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Particle Acceleration GeV-TeV Astrophysics in the GLAST era SLAC Sep 30 2004

Particle Acceleration GeV-TeV Astrophysics in the GLAST era SLAC Sep 30 2004. Roger Blandford KIPAC. Direct Evidence for SNR Acceleration. Cas A. Electron acceleration to TeV energies TeV gamma rays sometimes observed Model -dependent evidence for ion acceleration

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Particle Acceleration GeV-TeV Astrophysics in the GLAST era SLAC Sep 30 2004

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  1. Particle AccelerationGeV-TeV Astrophysics in the GLAST eraSLAC Sep 30 2004 Roger Blandford KIPAC

  2. Direct Evidence for SNR Acceleration Cas A • Electron acceleration to TeV energies • TeV gamma rays sometimes observed • Model -dependent evidence for ion acceleration • Efficient acceleration but not to knee • EGRET, GLAST (GeV) • Hess, VERITAS (TeV) • pion decay

  3. Proton spectrum ~MWB Energy Density E ~ 50 J c -1fm s-1 c -1km H0 ~ GRB Energy Density GeV TeV PeV EeV ZeV

  4. “Relativistic Jets”

  5. GeV-TeV electron emission • Tgyro < Tsynchrotron =>E<af-1~ 100 MeV => C-1 emission • If electrons radiate efficiently on time of flight and a<1 =>Modest electron energies in KN regime =>n(E-1)sTR<10, P2, relativistic sources quite likely Not a great challenge! Must beat radiative loss Protons?

  6. Particle Accleleration • Stochastic • Shocks • Reconnection • Electrostatic • Currents

  7. Traditional Fermi Acceleration • Magnetic field lazy-electric field does the work! • Frame change creates electric field and changes energy • CR bounce off magnetic disturbances moving with speed bc • |DE/E|~b • Second order process • dE/dt ~ b2(c/l)E - E/tesc • tacc>rL/cb2 • =>Power law distribution function IF l, tesc do not depend upon energy • THEY DO!

  8. u u / r u u / r L B B Shock Acceleration • Non-relativistic shock front • Protons scattered by magnetic inhomogeneities on either side of a velocity discontinuity • Describe using distribution function f(p,x)

  9. Transmitted Distribution Function where =>N(E)~E-2 for strong shock with r=4 Consistent with Galactic cosmic ray spectrum allowing for energy-dependent propagation

  10. Too good to be true! • Diffusion: CR create their own magnetic irregularities ahead of shock through instability if <v>>VA • Instability likely to become nonlinear - Bohm limit • Cosmic rays are not test particles • Include in Rankine-Hugoniot conditions • u=u(x) • Acceleration controlled by injection • Cosmic rays are part of the shock • What mediates the shock, • Parallel vs Perpendicular • Firehose? Weibel when low field? Ion skin depth

  11. Relativistic Shock Waves • Not clear that they exist as thin discontinuities • May not be time to establish subshock, magnetic scattering • Weibel instability => large scale field??? • Returning particles have energy x G2 if elastically scattered • Spectrum N(E)~E-2.3 G eg Keshet &Waxman 2-1/2 G

  12. Flares and Reconnection • May be intermittent not steady • Strong, inductive EMF? • Hall effects important • Most energy -> heat • Create high energy tail • Theories are highly controversial! • Relativistic reconnection barely studied at all

  13. B  Unipolar Induction M V~ n F ~ I Z0; P ~ V2 / Z0 • Crab Pulsar • B ~ 100 MT, n ~ 30 Hz, R ~ 10 km • V ~ 30 PV; I ~ 3 x 1014 A; P ~ 1031W • Massive Black Hole in AGN • B ~ 1T , n ~ 10 mHz, R ~1 Pm • V ~300 EV, I ~ 3EA, P ~ 1039 W • GRB • B ~ 1 TT, n ~ 1 kHz, R~10 km • V ~ 30 ZV, I ~ 300 EA, P ~ 1043 W

  14. Pictor A Active collimation?

  15. Nonthermal emission is ohmic dissipation of current flow? 1017 or 1018 A? (Implicit) conventional view is that electromagnetic energy dissipated close to source and outflow is fluid dynamical. Perhaps the transport is primarily electromagnetic.

  16. Is particle acceleration ohmic dissipation of electrical current? • Jets delineate the current flow? • Organized dominant electromagnetic field • Shocks weak and ineffectual • Tap electromagnetic reservoir • Poynting flux along jet and towards jet • Jet core has equipartition automatically • Unlikely to be direct electrostatic acceleration above GeV-TeV

  17. Let there be Light • Faraday • Maxwell • Initial Condition • Definition =>Maxwell Tensor, Poynting Flux

  18. Force-Free Condition • Ignore inertia of matter s =UM/UP>>G2, 1 • Electromagnetic stress acts on electromagnetic energy density • Fast and Alfven wave modes • B2-E2 > 0, normally

  19. Surfing • Relativistic wind eg from pulsar • V=ExB/B2 E->B, V->c • More generally force-free electrodynamics, limit of MHD when inertia ignorable evolves to give regions where E/B increases to, and sometimes through, unity • cf wake-field acceleration

  20. Stochastic acceleration • Line and sheet currents break up due to instabilities like filamentation? • Create electromagnetic wave spectrum • Nonlinear wave-wave interactions give electrons stochastic kicks • Inner scale where waves are damped by particles • Are there localpower laws? g2Ng kUk g k

  21. Summary • GeV-TeV emission - inverse Compton scattering by relativistic electrons • Protons? • Shock acceleration likely to be dominant in gas-dominated regions - eg SNR • Ultrarelativistic outflows may be electromagnetically dominated and require different acceleration mechanisms • Stochastic acceleration, Surfing, shear drift acceleration

  22. Abundances • (Li, Be, B)/(C, N, O) •  ~ 10 (E/1GeV)-0.6 g cm-2 • S(E) ~ E-2.2 • L ~ UCR Mgas c ~ 3 x 1040 erg s-1 ~ 0.03 LSNR ~ 0.001Lgal

  23. ACE ACE Sources • t ~ 15 Myr (eg 10Be etc) • t > 105 yr (eg 59Ni/Co) ACE • Heavy element injection Ionization Potential Volatility Injected as grains?

  24. UHE Cosmic Rays • Range ~30 Mpc (GZK) • luminosity density ~ GCR • Probably not Fe? • Probably not ``top down'’ • Excess  -ray background • Sources • Isotropy, clustering? - permanent not transient? Limits on B • Need better statistics • => Auger(S)...

  25. Luminosity density (cgs) -35 -36 -37 -38 Galactic Extragalactic 9 12 15 18 21 eV

  26. Cosmic Ray Propagation • Larmor radius: rL~E21/B pc • Larmor period:tL~20 E21/B yr • Resonant scattering by magnetic disturbances (wave modes) with k~rL-1 • Random walk in pitch angle • Mean free path: l~(B/dBres)2rL > rL • Non-resonant scattering: l~krL2 > rL • Diffusion coefficient: D~lc/3 • Bohm Diffusion: D~ 0.1 rLc

  27. Local Cosmic Ray Sources • Magnetosphere, Jupiter, Sun • Interplanetary traveling and standing shocks • Solar Wind Termination Shock • Galaxy (SNR, hot stars?)

  28. UHE Cosmic Rays • L>rL/b; b=u/c • BL>E21G pc=>I>3 x 1018E21 A! • Lateral diffusion • P>PEM~B2L2bc/4p > 3 x 1039 E212b-1 W • Powerful extragalactic radio sources, b ~1 • Relativistic motion eg gamma ray bursts • PEM ~ G2(E/eG)2/Z0 ~ (E/e)2/Z0 • Radiative losses; remote acceleration site • Pmw < 1036(L/1pc)W • Adiabatic losses • E~G/L • Observational association with dormant AGN?

  29. “Relativistic Jets”

  30. Pictor A Wilson et al

  31. UHE Cosmic Rays • L>rL/b; b=u/c • BL>E21G pc=>I>3 x 1018E21 A! • Lateral diffusion • P>PEM~B2L2bc/4p > 3 x 1039 E212b-1 W • Powerful extragalactic radio sources, b ~1 • Relativistic motion eg gamma ray bursts • PEM ~ G2(E/eG)2/Z0 ~ (E/e)2/Z0 • Radiative losses; remote acceleration site • Pmw < 1036(L/1pc)W • Adiabatic losses • E~G/L • Observational association with dormant AGN?

  32. Second order acceleration • N ~ b-2 • L ~ N1/2l ~ l/b • Similar requirements to shock model

  33. Electrostatic Acceleration • Establish static potential difference • Gaps • Double Layers • Aurorae • Pulsars • PEM > (E/e)2/Z0

  34. Pulsar Wind Nebulae

  35. Problems • Unlikely to make full potential difference available for electrostatic acceleration • 1TV creates pair discharge in pulsars • 1GV is all that is needed in AGN • The larger the EMF the harder it is to dissipate the current

  36. Solar Flares Magnetic instability and reconnection SOHO YOHKOH Trace

  37. Magnetars • Neutron stars with 1014-15G field • Repeating gamma ray sources • Magnetically powered • Radiative and adiabatic losses severe? • Galactic source as extragalactic, magnetar luminosity density too low • Fe?

  38. Magnetars • Slowly spinning, high field • neutron star • May spin rapidly at birth.

  39. E B x Shear Layer V • Ez=sinh kz; By=cosh kz; Vx=-c tanh kz • rE + j x B = 0 • Analytic solutions • eg ux=-(1-k2)1/2t; uz=(1-k2)1/2/k • Electric, gradient, polarization drifts • Energy bounded by electrostatic potential difference • Pem>(E/e)2/Z0

  40. Relativistic Outflows • Enormous particle energies possible in principle through “magnetic slingshot” • Can particles slide smoothly along field lines? • Radiative losses

  41. Summary • Basic astrophysical accelerators -shocks, holes, magnetars, winds- not ruled out • All pose serious problems • New idea or top down solution (also hard) • Auger should rule out many possibilities

  42. General References • Blandford (2000) Phys. Scripta T85 191-193 • Anchordoqui, Paul, Reucroft & Swain (2003) Int.J.Mod.Phys. A18 2229-2366 • Olinto (2004) astro-ph 0404114 (XXVIII ICRC) • Ong (2003) SSI03 vugrafs

  43. Acceleration Mechanisms • Statistical Acceleration • Shock Waves • Electrostatic Acceleration • Unipolar Induction • Flares • Magnetars • Surfing • Relativistic Outflows

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