Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating into
an ambient plasma. We find that the growth times of Weibel instability are
proportional to the Lorentz factors of jets. Simulations show that the Weibel
instability created in the collisionless shock front accelerates jet and
ambient particles both perpendicular and parallel to the jet propagation
direction.Comment: 4 pages, 2 figures, submitted to Il nuovo cimento (4th Workshop
  Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004

Fishman, G. J.

Hardee, P.

Hededal, C. B.

Nishikawa, K. -I.

Preece, R.

Richardson, G.

Sol, H.

English

arXiv

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating into an ambient plasma. We find that the growth times depend on the Lorenz factors of jets. The jets with larger Lorenz factors grow slower. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The small scale magnetic field structure generated by the Weibel instability is appropriate to the generation of "jitter" radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks

Nishikawa, K. I.

NASA Technical Reports Server

Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Pair Jets

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic

particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating into an ambient plasma. We find that the growth times of Weibel instability are proportional to the Lorentz factors of jets. Simulations show that the Weibel instability created in the collisionless shock front

accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The 

small-scale magnetic field structure generated by the Weibel

instability is appropriate to the generation of “jitter” radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation resulting

from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks

Nishikawa, K.-I.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10077-5IL NUOVO CIMENTO Vol. 28 C, N. 3 Maggio-Giugno 2005Particle acceleration, magnetic field generation, and emission inrelativistic pair jets(∗)K.-I. Nishikawa(1), P. Hardee(2), C. B. Hededal(3), G. Richardson(4)H. Sol(5), R. Preece(6) and G. J. Fishman(7)(1) National Space Science and Technology Center, Huntsville, AL 35805 USA(2) Department of Physics and Astronomy, The University of AlabamaTuscaloosa, AL 35487 USA(3) Niels Bohr Institute, Department of AstrophysicsJuliane Maries Vej30, 2100 København Ø, Denmark(4) Department of Mechanical and Aerospace Engineering University of Alabama in HuntsvilleHuntsville, AL 35899 USA(5) LUTH, Observatore de Paris-Meudon, 5 place Jules Jansen 92195 Meudon Cedex, France(6) Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899 andNational Space Science and Technology Center, Huntsville, AL 35805 USA(7) NASA-Marshall Space Flight Center, National Space Science and Technology CenterHuntsville, AL 35805 USA(ricevuto il 23 Maggio 2005; pubblicato online il 23 Settembre 2005)Summary. — Shock acceleration is a ubiquitous phenomenon in astrophysicalplasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibeland other two-stream instabilities) created in collisionless shocks are responsible forparticle (electron, positron, and ion) acceleration. Using a 3-D relativistic electro-magnetic particle (REMP) code, we have investigated particle acceleration associ-ated with a relativistic jet front propagating into an ambient plasma. We find thatthe growth times of Weibel instability are proportional to the Lorentz factors of jets.Simulations show that the Weibel instability created in the collisionless shock frontaccelerates jet and ambient particles both perpendicular and parallel to the jet prop-agation direction. The small-scale magnetic field structure generated by the Weibelinstability is appropriate to the generation of “jitter” radiation from deflected elec-trons (positrons) as opposed to synchrotron radiation. The jitter radiation resultingfrom small scale magnetic field structures may be important for understanding thecomplex time structure and spectral evolution observed in gamma-ray bursts orother astrophysical sources containing relativistic jets and relativistic collisionlessshocks.PACS 98.54.Cm – Active and peculiar galaxies and related systems.PACS 98.62.Nx – Jets and bursts; galactic winds and fountains.PACS 98.70.Qy – X-ray sources; X-ray bursts.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 01.30.Cc – Conference proceedings.(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.c© Societa` Italiana di Fisica 435436 K.-I. NISHIKAWA, P. HARDEE, C. B. HEDEDAL ETC.1. – IntroductionNonthermal radiation observed from astrophysical systems containing relativistic jetsand shocks, e.g., active galactic nuclei (AGNs), gamma-ray bursts (GRBs), and Galacticmicroquasar systems usually has power law emission spectra. In most of these systems,the emission is thought to be generated by accelerated electrons through the synchrotronand/or inverse Compton mechanisms. Radiation from these systems is observed in the ra-dio through the gamma-ray region. Radiation in optical and higher frequencies typicallyrequires particle re-acceleration in order to counter radiative losses.Particle-in-cell (PIC) simulations can shed light on the physical mechanism of particleacceleration that occurs in the complicated dynamics within relativistic shocks. RecentPIC simulations using injected relativistic electron-ion jets show that acceleration occurswithin the downstream jet, rather than by the scattering of particles back and forth acrossthe shock as in Fermi acceleration [1-7]. In general, these independent simulations haveconfirmed that relativistic jets excite the Weibel instability [8]. The Weibel instabilitygenerates current filaments with associated magnetic fields [9], and accelerates electrons[1-7].In this paper we present new simulation results of particle acceleration and magnetic(a)(b)Fig. 1. – 2D images show the current density (Jy) at t = 28.8/ωpe (a) γ = 5 and (b) γ = 15.Grey scales indicate the y-component of the current density, Jy [peak: (a) 15.63 and (b) 3.86],and the arrows indicate Jz and Jx .PARTICLE ACCELERATION, MAGNETIC FIELD GENERATION, AND EMISSION ETC. 437(a) (b)Fig. 2. – One-dimensional cuts along the z-direction (25 ≤ z/∆ ≤ 314) of a flat jet (a) γ = 5 and(b) γ = 15. Shown are the x-component of the magnetic field shown at t = 28.8/ωpe. Cuts aretaken at x/∆ = 38 and y/∆ = 33 (blue-dotted), 43 (red-solid), 53 (green-dashed) and separatedby about one electron skin depth.field generation for relativistic electron-positron shocks using 3-D relativistic electromag-netic particle-in-cell (REMP) simulations. In our new simulations, the growthrates withdifferent Lorentz factors of jets have been studied without an initial ambient magneticfield.2. – Simulation setup and resultsTwo simulations were performed using an 85×85×320 grid with a total of 180 millionparticles (27 particles/cell/species for the ambient plasma) and an electron skin depth,λce = c/ωpe = 9.6∆, where ωpe = (4πe2ne/me)1/2 is the electron plasma frequency and∆ is the grid size [6].The electron number density of the jet is 0.741nb, where nb is the density of ambient(background) electrons. The average jet velocities for the two simulations are vj =0.9798c, 0.9977c corresponding to Lorentz factors are 5 (2.5MeV) and 15 (7.5MeV),respectively. The jets are cold (vej,th = vpj,th = 0.01c and vij,th = 0.0022c) in the rest frameof the ambient plasma. Electron-positron plasmas have mass ratio mp/me = 1. Thethermal velocity in the ambient plasmas is ve,pth = 0.1c where c is the speed of light. Thetime step ∆t = 0.013/ωpe.Current filaments resulting from development of the Weibel instability behind thejet front are shown in figs. 1a and 1b at time t = 28.8/ωpe for unmagnetized ambientplasmas (a) γ = 5 and (b) γ = 15. The maximum values of Jy are (a) 15.63 and (b)3.86, respectively. The slower jet shows larger amplitudes than the faster jet at the samesimulation time. The effect of Lorentz factors of jets affects the growth rates of Weibelinstability as expected by the theory [8].The differences in the growthrates between the differerent Lorentz factors are seenmore clearly in the x-component of the generated magnetic fields as shown in fig. 2. Theamplitudes of Bx in the slower jet (a) are much larger than those in the faster jet (b).The comparisons in the saturated magnetic fields need much longer simulations, whichare in progress at the present time.3. – Summary and discussionWe have performed self-consistent, three-dimensional relativistic particle simulationsof relativistic electron-positron jets propagating into magnetized and unmagnetized438 K.-I. NISHIKAWA, P. HARDEE, C. B. HEDEDAL ETC.electron-positron ambient plasmas. The main acceleration of electrons takes place inthe region behind the shock front [5]. Processes in the relativistic collisionless shock aredominated by structures produced by the Weibel instability. This instability is excited inthe downstream region behind the jet head, where electron density perturbations lead tothe formation of current filaments. The nonuniform electric field and magnetic field struc-tures associated with these current filaments decelerate the jet electrons and positrons,while accelerating the ambient electrons and positrons, and accelerating (heating) thejet and ambient electrons and positrons in the transverse direction.The growthrates depend on the Lorentz factors of jet as expected by the theory [9].The e-fold time is written as τ  √γsh/ωpe, where γsh is the Lorentz factor of the shock.The simulation results show that τ ∝ γsh.Other simulations with different skin depths and plasma frequencies confirm that bothsimulations have enough resolutions and the electron Weibel instability is characterizedby the electron skin depth [5].These simulation studies have provided new insights for particle acceleration andmagnetic field generation. Further research is required to develop radiation models basedon these microscopic processes.∗ ∗ ∗K. Nishikawa is a NRC Senior Research Fellow at NASAMarshall Space Flight Center.This research (K.N.) is partially supported by the National Science Foundation awardsATM-0100997, and INT-9981508. P. Hardee acknowledges partial support by a NationalSpace Science and Technology (NSSTC/NASA) award. The simulations have been per-formed on IBM p690 (Copper) at the National Center for Supercomputing Applications(NCSA) which is supported by the National Science Foundation.REFERENCES[1] Frederiksen J. T., Hededal C. B., Haugbølle T., Nordlund A˚., Collisionless Shocks— Magnetic Field Generation and Particle Acceleration, in Proc. From 1st NBSI on Beamsand Jets in Gamma Ray Bursts, held at NBIfAFG/NORDITA, Copenhagen, Denmark,August, 1-6, 2002 (astro-ph/0303360).[2] Frederiksen J. T., Hededal C. B., Haugbølle T., Nordlund A˚., ApJ, 608 (2004)L13.[3] Hededal C. B., Haugbølle T., Frederiksen J. T., Nordlund A˚., ApJ, 617 (2004)L107.[4] Nishikawa K.-I., Hardee P., Richardson G., Preece R., Sol H., Fishman G. J., ApJ,595 (2003) 555.[5] Nishikawa K.-I., Hardee P., Richardson G., Preece R., Sol H., Fishman G. J., ApJ,622 (2005) 927 (astro-ph/0409702).[6] Nishikawa K.-I., Hardee P., Hededal C. B., Richardson G., Preece R., Sol H.,Fishman G. J., Relativistic Shocks: Particle Acceleration and Magnetic Field Generation,and Emission, in Proceeding of International Symposium on High Energy Gamma-RayAstronomy (July 26–30, 2004) edited by Aharonian F. A., Vo¨lk H. J. and Horns D.,AIP Conf.Proc., 745 (2005) 534 (astro-ph/0410193).[7] Silva L. O., Fonseca R. A., Tonge J. W., Dawson J. M., Mori W. B., MedvedevM. V., ApJ, 596 (2003) L121.[8] Weibel E. S., Phys. Rev. Lett., 2 (1959) 83.[9] Medvedev M. V. and Loeb A., ApJ, 526 (1999) 697.

Particle acceleration, magnetic field generation, and emission in relativistic pair jets

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Particle acceleration, magnetic field generation, and emission in relativistic pair jets

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