2 vi 2006Bethe Centenary Cornell1 The Future of High Energy Astrophysics Extreme Physics in an...

The Future of High Energy Astrophysics

Extreme Physics in an Expanding Universe

Roger BlandfordKIPAC

Stanford

Hans Bethe Centenary, Cornell

2 vi 2006 Bethe Centenary Cornell 2

Plan

• Electromagnetic extraction of energy from black holes

• Relativistic jets• Non-thermal emission • Other channels and future

possibilities


High Energy Sources• Compact Source

– Impulsive or Quasi-steady– White dwarf– Neutron star– Black hole

• Massive• Stellar

• Outflow– Often ultrarelativistic

• Dissipation region– Particle acceleration– Nonthermal emission

Expansion

Extreme Power


Jets are commonAGN, GSL, PWN, GRB, YSO…– TeV emission– 1 < < 300– AGN jets are not ionic?– Jets magnetically confined?– Structured jets– Disk-jet connection

~30c

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are needed to see this picture.


Black holes common

• Present in most galaxies– Sc, Irr, Pec?

• Hole-Bulge Relations– Co-evolution– Influence surroundings

• Binary black holes– Mergers– Low recoils


Progress - Galactic Center

• ~100 hot young stars in disk(s) within 106m– Approach ~ 104m

• mm size<10m?• X-ray, infrared flares,

QPOs?• Non-thermal emission

– 20 min

• mm polarization, 10-8 Ledd.• Accretion rate<<mass

supply

Chandra2-8 keV

2 hours

May 2002 campaign: ~0.6-1.2

flares/day


Unipolar Induction B

M

• V~ P ~ V2 / Z0

• Crab Pulsar – B ~ 100 MT, ~ 30 Hz, R ~ 10 km– V ~ 30 PV; I ~ 3 x 1014 A; P ~ 1031W

• Massive Black Hole in AGN– B ~ 1T , ~ 10 Hz, R ~1 Pm– V ~300 EV, I ~ 3EA, P ~ 1039 W

• GRB– B ~ 1 TT, ~ 1 Hz, R~10 km– V ~ 30 ZV, I ~ 300 EA, P ~ 1043 W

UHECR: Emax ~ ZeV


Three dynamical approaches

• Fluid dynamics +passive field– Fluid velocity, scalar + ram pressure

• Classical Electromagnetodynamics– Maxwell stress tensor + Poynting Flux

• Relativistic Magnetohydrodynamics– All of the above

Real sources require all three approaches


Let there be Light

• Faraday• Maxwell• Definition• Initial

Condition

€

∂B

∂t= −∇ × E

∂E

∂t=∇ × B − j

ρ =∇ ⋅E

∇ ⋅B = 0

=>Maxwell Tensor, Poynting Flux


Force-Free Condition

• Ignore inertia of matter =UM/UP>>2, 1

• Electromagnetic stress acts on electromagnetic energy density

• Fast and intermediate wave characteristics,

€

ρE + j × B = 0⇒ E ⋅B = E ⋅ j = 0

j =(∇ ⋅E)E × B + (B ⋅∇ × B − E ⋅∇ × E)B

B2


Electromagnetic Formation of Jets

For Cygnus A:B ~ 104 G; M ~ 109 MO

E ~ V Rin ~ Rout ~ 100I ~ V / R ~ 1018 AP ~ E I ~ 1038 W

Corotating observer seesenergy flow inwards athorizon; conserved energy flux in non-rotating frameis outward.


MHD/FF Simulations

• Time dependent GR calculations– Causality OK

• Power from spin of hole

• Collimating ~10 jets to r~1000m

• Marginally stable• Jet structured• Entrainment?

McKinney 2006


X

X

B>

JX

.

€

LH

LD

~1

α Dβ D

⎛

⎝ ⎜

⎞

⎠ ⎟ΩH

ΩD

⎛

⎝ ⎜

⎞

⎠ ⎟

2sD

c

⎛

⎝ ⎜

⎞

⎠ ⎟

On the Electrodynamics of Moving Bodies

Even field Odd current

Camenzind, Koide


Jet Expansion• From ~m to 106-11m • Initially electro/hydromagnetic

– Poynting dominated core • Plus pair plasma

• LEM>>Lfluid

– MHD disk wind• u < 1

• Hadrons

Where do the currents close?

Where does the jet become baryonic?


The case for large scale magnetic collimation

• Many jets do not appear to spread like observed fluid jets

• M ~ for fluid jets• Typically equipartition pressure

in jet exceeds maximum external gas pressure

• Some Faraday rotation evidence for toroidal field

nb DC not AC


Pictor A

Current FlowNonthermal emission is ohmic dissipation of current flow?

Electromagnetic Transport1018 not 1017 ADC not ACNo internal shocksNew particle acceleration mechanisms

Pinch stabilized by velocity gradient

Equipartition in core

Wilson et al


Elementary confinement by toroidal field

• Toroidal magnetic field B in jet frame• Current I in rest frame

– I=2rB/0

• Static equilibrium

• Jet Power

• Typically Mech power~ 3-10 EM power

€

dP

dr+

μ0

8π 2r2

d

dr

I

Γ

⎛

⎝ ⎜

⎞

⎠ ⎟2

= 0

€

LEM =Z0

2π

dr'

r'0

r

∫ (1− Γ−2)1/ 2 I2

LMech = 8πc dr'r'Γ(Γ 2

0

r

∫ −1)1/ 2 P

X B

I[r]

r

Return Current

Pext << Pjet

Shear jets stabler than pinches


Simple Example

• L=Lmech+Lem=3 x 1045(Ioo/1EA)2 erg s-1

• Ioo=0.15(B/1mG)(r/10pc)EA• Equipartition a natural consequence• Emission Model• Faraday Rotation Model• VIPS VLBI survey of 1000 radio sources

I/I00

P/P0

Lmech

Lem


Particle acceleration magnetic field amplification

and nonthermal emission

• Shock waves – Non-relativistic– Relativistic

• Reconnection• Electrostatic acceleration• Stochastic acceleration• Other possibilities


Proton spectrum

GeV TeV PeV EeV ZeV

~MWB Energy Density

E ~ 50 Jc -1fm s-1

c -1km H0

~ GRBEnergy Density


Supernova Remnants

• X-ray supernova remnants

• Shocks• Electron

acceleration• Proton

acceleration?• TeV gamma rays


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 c

E/E|~ – Second order process

• dE/dt ~ 2(c/)E - E/tesc

– tacc>rL/c2

• =>Power law distribution function IF , tesc do not depend upon energy

– THEY DO!


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) u u / r

B

u u / r

B

L

€

uf − D∂f

∂x= uf−

€

f = f+

€

f = f−

€

f[ ] = −u∂f

∂ ln p3− D

∂f

∂x

⎡

⎣ ⎢

⎤

⎦ ⎥= 0


Transmitted Distribution Function

€

f+(p) = qp−q dp' p'q−1 f−( p')0

p

∫where

€

q =3r

r −1

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


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


Progress - Cosmic Rays

• Sources of low energy particles

• t~15Myr,t>0.1Myr• Zevatrons• Protons, nuclei,

photons• GZK cutoff?• Bottom up???

ACE


UHE Cosmic Rays• L>rL/; =u/c

– BL>E21G pc=>I>3 x 1018E21 A!– Lateral diffusion

• P>PEM~B2L2c/43 x 1039 E212-1 W

– Powerful extragalactic radio sources, ~1

• Relativistic motion eg gamma ray bursts– PEM ~ 2(E/e)2/Z0 ~ (E/e)2/Z0

• Radiative losses; remote acceleration site – Pmw < 1036(L/1pc)W

• Adiabatic losses– E~/L

• Observational association with dormant AGN?


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


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 2 if elastically scattered

• Spectrum N(E)~E-2.3

eg Keshet &Waxman


Relativistic collisionless shocks in astrophysics

• Pulsars + winds (plerions) ~ 106??• Extragalactic radio sources ~ 10• Gamma ray bursts > 100• Galactic superluminal sources ~ few

Open issues:

• What is the structure of collisionless shock waves?• Particle acceleration -- Fermi mechanism? Something else?• Generation of magnetic fields (GRB shocks, primordial fields?)

Simulations of relativistic collisionless shocks

Anatoly Spitkovsky (KIPAC)


Why does a collisionless shock exist?

Particles are slowed down either by instability (two-stream-like) or by magnetic reflection. Unmagnetized shocks are mediated by Weibel instability, which generates magnetic field:

Simulations of relativistic collisionless shocks

Relativistic flow is reflected from a wall and sends a reverse shock through the simulation domain

Plasma density Field generation

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Shock structure with magnetic field

Magnetized perpendicular pair shock

Magnetized shock is mediated by Larmor reflections from compressed B field

3D density

Spectrum - Maxwellian

Pair shocks do not show nonthermal acceleration

Is Fermi acceleration really viable in magnetized shocks?

px

py

pz


Conclusions• Relativistic collisionless shocks exist, mediated by two-stream

instability (Weibel) in low magnetization flows, and coherent reflections in higher magnetization flows. Transition is observed in simulations:

• Efficient thermalization of the flow. In magnetized shocks not enough turbulence downstream for nonthermal acceleration. In unmagnetized shocks some evidence for diffusive nonthermal acceleration, although long-term large simulations still need to be done.

• Compositionally this suggests weak acceleration efficiency in pair plasma flows. Additional component of the ions may be necessary.

3D density

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=0 =0.003 =0.1


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– Automatically have kinetic energy to tap


GeV-TeV electrons in jets

• Tgyro < Tsynchrotron =>E<f~ 100 MeV

=> C-1 emission• If electrons radiate efficiently on time of flight and <1

=>Modest electron energies in KN regime=>n(E-1)TR<10, P2, relativistic sources quite likely• Not a great challenge!• No strong shocks when magnetically dominated• Must beat radiative loss• Hybrid (+e) bulk acceleration schemes tapping

kinetic energy of shear flow (Stern, Poutanen…)


Electrostatic Acceleration

• Establish static potential difference• Gaps • Double Layers• Charge-starved waves

(Thompson&Blaes)• Aurorae• Pulsars

• PEM > (E/e)2/Z0


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


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


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 local power laws?

kUk

k

2N


Observational Ramifications• VLBI observations

– Characteristic Field Geometry• Probe using Faraday rotation measurements

– Understand jet composition• Pairs vs ionic plasma

• X-ray/-ray observations– Are particles accelerated at strong shocks?

• Synchrotron vs inverse Compton emission

– TeV (HESS/VERITAS) vs GeV (GLAST)• Outside-in or inside-out emission


Prospects:GLAST (2007)

• Some key science objectives:– understand particle acceleration and high-energy emission

from neutron stars and black holes, including GRBs

– determine origin(s) of -ray extragalactic diffuse background

– measure extragalactic background starlight

– search for dark matter extra dimensions?n

• Multi-wavelength observations

• Interpolates discoveries of Chandra/XMM and HESS.

“.. GLAST will focus on the most energetic objects and phenomena in the universe…”


Prospects: High Energy Density Physics• High Power Lasers - 1023-25 W m-2

– > MT magnetic field – >100 MeV electron quiver energies– Collisionless shocks, particle acceleration, magnetic field

• Spheromaks – Dynamics and stability of force-free configurations– Magnetic reconnection

• High current e-beam experiments– Current instabilities

• Z-pinches– Stability


Computational Prospects

• Major advances in recent years • 3D RMD in Kerr-Schild

coordinates!• Increased dynamical range using

Adaptive Mesh Refinement– Jets, disks

• Improved treatment of energy• Radiative transfer• Kinetic problems…


Summary• Great progress in high energy astrophysics• Big astrophysics problems are multisource not

just multiwavelength• Major computational advances • Short term observational prospects very good

– Chandra, XMM, Swift, Suzaku…– HESS, MAGIC, (VERITAS?)– Auger– LIGO, Amanda, ICECUBE, NESTOR, Anita, LOFAR– GLAST (2007)

• Long term prospects in space problematic– NuSTAR, LISA, Constellation-X, XEUS…

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