Anisotropic Plasma Astrophysics Roger Blandford Paul Simeon Yajie Yuan KIPAC Stanford 27 i 2012 AronsFest Describing Cosmic Plasma • Fluid description – P, , v, B… – Magneto Fluid Dynamics • Flux-freezing,
Download ReportTranscript Anisotropic Plasma Astrophysics Roger Blandford Paul Simeon Yajie Yuan KIPAC Stanford 27 i 2012 AronsFest Describing Cosmic Plasma • Fluid description – P, , v, B… – Magneto Fluid Dynamics • Flux-freezing,
Anisotropic Plasma Astrophysics Roger Blandford Paul Simeon Yajie Yuan KIPAC Stanford 27 i 2012 AronsFest 1 Describing Cosmic Plasma • Fluid description – P, , v, B… – Magneto Fluid Dynamics • Flux-freezing, conservation of mass, momentum, energy • P ~ isotropic! – Relativistic flows – Electromagnetic Flows • Kinetic description Need a hybrid approach to tackle contemporary problem – f(p,x,t), E, B… – Collisionless plasmas • Vlasov equation for f – Nonthermal distributions – Transport effects – Ultrarelativistic plasmas 27 i 2012 AronsFest 2 Anisotropic MHD • MHD commonly developed and computed assuming isotropic pressure • Collisionless plasmas are observed and expected to be anisotropic • How and when can we discuss this at the fluid level using a pressure tensor? • Interesting and possibly different when dominant pressure (and current) is due to ultrarelativistic particles while the flow is locally non-relativistic • Dissipation due to radiation not collisions or plasma instabilities • Diffusive Shock Acceleration – SNR, clusters… – Pinches - PWN, jets… 27 i 2012 AronsFest 3 Particle Acceleration Unipolar Induction Stochastic Acceleration U V ~ I ~ V / Z0 Z0~100 P ~ V I ~ V2/Z0 c 27 i 2012 DE/E ~ +/-u/c ln(E) ~ u/c (Rt)1/2 AronsFest 4 Cosmic Rays protons heavies? protons?? 27 i 2012 GeV Cahill Dedication, Caltech TeV PeV AronsFest EeV ZeV 5 Cosmic Rays solar system supernova remnants pulsars? active galactic nuclei ?? 27 i 2012 GeV Cahill Dedication, Caltech TeV PeV AronsFest EeV ZeV 6 Supernova remnants SN1006 X-ray Cassiopeia A X-ray SN1006 Cas A Radio Tycho Cosmic rays Crab Nebula W44 . 27 i 2012 Chandra AronsFest 7 Particle acceleration in SNR • ~ 100TeV gamma rays SN1006 – ~0.3 PeV cosmic rays – Hadronic and leptonic (Fermi) • Variable X-rays – 100 TeV electrons – ~ 1 mG magnetic field Tycho Cas A • Shocks also amplify magnetic field – Details controversial Perseus Cluster 27 i 2012 AronsFest 8 Ultra High Energy Cosmic Rays • Zevatrons? – Top down exotica • GZK cutoff • EM channel not seen and hard to avoid – Massive BH in AGN (~30-50 Mpc) • AGN may be too weak • Acceleration must be remote from BH – Gamma-Ray Bursts • Stellar BH or millisecond magnetar? • Too distant? Too much radiation? – Cluster Shocks (Norman,Ryu,Bohringer…) • • • • • • High Mach accretion shocks Hard to accelerate p to ZeV energy Heavy elements may be predicted e.g. Fe; range ~ 10 Mpc? Composition controversial Analysis should be aided by LHC 27 i 2012 AronsFest 9 Entropy Matters • Sgas=1.5 ln[(T/Trec)(n/nrec)-2/3]k (relative to recombination) – Much more in CMB • Shocks create gas entropy DS[M]=1.5ln[(5M2/4-1/4)(1/4+3/4M2)5/3]k • Before reionization – Weak shocks M ~ 1-3 DS < k • During reionization (z~10) – Ionization entropy – Moderate shocks M ~1-20 DS <3k • After reionization – May need DS as large as 10k – Would imply M~100 • e.g. V~1000, s~10 km s-1 M Simionescu et al Perseus Sgas/k 18 r 17 16 15 14 13 27 istrong 2012 AronsFest 10 Recent evidence for presence of high M accretion shocks around clusters Shocks in Structure Formation Simulations (Ryu et al 2003) Lx pancake gas filament cluster (100 Mpc/h)2 2D slice T Ms Simulations exhibit high M shocks LCDM simulation with 10243 cells, computational box: (100h-1 3 , TVD: grid-based Eulerian 27 i 2012 AronsFesthydro code 11 Mpc) GeV -rays from Clusters of Galaxies Keith Bechtol •Active Galactic Nuclei •Primordial cosmic rays •Dark Matter Annihilation Han et al 2012 27 i 2012 Upper limits are interesting! AronsFest 12 • • • • • • • • UHECR at accretion shocks?? Accretion shocks Norman Invisible Ryu v~500-1000 km/s? Bohringer… R ~ 3-10Mpc B ~ 3 x 1020V/v R ~10mG! However, SNR generate phenomenal fields OK if UHECR is Fe Counter-evolutionary tendency 27 i 2012 AronsFest 13 Collisionless Shocks Scatter off magnetic waves f ( p) qp q p 0 dp' p'q1 f ( p');q 3r /(r 1) Explain intensity and spectrum of Galactic cosmic rays 27 i 2012 AronsFest 14 Too good to be true! • Diffusion: CR create their own magnetic irregularities ahead of shock through instability if <v>>a – Instability likely to become nonlinear - Bohm limit – What happens in practice? – Parallel vs perpendicular diffusion? • Cosmic rays are not test particles – Include in Rankine-Hugoniot conditions – u=u(x) – Include magnetic stress too? • Acceleration controlled by injection – Cosmic rays are part of the shock • What happens when v ~ u? – Relativistic shocks • Energy cutoff? – E < euBR ~ PeV for mG magnetic field 27 i 2012 AronsFest 15 Particle Transport • Alfven waves scatter cosmic rays ~ (B/B)2rL • Bohm? Cosmic Ray Pressure dominates Magnetic and Gas Pressure far ahead of Shock – D ~ c/3 • Parallel vs perpendicular – L ~ D/u > 100rL ~ 100 EPeVBmG-1Z-1pc • RSNR < 10pc – Highest energy cosmic rays stream furthest ahead of shock • L~E? – “Magnetic Bootstrap” • Firehose and other instabilities P(E) / GeV 27 i 2012 0.1 P(E) / u2 u2 TeV AronsFest E PeV PeV TeV X GeV Shock 16 Magnetic Bootstrap RB Funk 2007 • Assume: – – – – Cosmic rays accelerated by DSA at shock front to ~PeV energy PCR ~ 0.1u2 E9-0.2 Magnetic field amplified upstream > rL Dynamical effects on background small • Wave turbulence maintained at Bohm level mainly by Firehose modes – Also mirror, gyroresonant, Bell-Lucek – Transport by field line modeling – “Uniform” field is turbulent field created by higher energy upstream • Cosmic rays with energy ~ Emax stream away from the shock – Firehose dominates if u > (aISM c)1/2 (PCR/u2)-1/4 ~1000 km s-1 • Fermi observations are instructive! 27 i 2012 AronsFest 17 Magnetic Bootlaces th • How can a small magnetic pressure mediate the interaction between two particle “fluids”? PP j P B PCR jCR B dB j P jCR dX 27 i 2012 CR P mag X j X AronsFest 18 Extragalactic Jets M87 Cygnus A PictorA 3C273 3C31 Pictor A NGC 326 3C75 27 i 2012 AronsFest 19 McKinney Tschekhoskoy RB 2012 27 i 2012 AronsFest 20 Crab Nebula 27 i 2012 AronsFest 21 Flaring behavior Buehler et al April 2011 Power~1029W Singular events or power spectrum? 27 i 2012 AronsFest No variation seen in other bands 22 Spectrum of “Flare” synchrotron spectrum 27 i 2012 AronsFest Electron synchrotron radiation: E~PeV, ~109; B~100nT 23 Where does the variation originate ? • Long term variation of nebula likely due to changes in magnetic field • Peak power is ~ 3 percent of nebular power • Flare energy equals that stared in a region of size L~ 20B-71/2 lt d ~ 2B-71/2 arcsec • We want to learn where and how nature accelerates particles to high energy • Not the Pulsar =10,000mas W J P S T – No correlation with rotation frequency • Wind shocks when momentum flux equals nebular pressure • Wind, Shock, Jet, Torus are all possibilities 27 i 2012 AronsFest 1 lt hr = 3 mas Larmor radius= 609B-7-1mas 24 Pinch? • Resistance in line current – Current carried by high energy particles – Resistance due to radiation reaction – Pairs undergo poloidal gyrations which radiate in all directions – Relativistic drift along direction of current Jet!! – Compose current from orbits selfconsistently – Illustration of Poynting’s theorem! – Variation intrinsic due to instability E j X Bf r E j N 1 jz E cm 0 j 27 i 2012 AronsFest Prr dB Pff B 2 d B 25 MHD with Pressure Tensor Linear Perturbations etc Double Adiabatic Ansatz (CGL) etc First invariant has validity but second is questionable in real plasmas (Kulsrud…). However something like this may be a reasonable approximation for some problems including those of current interest 27 i 2012 AronsFest 26 Friedrich Diagrams Vf(q) 27 i 2012 AronsFest 27 Firehose (and mirror) instabilities 27 i 2012 AronsFest 28 Conjectures • Relativistic CR ahead of nonrelativistic shock wave behave like a highly anisotropic fluid with an equation of state and this dictates the growth of magnetic field • Firehose dominates, resonant, Bell, Weibel… • Cosmic ray acceleration kinetics can be solved self-consistently in this background 27 i 2012 AronsFest 29 Particle drifts and current Normal approach is to analyze particle orbits and deduce currents Can also start from static equilibrium and understand what is happening Curvature perpendicular magnetization gradient ExB Parallel current contains a part that is magnetization drift Also a part that is resistive – collisional/radiative 27 i 2012 AronsFest 30 Conjectures • Unipolar inductor potential differences maintained into jets and nebulae • Pinches (relatively) stabilized by flow, expansion… • Highest energy particles with largest gyro radii carry current and dissipate resistively through radiation • Steady and explosive instability generic leading to in situ acceleration and emission 27 i 2012 AronsFest 31 Summary • Major challenges to particle acceleration from Crab, blazar flares UHECR… • Hybrid – fluid/kinetic problems • Currents may be carried by highest energy particles and dissipation can be primarily radiative • May be possible to gain insight at fluid level using anisotropic RMHD • Drift currents etc can emerge from MHD rather than vice versa • Particle trajectories must still be solved in this background 27 i 2012 AronsFest 32