Particle Acceleration and Magnetic Field Generatio

arXiv:astro-ph/0409702v210Dec2004acceptedforApJParticleAccelerationandMagneticFieldGenerationinElectron-PositronRelativisticShocksK.-I.Nishikawa1NationalSpaceScienceandTechnologyCenter,Huntsville,[e-mail]sandAstronomy,TheUniversityofAlabama,Tuscaloosa,[e-mail]chanicalandAerospaceEngineeringUniversityofAlabamainHuntsvilleHuntsville,[e-mail]s,UniversityofAlabamainHuntsville,Huntsville,AL35899andNationalSpaceScienceandTechnologyCenter,Huntsville,AL35805H.SolLUTH,ObservatoredeParis-Meudon,5placeJulesJansen92195MeudonCedex,FranceandG.J.FishmanNASA-MarshallSpaceFlightCenter,NationalSpaceScienceandTechnologyCenter,Huntsville,AL358051NRCAssociate/NASAMarshallSpaceFlightCenter–2–ABSTRACTShockaccelerationisanubiquitousphenomenoninastrophysicalplasmas.Plasmawavesandtheirassociatedinstabilities(e.g.,Buneman,Weibelandothertwo-streaminstabilities)createdincollisionlessshocksareresponsibleforparticle(electron,positron,andion)acceleration.Usinga3-Drelativisticelectromag-neticparticle(REMP)code,wehaveinvestigatedparticleaccelerationassoci-atedwitharelativisticelectron-positronjetfrontpropagatingintoanambientelectron-positronplasmawithandwithoutinitialmagneticfields.Wefindsmalldifferencesintheresultsfornoambientandmodestambientmagneticfields.NewsimulationsshowthattheWeibelinstabilitycreatedinthecollisionlessshockfrontacceleratesjetandambientparticlesbothperpendicularandparalleltothejetpropagationdirection.Furthermore,thenon-linearfluctuationampli-tudesofdensities,currents,electric,andmagneticfieldsintheelectron-positronshockarelargerthanthosefoundintheelectron-ionshockstudiedinapreviouspaperatthecomparablesimulationtime.ThiscomesfromthefactthatbothelectronsandpositronscontributetogenerationoftheWeibelinstability.Addi-tionally,wehaveperformedsimulationswithdifferentelectronskindepths.Wefindthatgrowthtimesscaleinverselywiththeplasmafrequency,andthesizesofstructurescreatedbytheWeibelinstabilityscaleproportionaltotheelec-tronskindepth.Thisistheexpectedresultandindicatesthatthesimulationshavesufficientgridresolution.WhilesomeFermiaccelerationmayoccuratthejetfront,themajorityofelectronandpositronaccelerationtakesplacebehindthejetfrontandcannotbecharacterizedasFermiacceleration.ThesimulationresultsshowthattheWeibelinstabilityisresponsibleforgeneratingandampli-fyingnonuniform,small-scalemagneticfieldswhichcontributetotheelectron’s(positron’s)transversedeflectionbehindthejethead.Thissmallscalemagneticfieldstructureisappropriatetothegenerationof“jitter”radiationfromdeflectedelectrons(positrons)asopposedtosynchrotronradiation.Thejitterradiationhasdifferentpropertiesthansynchrotronradiationcalculatedassumingaauni-formmagneticfield.Thejitterradiationresultingfromsmallscalemagneticfieldstructuresmaybeimportantforunderstandingthecomplextimestructureandspectralevolutionobservedingamma-rayburstsorotherastrophysicalsourcescontainingrelativisticjetsandrelativisticcollisionlessshocks.Subjectheadings:relativisticjets:Weibelinstability-shockformation-electron-positronplasma,particleacceleration-particle-in-call–3–1.IntroductionNonthermalradiationobservedfromastrophysicalsystemscontainingrelativisticjetsandshocks,e.g.,activegalacticnuclei(AGNs),gamma-raybursts(GRBs),andGalacticmicroquasarsystemsusuallyhaspower-lawemissionspectra.Inmostofthesesystems,theemissionisthoughttobegeneratedbyacceleratedelectronsthroughthesynchrotronand/orinverseComptonmechanisms.Radiationfromthesesystemsisobservedintheradiothroughthegamma-rayregion.Radiationinopticalandhigherfrequenciestypicallyrequiresparticleaccelerationinordertocounterradiativelosses.Ithasbeenproposedthattheneededparticleaccelerationoccursinshocksproducedbydifferencesinflowspeed.Fermiaccelerationisthemostwidelyknownmechanismfortheaccelerationofparticlesinastrophysicalenvironmentscharacterizedbyapower-lawspectrum.Thismechanismforparticleaccelerationreliesontheshockjumpconditionsinrelativisticshocks(e.g.,Gallant2002;Niemiec&Oskowski2004).Mostastrophysicalshocksarecollisionlesssincedissi-pationisdominatedbywave-particleinteractionsratherthanparticle-particlecollisions.Diffusiveshockacceleration(DSA)reliesonrepeatedscatteringofchargedparticlesbymag-neticirregularities(Alfv´enwaves)toconfinetheparticlesneartheshocks.However,particleaccelerationnearrelativisticshockscannotbecharacterizedasDSAbecausethepropaga-tionofacceleratedparticlesaheadoftheshockcannotbedescribedbyspatialdiffusion.Anisotropiesintheangulardistributionoftheacceleratedparticlesarelarge,andthediffu-sionapproximationforspatialtransportdoesnotapply(Achterbergetal.2001).Previousmicrophysicalanalysesoftheenergyconversioninrelativisticpairoutflowsin-teractingwithaninterstellarmediumconsistingofcoldprotonsandelectrons(e.g.,Brainerd2000;Schlickeiseretal.2002)havedemonstratedthatthebeamexcitesbothelectrostaticandlow-frequencymagnetohydrodynamicAlfv´en-typewavesviaatwo-streaminstabilityinthebackgroundplasma.This