanism is DSA. If the acceleration is efficient, nonlinear effects will be important and, in DSA, these effects include a modification of the shock structure from the backpressure of CRs, an amplification of the ambient magnetic field above simple compression, and the production of a sizable flux of the highest energy CRs which escape upstream. Since all of these processes are strongly coupled, semi-analytical or fully numerical techniques are required to describe the important nonlinear effects. I will show some recent Monte Carlo results where the escaping particle flux is calculated explicitly and where the particle diffusion coefficient is determined in self-generated magnetic turbulence. These results will be compared to other models of nonlinear DSA.
NNiels Bohr Institute
Split dynamics plasma simulations
I will talk about some aspects of a trans-Debye kinetic plasma modeling. A split dyna-mics scheme is employed for the simulation of processes complementary to PIC codes. Furthermore, I will briefly touch upon a recent result regarding the interaction between a prompt GRB photonic pulse and an assumed circumburst medium.
LPTA Montpellier, France
TeV and lower-energy observations of accelerated particles in shell-type SNRs
I will review the current status of observational knowledge about particle acceleration in shell-type supernova remnants (SNRs), highlighting in particular the contribution of the current generation of TeV gamma-ray observatories, which provide the most direct probe of particles accelerated in these objects up to energies of hundreds of TeV. HESS and other TeV gamma-ray telescopes have revealed emission from a number of sources which can be clearly identified with known shell-type SNRs. These include SNRs such as G347.5-0.3, G266.2-1.2, RCW 86 and most recently SN 1006, in which there is a remarkable morphological correspondence between the TeV gamma-ray image and non-thermal X-ray emission. Another class of TeV sources are found in SNRs which are interacting with molecular clouds, such as IC 443, W28 and CTB 37A. Uncertainties concerning the hadronic or leptonic nature of the observed TeV emission will be discussed, in relation to the question of the origin of Galactic cosmic rays.
Lower-energy observations which shed light on processes related to particle acceleration at SNR shocks will also be reviewed. The non-thermal X-ray rims revealed in many young SNRs will be discussed, along with their implications for magnetic field amplification and the maximum particle energy attainable by acceleration at the blast wave. Current examples of indirect evidence for accelerated nuclei from the hydrodynamics of these objects will be reviewed.
University of Arizona
Injection and acceleration of thermal plasma at perpendicular shocks
It has long been thought that nearly perpendicular shocks, or those that move normal to the average magnetic field, do not accelerate low-energy particles. This has been known as the injection problem. Here we show that such shocks are indeed very efficient accelerators of low-energy particle. Under many physically realistic and reasonable circumstances, they can even extract particles directly from the thermal population. The basic physics will be addressed in this talk, which will emphasize the importance of pre-existing large-scale magnetic fluctuations that are known to exist in turbulent astrophysical plasmas. We will also present results from recent numerical simulations that support this conclusion. The application of these results to astrophysical shock waves such as at the termination of the solar wind and those associated with supernovae blast waves, will also be discussed.
University of Tokyo
Particle acceleration and injection problem in relativistic and nonrelativistic shocks
Acceleration of charged particles at the collisionless shock is believed to be responsible for production of cosmic rays in a variety of astrophysical objects such as supernova, AGN jet, and GRB etc., and the diffusive shock acceleration model is widely accepted as a key process for generating cosmic rays with non-thermal, power-law energy spectrum. Yet it is not well understood how the collisionless shock can produce such high energy particles. Among several unresolved issues, two major problems are the so-called "injection" problem of the supra-thermal particles and the generation of plasma waves and turbulence in and around the shock front. With recent advance of computer simulations, however, it is now possible to discuss those issues together with dynamical evolution of the kinetic shock structure. A wealth of modern astrophysical observations also inspires the dynamical shock structure and acceleration processes along with the theoretical and computational studies on shock. In this presentation, we focus on the plasma wave generation and the associated particle energization that directly links to the injection problem by taking into account the kinetic plasma processes of both non-relativistic and relativistic shocks by using a particle-in-cell simulation. We will also discuss some new particle acceleration mechanisms such as stochastic surfing acceleration and wakefield acceleration by the action of nonlinear electrostatic fields.
Unit for Space Physics, NWU, South Africa
Pulsar Wind Nebulae in TeV gamma-ray emission
In this presentation we review the observational properties of pulsar wind nebulae (PWN) detected at VHE gamma-ray energies. Whereas we have evidence for dynamic plerions in binary systems, we will focus on those which resulted from supernova explosions, leaving an expanding pulsar driven bubble and a supernova shell. For these we see two types: (1) The younger Crab-like PWN, which are mostly unresolved and composite and (2) the evolved Vela-like PWN which show resolved, but offset morphologies. A shell is seldom seen. We also notice an evolution in the ratio of X-ray to VHE fluxes from PWN. A general theoretical framework will also be presented within which these observations can be understood.
University of Tokyo
PRadiation dominated relativistic current sheets
Relativistic Current Sheets (RCS) feature plasma instabilities considered as potential key to magnetic energy dissipation and non-thermal particle generation in Poynting flux dominated plasma flows. We show in a series of kinetic plasma simulations that the physical nature of non-linear RCS evolution changes in the presence of incoherent radiation losses: In the ultra-relativistic regime (i.e. magnetization parameter sigma = 104 defined as the ratio of magnetic to plasma rest frame energy density) the combination of non-linear RCS dynamics and synchrotron emission introduces a temperature anisotropy triggering the growth of the Relativistic Tearing Mode (RTM). As direct consequence the RTM prevails over the Relativistic Drift Kink (RDK) Mode as competitive RCS instability. This is in contrast to the previously studied situation of weakly relativistic RCS (sigma ~ 1) where the RDK is dominant and most of the plasma is thermalized. The simulations witness the typical life cycle of ultra-relativistic RCS evolving from a violent radiation induced collapse towards a radiation quiescent state in rather classical Sweet-Parker topology. Such a transition towards Sweet-Parker configuration in the late non-linear evolution has immediate consequences for the efficiency of magnetic energy dissipation and non-thermal particle generation. Ceasing dissipation rates directly affect our present understanding of non-linear RCS evolution in conventional striped wind scenarios.
University of Arizona
Pre-existing turbulence, magnetic fields and particle acceleration at a supernova blast wave
We consider effects of pre-existing, large-scale turbulence, upstream of a shock, on the magnetic field and the acceleration of charged particles. Turbulent density fluctuations upstream of the shock have a large effect on the magnetic field downstream (Giacalone & Jokipii, ApJ 633, L41, 2007). For high Alfven Mach number shocks, the downstream magnetic field is amplified considerably above the value obtained from the shock jump conditions. These effects may provide a robust and natural understanding of recent observations at supernova shocks.
The magnetic field amplification implied by our simulations should exceed factors of 100, consistent with observed X-rays from supernova remnants, which require magnetic fields of 100 \muG. These are much larger than expected from the shock jump conditions. The upstream field is not amplified, so cosmic-rays with energies approaching the ''knee'' in the spectrum require rapid acceleration, which can occur at the quasi-perpendicular part of the supernova blast wave, where the turbulent field-line mixing plays a large role.
We have carried out a simple global test-particle simulation of acceleration at a spherical blast wave propagating into a uniform magnetic field. We find that although most of rapid particle acceleration occurs in the "equatorial" band, where the upstream magnetic field is quasi-perpendicular, the ongoing temporal evolution of the shock brings most of the particles to the quasi-parallel "polar" part of the shock. This is in agreement with the observational constraints reported by Rothenflug et al. (A&A 4225, 121, 2004), and allows the rapid acceleration at the quasi-perpendicular shock.
We conclude that a model in which the magnetic-field amplification occurs because of the upstream turbulence and rapid acceleration to the knee occurs at the quasi-perpendicular part of the shock is consistent with the observations. Amplification of the upstream magnetic field is not necessary in this model.
Relativistic shocks and particle acceleration
University of Wisconsin-Madison
Numerical simulations of astrophysical turbulence
Turbulence is a crucial component of dynamics of astrophysical fluids dynamics, including those of ISM, clusters of galaxies and circumstellar regions. The knowledge of turbulence properties is essential for understanding star formation, molecular chemistry, cosmic ray transport and acceleration, transport of heat and angular momentum etc. I shall show that a substantial progress in understanding major astrophysical processes is possible if we know very basic properties of the magnetized turbulent flow, e.g., its spectrum. I shall show that more subtle properties can be obtained if we also know turbulent intermittency. I shall discuss theoretical models, their numerical testing and the practically important astrophysical implications of the magnetized turbulence.
Tel Aviv University
Relativistic photon-mediated shocks
PIC simulations of relativistic magnetized plasmas
This talk will summarize several recent results from the PIC simulations of strongly magnetized relativistic plasmas. These include particle acceleration in EM-dominated expansion, collisions of relativisitic plasmas, relativistic Weibel and 2-stream instabilities, and magnetic turbulence cascade. We will present results of both 2.5-D and 3-D PIC simulations. The radiation power output of some of these plasmas will also be discussed.
University of California, Berkeley
The solar system: a laboratory for the study of the physics of particle acceleration
A remarkable variety of particle acceleration occurs in the solar system, from lightning-related acceleration of electrons to tens of MeV energy in less than a millisecond in planetary atmospheres; to acceleration of auroral and radiation belt particles in planetary magnetospheres; to acceleration at planetary bow shocks, co-rotating interplanetary region shocks, shocks driven by fast coronal mass ejections, and at the heliospheric termination shock; to acceleration in magnetic reconnection regions in solar flares and at planetary magnetopause and magnetotail current sheets. These acceleration processes often occur in conjunction with transient energy releases, and some are very efficient, with the accelerated particles containing ~10-50% of the total energy released. Others are highly selective; for example, the acceleration in 3He-rich solar particle events enriches 3He by a factor of up to 10,000 or more relative to 4He. Unlike acceleration processes outside the solar system, the accelerated particles and the physical conditions in the acceleration region can be studied through direct in situ measurements, and/or through detailed imaging and spectroscopy. Here I review recent observations of these acceleration phenomena, our current understanding of the physics involved, and the applicability to particle acceleration elsewhere in the universe.
Los Alamos National Laboratory
Stochastic particle acceleration
Free energy dissipation in collisionless astrophysical plasmas is mediated by turbulence and/or plasma waves. Waves have been introduced to scatter particles in the diffusive shock models and to induce anomalous viscosity for magnetic reconnection. Waves can also selectively accelerate some of the background particles to high energy through resonant wave-particle interactions, which corresponds to the stochastic particle acceleration. There are increasing pieces of evident for such interactions in space and solar physics observations. In this talk, major achievements of this theory in explaining observations will be reviewed. We will also discuss what needs to be done to build more realistic and testable models.
Ben-Gurion University, Beer-Sheva, Israel
Magnetic reconnection at the termination shock in a striped pulsar wind
As the plasma is sharply compressed at the shock front, one can expect forced reconnection of the magnetic fields frozen into the plasma. If the upstream flow is highly relativistic, the compression (defined as the ratio of the proper plasma densities downstream and upstream of the shock) could be very high so that the total annihilation of the alternating magnetic field is possible in this case. I discuss the extreme case when the upstream flow is both relativistic and Poynting dominated so that the energy transferred by alternating magnetic fields exceeds the energy transferred by the plasma. Such outflows are emanated from pulsars; they are called striped winds. Some models of jets in gamma-ray bursts and active galactic nuclei also incorporate relativistic, Poynting dominated outflows. The basic challenge for all these models is deducing how the Poynting flux is eventually converted into the plasma energy. I show that there is a simple criterion for dissipation of alternating fields at the relativistic shock front. Applying this criterion to plerions, one sees that the Poynting flux may be easily converted into ultra-relativistic particles just at the termination shock. This conclusion could resolve a long-standing difficulty with transformation of the electro-magnetic energy of the pulsar wind (the so called sigma-problem). Applying this criterion to the intra-binary shocks in double pulsar systems, one can place limits on the pair multiplicity in the pulsar wind. I also discuss the particle acceleration at the shock in a striped wind. Particle-in-cell simulations show that in the course of compression-driven magnetic annihilation, the particles acquire a non-thermal spectrum, which could be approximated by a power-law with an index of -1 and an exponential cutoff. I argue that the observed flat radio spectra of plerions could be attributed to the dissipation of the Poynting flux at the pulsar wind termination shock.
University of Southampton
Observations of microquasars
I will review the observations of jets from X-ray binaries. I will discuss correlations between radio emission and X-ray properties (e.g. luminosity, timing behavior, and shape of the spectral energy distribution) of black hole and neutron star X-ray binaries, and will compare the results from the two. I will discuss the implications for jet production - notably the possible importance of black hole spin and the effects of a solid surface and magnetic field in the neutron star case.
ESST Kyushu University
Relativistic particle acceleration in a developing turbulence
An efficient particle acceleration process in the course of parametric instabilities of large amplitude Alfven waves in a relativistic pair plasma is investigated by utilizing one dimensional full particle-in-cell (PIC) code. Although MHD turbulence is ubiquitous in space and astrophysical environments, particle acceleration processes in turbulent MHD waves such as diffusive shock acceleration often assume fully developed, or incoherent, turbulence. However, at least in the vicinity of a source region which may not be small compared with whole acceleration site, an MHD turbulence may have not fully developed but be rather coherent. Such a developing MHD turbulence is reproduced in PIC simulation as parametric instabilities of large amplitude Alfven waves. When amplitude of a parent Alfven wave is large as same as an ambient magnetic field, successive decay instabilities result in rapid spectral cascade. In such a developing turbulence coherent wave-particle interactions lead to strong particle acceleration. The acceleration process is analyzed in detail and it is shown that relativistic effects in wave-particle interactions play decisive roles in the process.
University of Kansas
Radiation processes in relativistic shocks
Radiation from GRBs is thought to be produced by accelerated electrons in magnetic fields. Such emission may be produced at collisionless shocks of baryonic outflows or at reconnection sites (at least in the prompt phase) of the magnetically dominated (Poynting flux driven) outflows, where no shocks presumably form at all. It has recently been found that during reconnection strong small-scale magnetic fields are produced via the Weibel instability, very much like they are produced at relativistic shocks. The relevant (Weibel) physics has been successfully and extensively studied with the PIC simulations in 2D and, to some extent, in 3D for the past few years; some of the results will be presented on this conference. We discuss how these simulations predict the existence of multi-MeV synchrotron/jitter emission in some GRBs, which can be observed with GLAST. Recent very interesting results on modeling of the spectral variability and spectral correlations of the GRB prompt emission in the Weibel-jitter paradigm applicable to both baryonic and magnetic-dominated outflows will be reviewed with the emphasis on predictions for GLAST. An alternative model of the vorticity-generated magnetic field at a shock in a clumpy external medium is also discussed. Further observations can, hopefully, discriminate between the models and lead to ultimate understanding of the nature of GRB progenitors.
Gamma-ray bursts and collisionless shocks
During Gamma-Ray bursts (GRBs) plasma is accelerated to ultra-relativistic velocities, making GRBs premium astrophysical sites to explore relativistic collisionless shocks. Especially, the interaction of the accelerated plasma with the external medium drives a relativistic blast-wave into the weakly magnetized surrounding circum-burst proton-electron plasma. This blast wave is believed to be the source of the observed GRB afterglow emission. Observations of these afterglows indicate that weakly magnetized relativistic collisionless shocks efficiently accelerate electrons, and generate strong and long lasting magnetic fields. I will give a brief review of the observations and their constraints on relativistic collisionless shocks, and discuss various processes that may generate long lasting magnetic fields in these shocks.
New relativistic particle-in-cell simulation studies of prompt and early afterglows from GRBs
Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations of relativistic electron-ion (electro-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In the collisionless relativistic shock particle acceleration is due to plasma waves and their associated instabilities (e.g., the Buneman instability, other two-streaming instability, and the Weibel (filamentation) instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electrons' transverse deflection behind the jet head. The ''jitter'' radiation from deflected electrons has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.
The beginning of gamma-ray astronomy with Fermi
The Fermi observatory is designed to perform gamma-ray astronomy in the energy range 20 MeV to 300 GeV, with supporting measurements for gamma-ray bursts from 10 keV to 30 MeV. Fermi was successfully launched on June 11 (2008) from the Kennedy Space Center at Cape Canaveral. The main instrument of Fermi is the Large Area Telescope (LAT), which provides break-through high-energy measurements using techniques typically used in particle detectors for collider experiments. The LAT consists of 16 identical towers in a four-by-four grid, each one containing a pair conversion tracker and a hodoscopic crystal calorimeter, all covered by a segmented plastic scintillator anti-coincidence shield. The LAT is currently monitoring the GeV gamma-ray sky with rather uniform exposure (covering 20% of the sky at any instant and the entire sky on a timescale of a few hours) and a sensitivity ~30 times better than its predecessor, EGRET. The large performance improvement of LAT opens a new and important window on a wide variety of high-energy astrophysical phenomena, as well as potential to discover/study non-conventional physics. In the talk I will report the instrument performance, the mission status and science opportunities and will present some results derived from the first months of operation, which includes astronomical telegrams on AGN flares, 2 GCN circulars on LAT-detected GRBs and the monitoring of some selected sources (22 blazars and 1 high mass X-ray Binary).
Iowa State University
PIC simulations of magnetic field production by cosmic rays drifting upstream of SNR shocks
Turbulent magnetic-field amplification appears to operate near the forward shocks of young shell-type SNR. I review the observational constraints on the spatial distribution and amplitude of amplified magnetic field in this environment. I also present new PIC simulations of magnetic-field growth due to streaming cosmic rays. While the nature of the initial linear instability is largely determined by the choice of simulation parameters, the saturation always involves changing the bulk motion of cosmic rays and background plasma, which limits the field growth to amplitudes of a few times that of the homogeneous magnetic field.
Naval Research Laboratory
Balmer line diagnostic of electron heating at collisionless shocks in supernova remnants
The mechanism and extent of electron heating at collisionless shocks has recently been under intense investigation. H\alpha Balmer line emission is excited immediately behind the shock front and provides the best diagnostic for the electron to proton temperature ratio at supernova remnant shocks. Two components of emission are produced, a narrow component from electron and proton impact excitation of cold neutrals, and a broad component produced through charge exchange between the cold neutrals and the shock heated protons. Thus the broad and narrow component fluxes reflect the competition between electron and proton impact ionization, electron and proton impact excitation and charge exchange. This diagnostic has led to the discovery of an approximate inverse square relationship between the electron to proton temperature ratio and the shock velocity. In turn, this implies a constant level of electron heating, independent of shock speed above ~450 km/s. In this talk I will present the observational evidence to date. Time permitting, I will introduce how lower-hybrid waves in an extended cosmic ray precursor could explain such a relationship, and how this and other parameters in the H\alpha profile might relate to properties of cosmic rays and magnetic field amplification ahead of the shock.
A cosmic-ray current driven instability in parallel shocks
A kinetic description of the role of cosmic-ray streaming upstream of parallel shocks is presented in a relativistic framework. It is demonstrated that efficient cosmic-ray acceleration leads to the development of a rapidly growing non-resonant instability. Assuming the field is amplified via this instability, we discuss its role in the confinement of cosmic rays and how to incorporate this into stationary kinetic solutions of modified shocks.
Generation of magnetic field by the current-driven instability near cosmic-ray modified shocks
We study the non-linear properties of the current-driven instability predicted by Bell (2004). In the first part, we combine an analytical modeling plus particle-in-cell (PIC) simulations to make a one-dimensional study of the waves as they were isolated plane waves subject to a constant cosmic ray (CR) current. In the second part, we relax these conditions by including multidimensional effects and also the back-reaction on the CRs. In the idealistic case, we find that the current-driven waves can grow exponentially until the Alfven velocity of the plasma, Va, becomes comparable to the drift velocity of the CRs, Vcr. We also find that, in the exponential growth regime, the current-driven waves will move the plasma along the direction of propagation of the CRs at a speed ~Va^2/Vcr. Also, they will induce transversal plasma motions of about the "transversal Alfven speed", which is the Alfven speed calculated only with the fields perpendicular to the CR curent. The multidimensional evolution of the instability is studied making use of two- and three-dimensional simulations. First, we find that a condition for the growth of the instability is that Vcr(Ncr/Ni) << Vai, where Ncr and Ni are the density of CR and background ions, respectively, and Vai is the initial Alfven velocity of the plasma. If this condition is not met, the instability will be suppressed by the formation of strong plasma filaments of the scale of the ions skin depth. We also find that, even if the current-driven waves can grow, they will produce significant density fluctuations in the plasma shortly after they become non-linear, which confirm previous numerical MHD studies. The formation of these fluctuations will decrease the growth rate of the instability and will enlarge its dominant wavelengths. We also study the effect of the back-reaction on CR. We find that, if the Larmor radii of the CR become comparable to the size of the magnetic fluctuations, the deflection of the CR can saturate the instability before the regime Va ~ Vcr is reached. We discuss applications of this instability to both relativistic and non-relativistic shock environments.
Imperial College, London
Kinetic turbulence in magnetized space and astrophysical plasmas
I will first discuss how the gyrokinetic theory, originally developed for fusion applications, is applicable to astrophysical and space plasma turbulence problems. I will then explain how the familiar "fluid" turbulence ideas such as the energy cascade are generalized for a kinetic turbulence in a weakly collisional plasma. I will introduce the concept of a cascade of generalized energy in a 5D phase space and explain how small-scale structure develops both in the real and velocity space ("the entropy cascade") and how this process is fundamental in the conversion of electromagnetic fluctuation energy into heat. I will discuss astrophysical applications: inertial and "dissipation" range turbulence in ISM and the solar wind, etc.
A. A. Schekochihin, S. C. Cowley, W. Dorland, G. W. Hammett, G. G. Howes. E. Quataert, T. Tatsuno, ApJS submitted (2007) [e-print arXiv:0704.0044]
Kinetic modelling of MHD turbulence and particle acceleration in nonuniform magnetic fields
A new distributed 1st-order Fermi acceleration process of relativistic particles in nonuniform guide magnetic fields is proposed. Preferential acceleration of cosmic ray hadrons occurs for negative values of the product of the cross helicity state H of the Alfvenic slab magnetic turbulence and the focusing length L of the nonuniform guide field. Kinetic models of the turbulence in different cosmic sites with nonuniform guide fields are investigated.
Max-Planck-Institut fuer extraterrestrische Physik
Collisionless nonrelativistic shocks
The theory and simulations of collisionless nonrelativistic shocks will be reviewed. The physical properties of collsionless shocks depend strongly on Mach number, angle between upstream magnetic field and shock normal, and on ion/electron beta. We will discuss the concept of criticality and the fundamental differences between the quasi-parallel and quasi-perpendicular shock structure. In the supercritical Mach number regime specularly reflected ions at the quasi-perpendicular shock lead to a foot in which these backsteaming ions excite various ion and electron instabilities. Additionally, due to wave excitation in the ramp the shock surface is not planar but has a rippled structure. At quasi-parallel shocks backstreaming ions excite low frequency upstream waves which will, depending on Mach number, have upstream or downstream directed group velocities. These waves can steepen upstream of the ramp to large amplitude pulsations which take over the role of a newly reformed shock front. Finally, we will also address the problem of injecting ions into a diffusive shock acceleration mechanism.
Department of Physics, Yamagata University
Particle simulation of the global pulsar magnetosphere
Particle simulation has been applied to understand the activity of the pulsar magnetosphere in use of the special purpose computer for gravitational N-body problem. We confirm the existence of the outer gap, which is a linear-accelerator in the magnetosphere. If electron-positron pair creation is induced, the magnetosphere becomes to have winds with trans-field drift motion by radiation drag. We suggest that the basic activities of the pulsar magnetosphere, gamma-ray pulses and pair winds, can be understood by the global particle simulation.
*Instituto Superior Tecnico, Lisbon, Portugal
Particle dynamics in relativistic shocks
Taking advantage of particle tracking, results of ab-initio simulations of electron/light-ion relativistic flows with shock formation will be discussed focusing on the analysis of the full particle dynamics and acceleration mechanisms. Comparisons with hybrid simulations of collisionless shocks will also be presented to highlight similarities in the acceleration mechanisms. We also explore the possibility to post-process several tracked quantities for the most energetic particles.
* University of Oslo, ** University of Melbourne
Particle-in-cell simulations of wave-like pulsar winds
We review recent theoretical work on the electrodynamics of wave-like relativistic outflows emitted by a rapidly spinning compact object. By combining analytic arguments with particle-in-cell (PIC) simulations, we investigate whether a relativistically stream-ing, electron-positron plasma can support a nonlinear, electromagnetic, antenna-driven wave that is generated by a realistic inner boundary condition. The answer seems to be yes: a coherent, large-amplitude electromagnetic wave propagates stably within the outflow under a range of conditions, transporting most of the wind energy in the form of oscillatory Poynting flux rather than mechanical energy. Contrary to first expectations, there exist a number of viable formation scenarios if the wave is injected far above the cut-off frequency by a sensibly phased antenna into an ultrarelativistically streaming plasma. The particles and fields achieve phase coherence within the launch zone without excessive thermalization, via a process that can be understood in terms of adiabatic changes in the plane-wave constants of the motion. (Near the cut-off, electrostatic thermalization disrupts coherence.) Downstream, the streaming decelerates as the plasma is energized transversely (but not thermalized), and the wave becomes superluminal. This is a semi-cyclic process which, depending on the parameters, can settle down and yield an Akhiezer-Polovin-type electromagnetic wave. The electromagnetic field spectrum and particle distribution function depend on the polarization (linear versus circular) of the "antenna" (i.e. the near zone of the magnetosphere). As the outflow travels away from its source, the Poynting flux is converted gradually to kinetic energy flux by adiabatic mode conversion without significant radiation losses. This offers a possible resolution of the sigma paradox in the Crab pulsar wind. Under certain conditions, parametric instabilities can disrupt this "silent" energy conversion. An observational test of these ideas has been carried out recently with X-ray timing data from the relativistic double pulsar J0737-3039. The absence of orbital modulation of X-rays from the pulsar wind termination shock indicates a high-sigma outflow at the shock, unlike in the Crab pulsar wind. This is consistent with theoretical predictions from the wave-like wind picture. The results are applied briefly to gamma-ray-burster models where the central engine is a rapidly spinning magnetar. Our work differs from most previous studies of pulsar winds in that the proper-frame electric field deviates slightly from zero and the wave is almost luminal in both the laboratory and proper frames.
Harvard-Smithsonian Center for Astrophysics
Observations of pulsar winds and jets
The extended nebulae formed as pulsar winds expand into their surrounding supernova remnants provide information about the composition of the winds, the injection history from the host pulsar, and the material into which the nebulae are expanding. Observations from across the electromagnetic spectrum provide constraints on the evolution of the nebulae, the density and composition of the surrounding ejecta, the geometry of the systems, the formation of jets, and the maximum energy of the particles in the nebulae. Here I provide a broad overview of the structure of pulsar wind nebulae, with specific examples of recent work that demonstrate our ability to constrain the above parameters.
Relativistic collisionless shocks
I will present a summary of recent progress in theoretical modeling of relativistic collisionless shocks, focusing on the first-principles simulations of shocks using particle-in-cell codes. Such simulations allow self-consistent calculations of the structure of shocks and the generated turbulence. I will discuss the internal structure of shocks in different regimes of flow magnetization and composition, concentrating on the conditions necessary for particle acceleration. I will present simulations which show ab-initio Fermi acceleration of particles from the thermal pool to power-law distributions, setting constraints on the shock acceleration efficiency and geometry. Other results that will be discussed include the amplification of the upstream magnetic field by accelerated particles through streaming instabilities, and the electron-ion temperature equilibration in collisionless shocks. I will conclude with the applications of shock simulations to the physics of gamma-ray bursts, pulsar wind nebulae, and supernova remnants.
KIPAC, Stanford University & Astronomical Observatory, Jagiellonian University
Stochastic electron acceleration in astrophysical jets
Scattering length of relativistic particles in aperiodic fluctuations
Aperiodic fluctuations are discussed as a significant contribution to the magnetic field generation process in anisotropic astrophysical plasmas. Due to the Lorentz force the plasma particles are spatially separated and collimated into current filaments. The induced magnetic field leads to an amplification of the initial fluctuation. In order to gain information if an interaction between the plasma particles and the magnetic field fluctuations is possible, an appropriate model for the scattering length of the particles is necessary. The quasi-linear theory (QLT) has been applied to calculate the pitch-angle Fokker-Planck coefficient, which is required for the calculation of the parallel mean free path length. A perpendicular turbulence model has been considered as the focus was on fluctuations of Weibel-type. It will be shown that the choice of the wave number dependence is a crucial point leading to differing results regarding the interaction between field and particles.
University of Maryland
The role of the Weibel instability in electron-positron reconnection
The mass symmetry between the two species in electron-positron (pair) plasmas has interesting consequences for collisionless magnetic reconnection because the Hall term, which plays a crucial role in supporting fast reconnection in electron-proton plasmas, vanishes. We perform kinetic simulations of pair reconnection in systems of various sizes, show that it remains fast, and identify the reason why this occurs. For sufficiently large systems a Weibel-like temperature anisotropy instability develops in the outflow from the X-point that causes the current layer to broaden and form a Petschek-like open outflow. We discuss the parameter regimes in which pair reconnection should be fast and the implications for astrophysical pair plasmas.
Buneman and ion two-stream instabitities in the foot of collisionless shocks
Two-dimensional electrostatic PIC simulations as well as linear analysis have been made for double periodic boundary conditions mimicking the shock foot region of supernona remnants. We found that modes propagating obliquely to the beam direction grow fast enough so that no surfing acceleration occurs. We also found that a new type of instability called ion two-stream instability is excited after the Buneman instability saturated instead of the ion acoustic instability. Implications for electron heating are shortly discussed.
Ruhr-Uni Bochum, Germany
Kinetic instabilities in relativistic plasmas: the Harris instability revisited
Plasma instabilities that generate aperiodic fluctuations are of outstanding importance in the astrophysical context. Two prominent examples are the electromagnetic Weibel instability and the electrostatic Harris instability, which operate in initially non-magnetized and magnetized plasmas, respectively. In this talk, the original formulation of the Harris instability will be reviewed and generalizations will be presented such as the inclusion of (1) relativistic effects, (2) ion effects, and (3) mode coupling. It will be shown that, with these modifications, a powerful method has been developed for the determination of both the existence and the growth rate of low-frequency instabilities. Applications can be found in astrophysical jets, where the rest frame can be used and so no parallel motion is present. At the end of the talk, how the particle composition of gamma-ray burst jets can be predicted using the Harris technique.
University of California, Berkeley
Time-dependent PIC - Monte Carlo modeling of electron-positron cascade in the polar cap of pulsar
Many previously proposed models for polar cap cascades (and almost all quantitative models) assumed stationary particle outflow. Predictions of such models disagree with both observational data (e.g. the number of electron-positron pairs in the Crab nebula is ~100 higher than predicted) and results of numerical models of force-free pulsar magnetosphere (the current density required to support force-free magnetosphere differs substantially from what stationary model for PC cascade predicts). On the other hand, the stability of stationary models has not been quantitatively studied. We decided to study the problem from the first principles, namely to model the accelerating electric field, particle acceleration and pair production simultaneously. We developed a hybrid Particle-In-Cell-Monte Carlo code for direct self-consistent time-dependent modeling of polar cap cascades. Here we report the first results of cascade modeling using the current 1D version of the code.
GRPIC modeling of jets from accretion disks
An algorithm is presented that incorporates the tensor form of Maxwell\'s equations in an electromagnetic particle-in-cell algorithm. The code simplifies to Schwartzschild space-time for the absence of a spinning central mass and to Minkowski space-time if no central mass is present. The current density is calculated using the curved space-time of the metric. The algorithm described here is part of a core software engine developed for plasma simulation in an environment around a spinning central mass. The versatility of the algorithm allows for calculations without spin. Because the algorithm uses a general metric explicitly for the description of the space-time, this algorithm can be used as a general relativistic particle-in-cell (GRPIC) code. We have studied the particle dynamics within the negative energy region of the ergosphere.
NASA/Goddard Space Flight Center
Relativistic current sheets in electron-positron plasmas
The current sheet structure with magnetic field reversal is one of the fundamental structure in space and astrophysical plasmas. It draws recent attention in high-energy astrophysical settings, where relativistic electron-positron plasmas are considered. In this talk we will review the recent progress of the physical processes in the relativistic current sheet. The kinetic stability of a single current sheet, the nonlinear behavior of these instabilities, and recent challenges on the multi current sheet systems are introduced. We will also introduce some problems of magnetic reconnection in these relativistic environments.
Institute of Experimental Physics SAS, Kosice
Supernova blast wave particle acceleration stochastic method
Stochastic integration is well-known method for solving diffusion problems. We presented a two new methods for solving Parker's diffusion-convection transport equation in the presence of discontinuities such as shock. We follow pseudo particles injected at the termination shock with a monoenergetic spectrum. The random diffusive motion is determined from the diffusion coefficient, with a specified energy change when the particle crosses the shock. The resulting solution agrees well with the corresponding analytic solutions. The method was applied to acceleration of particles at supernova blast waves. Distribution of particles for different periods and parameters of explosion is presented.
Tel Aviv University
The effect of radiative cooling on the collimation of relativistic jets
Recent analysis of gamma-ray AGNs indicate that the gamma-ray emission may originate in a recollimation nozzle located far from the central active black hole. An example is the HST-1 knot in M87. We present a model of jet recollimation by radiative cooling, and show that effective jet focusing can occur under certain conditions. The application to TeV blazars is shortly discussed. In particular it is shown that such structures can naturally explain the apparent discrepancy between the large Doppler factors inferred from TeV variability and the much smaller Doppler factors inferred from superluminal motions.
INAF/ Osservatorio Astrofisico di Arcetri
Azimuthal variation in the properties of cosmic ray acceleration at the blast wave of SN 1006
We present evidence for morphological changes due to efficient cosmic ray ion acceleration in the structure of the southeastern region of the supernova remnant SN 1006 using a combination of multi-wavelength observations. The forward shock front is clearly traced by a filament of Balmer emission in the southeast. This optical emission allows us to trace the location of the blast wave (BW) even where the synchrotron emission from relativistic electrons is either absent or too weak to detect. The contact discontinuity (CD) is traced using the low-energy X-ray (i.e., oxygen band) image which we argue reveals the distribution of shocked ejecta. We interpret the azimuthal variations of the ratio of radii between the BW and CD plus the X-ray and radio synchrotron emission at the BW using CR-modified hydrodynamic models. Different azimuthal profiles are assumed for the injection rate of particles into the acceleration process, magnetic field and level of turbulence. We found that the observations are consistent with a model in which these parameters are all azimuthally varying, being largest in the brightest regions.
* GoLP/IPFN Instituto Superior Tecnico
Exploration of the nonlinear saturation mechanisms of cosmic ray-driven magnetic field amplification with hybrid simulations
It has been proposed that cosmic rays (CR) streaming through a background magnetized plasma in supernovae remnant shocks drive a non-resonant instability that amplifies the seed magnetic field to values exceeding the background magnetic field .
We analyze this instability by means of hybrid simulations, with kinetic ions and fluid electrons, in order to understand the nonlinear saturation mechanism that stops magnetic field growth. In our simulations we use either a stream of particles to drive the instability, or an externally imposed current, as in the original work by Bell .
Our results show magnetic field growth beyond the background magnetic field intensity level, and exhibit growth rates in agreement with the linear theory. In the nonlinear regime different behaviors regarding energy transfer between the driver (CR or external current), and the magnetic field and the background plasma are observed. We explore this saturation phase in detail, and present the main differences between the two scenarios, including the effects of the generated magnetic field on the CR population, and the heating of the background plasma. A comparison of our results with previously published PIC simulations and MHD simulations is also provided.
 A. R. Bell, Mon. Not. R. Astron. Soc. 353, 550, 2004
Karlsruhe University, KIT
An anisotropic convection driven model for CR transport
The ROSAT Galactic wind observations confirm that our Galaxy launches SN driven Galactic winds with wind speeds of about 150km/s in the Galactic plane. Galactic winds of this strength are incompatible with current isotropic models for CR transport as implemented in the GALPROP code. In order to reproduce our local CRs in the presence of strong Galactic winds, charged CRs are required to be much more localized than in the standard isotropic GALPROP models. This requires that anisotropic diffusion is the dominant diffusion mode in the ISM. In addition small scale phenomena such as trapping by molecular cloud complexes and our local bubble might influence the secondary CR production rate and our local CR density gradients. We present an anisotropic convection driven model for CR transport, capable of reproducing our local charged CRs as well as the interstellar diffuse Galactic gamma-rays.
University of Tokyo
Suprathermal ion acceleration in magnetic reconnection observed in the Earth's magnetosphere
The origin of nonthermal, energetic particles is a long-standing unsolved problem in space. Magnetic reconnection is believed to be particularly important for producing energetic particles, because the magnetic field energy can be rapidly released to the particle energy. The Earth's magnetosphere, where we can obtain abundant and precise information on both fields and particles from in-situ observation by satellite, gives important clues to understand nonthermal particle acceleration in magnetic reconnection. We studied suprathermal ion acceleration in magnetic reconnection observed in the Earth's magnetotail. Suprathermal ions are accelerated in the energy up to 1 MeV when we observed ion beams ejected from the reconnecting X-point. Suprathermal ions exhibit power law spectrum with spectrum index ~ 4. At the same time, strong broadband low-frequency electromagnetic waves generated by ion/ion beam instabilities are observed. We found a good correlation between the flux of suprathermal ions and the low-frequency wave power, indicating the involvement of low-frequency waves to the ion acceleration. Furthermore, we found that suprathermal ions are accelerated preferably in the same direction of the ion beams. These observations suggest the generation of suprathermal ions by stochastic acceleration of the pre-accelerated ion beams by the self-generated broadband low-frequency electromagnetic waves, other scenarios such as the acceleration around the pileup magnetic field region due to the gradient and curvature drift under the nonadiabatic motion of kappa ~ 1 should be also discussed though.
ESST Kyushu University
Quasilinear analysis on electron heating in a foot of a high Mach number shock
In a transition region of a high Mach number quasi-perpendicular shock the presence of reflected ions leads to a variety of microinstabilities. Heating processes of electrons have not been paid much attention in contrast to some acceleration processes producing nonthermal populations. In this study quasilinear heating of electrons through microinstabilities in a shock transition region is investigated. An evolution equation of kinetic energy obtained by taking the second order moment of the Vlasov equation under the quasilinear assumption is numerically solved to discuss a saturation level of electron temperature. Dependencies of electron heating rate on types of instabilities and upstream plasma parameters are reported.
Gamma ray emission from SNR RX J1713.7-3946 and the origin of galactic cosmic rays
Using a nonlinear theory for particle acceleration at shock waves in supernova remnants, we investigate the multi wavelength photon spectra produced by electrons and protons via several different emission processes. The acceleration model includes magnetic field amplification and the self consistent computation of the maximum particle energy. The latter is a key parameter to understand the nature of TeV gamma-ray emission. We compare the model with the observations of SNR RX J1713.7-3946 finding that the electron inverse Compton scattering cannot explain photon energies beyond ~ 1TeV, unless an unrealistically low background magnetic field is assumed. On the other hand, &Pi 0 decay accounts very well for the TeV radiation if the proton spectrum extends up to 10^14 eV. The X-ray radiation observed by Suzaku is well fitted by the electron synchrotron emission if one assumes Kep &asymp 10^-4 and a few &mu G background magnetic field. The resulting thickness of the X-ray emitting rims is also compatible with CHANDRA observations.
Yamagata Junior College
Effects of small wavenumber Alfven waves on particle acceleration
Energetic charged particles are accelerated by turbulent Alfven waves via resonant interaction. We discuss effects of nonresonant Alfven waves on energy diffusion by using test particle simulations. When the Alfven waves are given at wavenumbers larger than the resonant wavenumber with small amplitude, simulated diffusion coefficient is similar to that by the quasi-linear theory. If the Alfven waves are added at wavenumbers smaller than the resonant wavenumber, it is found that the simulated diffusion coefficient exceeds the quasi-linear one and becomes larger with increasing the energy density of the nonresonant Alfven waves.
Gamma-ray binary systems: pulsar wind and shock emission model
Based on two gamma-ray binaries LS 5039 and LS I +61 303 recently detected by the High-Energy Stereoscopy Array (H.E.S.S.) and the Major Atmospheric Imaging Cerenkov (MAGIC) telescope we investigate the models for high energy gamma emission assuming pulsar as a companion object. The nature of those binaries is still unknown, since a final observational feature for a black hole or a pulsar compact object companion is still missing. For the pulsar scenario, we compare the properties of the pulsar wind, the termination shock and the massive star wind regions which gives then the full picture of such a binary. Our model includes a detailed account of the system geometry, the angular dependence of processes such as Klein-Nishina inverse Compton and gamma-gamma absorption, and a Monte Carlo simulation of cascading in the anisotropic radiation of the massive star. For the case of LS 5039, we show that the H.E.S.S. phenomenology at all scales (spectra along the orbit in both broad and short phase-bins and lightcurve) is explained already by interactions in the pulsar wind zone.
Particle acceleration in relativistic magnetized collisionless pair shocks: dependence of shock acceleration on magnetic obliquity
We study particle acceleration in relativistic magnetized collisionless pair shocks by means of two-dimensional particle-in-cell numerical simulations. For fixed bulk Lorentz factor \gamma=15 and magnetization \sigma=0.1 of the upstream flow, we explore a range of inclination angles θ between the magnetic field and the shock normal. The inclination is measured in the downstream rest frame and the magnetic field lies in a plane perpendicular to the simulation plane. We define a critical obliquity angle \theta_crit so that in "superluminal" shocks (\theta>\theta_crit) downstream-transmitted particles heading upstream along magnetic field lines cannot re-cross the shock. The downstream energy spectrum for "subluminal" shocks (\theta<\theta_crit) consists of a relativistic Maxwellian and a high-energy power-law tail modified by an exponential cutoff. For parallel shocks (\theta=0), the tail accounts for 1% of the downstream particle number and 5% of the energy, and its energy spectral index is 7۪.1. Accelerated particles bounce between the upstream and the shock, and the upstream scattering is provided by oblique filaments, which have both an electromagnetic and an electrostatic component. Such filaments propagate towards the shock and are generated by the accelerated particles that escape upstream. For larger inclination angles the acceleration efficiency increases, and particles are efficiently boosted by the motional upstream electric field when gyrating across the shock. For \theta=30 deg, close to the superluminality threshold \theta_crit, the number and energy fractions of downstream accelerated particles are 2% and 12% respectively; the spectral index of the corresponding power-law tail is 4.1. When the shock becomes superluminal (\theta>\theta_crit), the acceleration efficiency abruptly drops. Our results show that the range of upstream-frame inclination angles producing efficient acceleration in relativistic magnetized pair shocks is indeed very small < 32 deg, as suggested by previous Monte-Carlo simulations. Self-generated shock turbulence is not large enough to overcome the kinematic constraints for superluminal shocks. These findings place constraints on the models of AGN jets, pulsar wind nebulae and GRBs that require particle acceleration in magnetized relativistic shocks.
Iowa State University
Kinetic simulation of turbulent magnetic field growth by streaming cosmic rays
I will present results of 2.5-dimensional PIC studies of the non-resonant instability driven by a cosmic ray current as predicted by Bell (2004). Our earlier work (Niemiec et al. 2008) observed moderate turbulent magnetic field amplification, saturating with \delta B~ B when the cosmic rays no longer had significant drift with respect to the background plasma. The more recent study, whose parameters better satisfy the analytical requirements of Bell\'s theory, supports the predictions of a dominant parallel mode and marginally larger amplification, but one which is still subject to saturation when the back-reaction on the cosmic rays and background plasma greatly weakens their relative drift.
University of Oulu
Time-dependent modelling of radiative processes in hot magnetized plasma
We have developed a computer code for time-dependent simulations of radiative processes in magnetized compact sources such as hot accretion disks around black holes, relativistic jets in active galaxies and gamma-ray bursts. The processes taken into account are Compton scattering, Coulomb collisions, electron-positron pair production and annihilation as well as synchrotron emission and absorption. No approximation has been made on the corresponding rates. For the first time, we solve coupled integro-differential kinetic equations for photons and electrons/positrons without any limitations on the photons and lepton energies. A numerical scheme is proposed to guarantee energy conservation when dealing with synchrotron processes in electron and photon equations. We apply the code to model non-thermal pair cascades in the blackbody radiation field, to study the synchrotron self-absorption as particle thermalization mechanism, and to simulate time evolution of stochastically heated pairs and corresponding synchrotron self-Compton photon spectra which might be responsible for the prompt emission of gamma-ray bursts. Good agreement with previous works is found in the parameter regimes where comparison is feasible, with the differences caused by our improved treatment of the microphysics.
Reconfinement shocks in relativistic AGN jets
Stationary knots observed in many AGN jets can be explained in terms of a reconfinement shock that forms when relativistic flow of the jet matter collides with the external medium. The position of these knots can be used, together with information on external pressure profile, to constrain dynamical parameters of the jet. We present a semi-analytical hydrodynamical model for the dynamical structure of reconfinement shocks, taking into account exact conservation laws both across the shock surface and in the zone of the shocked jet matter. This approach allows us to include variations of the flow parameters along the radial coordinate. As the pressure just behind the shock is lower than the external pressure, the position of the reconfinement is larger than predicted by simple models. A portion of kinetic energy is converted at the shock surface to internal energy, with efficiency increasing strongly with both bulk Lorentz factor of the jet matter and the jet half-opening angle. Our model may be useful as a framework for modeling non-thermal radiation produced within the stationary features.
|Last update October 2, 2008|