Fundamental Issues in High Energy Astrophysics Roger Blandford KIPAC Stanford.

Fundamental Issues in High Energy Astrophysics

Roger BlandfordKIPAC

Stanford

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The Electromagnetic Spectrum

r mmsmm

ir o uv x

100neV 100TeV

mec2 mpc2

High Energy Astrophysics

MeV GeV TeV

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Other spectra

Electromagnetic

Cosmology

Gravitational Radiation

Dark Matter

Neutrinos

10-32eV 1028eV

Hubble Planck

Cosmic Rays

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Radio Astronomy

VLA

100 mas

VLBA0.0001arc secGreen BankTelescope

10 arc sec

0.1 arc sec

Radio telescopes are extremely sensitive and can resolve fine detail

Cygnus A

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Mm waves

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ALMA (Chile)

Owen’s Valley

Radio source associated with the first“Quasar” discovered that appears toexpand “faster than light”!

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Far InfraredRadiation

Infrared radiation comes from dusty, star forming regions It is possible to distinguish infrared colors just like optical

colors

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Spitzer

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It is possible to removethe blurring of theatmosphere

Near Infrared EmissionKeck





Keck

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Optical EmissionQuickTime™ and a

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Centaurus A

Hubble Space Telescope orbits above the Earth’s atmosphere and can resolve 0.1”

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Ultra Violet Radiation

Ultraviolet telescopes trace hot stars

and measure colors too.



GALEX

M51

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X-rays

X-ray telescopes observe hot gas and high energy particles

Pictor A

Chandra

XMM-Newton

Perseus

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MeV -raysINTEGRAL

Gamma rays are created by nuclear processes and particles moving with “relativistic” speed

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GeV-rays

Compton

GLAST

GeV -rays are created mainly by relativistic electrons and protons

Dec. 2007AGILE launched!

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TeV -rays

Observed by the optical light they produce

HESS (Namibia)

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Radio Continuum (408 MHz)

Atomic Hydrogen

Radio Continuum (2.4–2.7 GHz)

Molecular Hydrogen (CO)

Infrared

Mid-infrared

Near-infrared

Optical

Soft X-ray

Gamma ray

Hard X-rayHard X-ray

The Milky Way

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Cosmic Rays

Fastest cosmic rays have energy of a well-hit baseball

ACE

SN1006

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Very High Energy Neutrinos

Nothing yet!

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Gravitational Radiation

LIGO

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.Merging neutron stars, black holes

LISA

High frequencyLow frequency

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The Multiwavelength Challenge



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Fundamental Issues

• What Powers High Energy Sources?– Does standard (experimentally verified) physics suffice?– As a practical matter have to understand secondary problems

• Energy transport• Emission mechanisms• Particle acceleration

• What can High Energy Astrophysics tell us about Particle Physics Beyond the Standard Model?

– Neutrinos, dark energy, dark matter, >TeV collisions…

• Is Classical GR Correct?– Minimally coupled Einstein-Maxwell Equations?

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Nonthermal Power

• Magnetic Flares– Sun – SGR

• Unipolar Induction– PWN– AGN– GRBs– XRBs

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Stars

• Sun– Flares – Solar minimum->maximum– Observe neutrons– Radiation hazard

• Minutes!



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Magnetar (Soft Gamma Ray Repeater)

• SGR 1806-20. Giant burst, Dec 27 2004• Source in our Galaxy; ~1040J• 300s rise time; 7s period in tail• Relativistically expanding, anisotropic, radio

remnant

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B

M

• Rules of thumb:

B R2 ; V ~ ;

– I~ V / Z0; P ~ V I

PWN AGN GRB

B 100 MT 1 T 1 TT

10 Hz 10 Hz 1 kHz

R 10 km 10 Tm 10 km

V 3 PV 300 EV 30 ZV

I 300 TA 3 EA 300 EA

P 100 XW 1 TXW 10 PXW

Unipolar Induction

UHECR!

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Consequences of large EMFs

• Particle energy density / EM energy density– > r

L/L , m

ec2/eV

• Electric field => rapid breakdown – accelerate electrons– scatter photon – produce electron-positron pair

• Vacuum is an excellent conductor thanks to QED– B2-E2>0

• Very hard to produce entropy– Not a criticism of neutrino models!

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The ElectromagneticView

• Pulsar powers nebula ~ 1031 W • Mechanical energy =>

electromagnetic energy at the pulsar (10km)• Jets are Ohmic dissipation of current flowing through the nebula (1012km)• Accelerate electrons• Jets are highly unstable

X X

.

OO

O

O.

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Black Holes Spin



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Electromagnetic Jet Model

• Black Hole plus magnetised disk act as unipolar inductor creating ~ 1020V, 1018A -> 1038W

• Where does current complete?– Close to hole => emission due to internal shocks in a fluid

– In radio source => emission due to ohmic dissipation of current

Largeelectrical resistance

E x B

J

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Gamma Ray Bursts

• Long bursts– ts~3-100 s E ~ 1051 erg (beaming)– ~1d -1yr afterglows– Associated with SNIc; BH/NS formation– Achromatic breaks => jets?– Gamma ray escape => > 300?

• Short bursts– ts~ 0.1-3 s – Coalescing neutron stars

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Active Galactic Nuclei ~ 3-30

Are these fluid jets?

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Pulsar Physics

• Detection– 100s pulsars?– 50 RQ pulsars?– 10 MSP– RRATS– Blind searches

• How do pulsars shine?– Polar cap vs slot gaps vs outer gaps– Locate gamma ray and radio emission– Does gamma ray power ~ V?

• Force free models – Compute pulse profiles for different emission

sites and fit to radio, gamma ray observations– Is the rotating vector model really supported by

observations?

• Orthogonal polarization!

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Harding

JohnstonRansom

Spitkovsky

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Particle Acceleration

• 100 yr question of acceleration of cosmic rays – GeV-TeV– PeV– EeV

• Relativistic outflows dissipate into high energy electrons with high efficiency

– PWN, AGN, GRB, XRB– Volumetric?

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Proton spectrum

GeV TeV PeV EeV ZeV

~MWB Energy Density

E ~ 50 Jc -1fm s-1

c -1km H0

Iron??

Can one mechanism Accelerate up to the knee?

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Nonthermal electron acceleration

• Diffusive Shock Acceleration– Transmit CR protons with PCR ~E2N(E)~ E-.2

~0.1u2 – Pe ~ 0.03 Pp – Accounts for GCR after including

propagation – Observed in IPM– Generic - eg clusters of galaxies

• Radio observations of SNR– Relativistic electron spectrum– Tycho, Cas A….

• X-ray observations of SNR– 2-100 keV– 100TeV electrons– >100G fields

Thermal, Thermal, ExtendedExtended

Non-thermal, sharpNon-thermal, sharp

Bamba SN1006

Cas ATycho

5 v 07 INPAC 34PeV CR => >100G field

10’Nonthermal Proton

Acceleration?

• RX J1713.7-3946 – AD385, R ~ 10pc, u~3000 km s-1– ~ 10-25 g cm-3 ; P- ~ 10-12 dyne cm-2; – P+ ~ 10-8 dyne cm-2; M ~ 150

• ~O.1 PeV -rays– Inverse Compton by electrons?– Pion decay from protons?– Accelerate ~0.3 PeV protons?– Explain knee in GCR spectrum

• Lx/L ~ 3 => hadronic emission?– =>P+(100TeV) ~ 10-10 dyne cm-2

– =>P+(GeV) ~ 10-9 dyne cm-2 ~0 1 P+

– P+(e) ~ 3 x 10-11 dyne cm-2

• Particle transport– rL ~ 4 x 1012EGeVBG

-1Z-1cm– <u R/c

Aharonian et al

GeV TeV PeV

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Diffusive 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

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Transmitted Distribution Function

€

f = f− + ( f+ − f−)exp[ dx'u /D0

x

∫ ];x < 0

f = f+;x > 0

f+(p) = qp−q dp' p'q−1 f−( p')0

p

∫ ;q = 3r /(r −1)

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

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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

• How do you accelerate ~PeV cosmic rays?– E < euBR ~ TeV for G magnetic field

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Particle Transport

• Alfven waves scatter cosmic rays ~ (B/B)2rL

• Bohm? – D ~ c/3

• Parallel vs perpendicular– L ~ D/u > 100rL ~ 100 EPeVBG

-1Z-1pc

• RSNR < 10pc– Highest energy cosmic rays stream furthest ahead of shock

• L ~ E ?– Wave Turbulence spectrum

• 3, 4 Wave processes.• Transit time damping?• Nonlinear P(E) / u2

GeVTeVPeV

0.1

Shock

X

E

P(E) / u2

GeV TeV PeV

Cosmic ray pressure dominates magnetic and gas pressure far ahead of the shock.Furthermore the pressure drives instability and magnetic field growthleading to efficient particle acceleration

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Particle acceleration in relativistic outflows may

not be due to shaocks

•Inter-knot X-ray emission in M87•Magnetic shocks are weak•Stochastic acceleration (by em wave) is efficient

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UHE Cosmic Rays

• GZK cutoff?– Distant or local

• Clusters– CR astronomy or physics

• Composition– Protons in EeV range?

• 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 ~

• Leat unpromising astronomical sources– Dormant AGN– GRBs

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Summary

• Great opportunities in High Energy Astrophysics• Opening up astronomical energy frontiers in

electromagnetic, neutrino, cosmic ray, gravitational radiation channels

– Plausible sources in all of these bands but no guarantees

• No compelling evidence yet that “new” physics needed for any high energy astronomical sources

– cf cosmology where dark matter and energy were discovered – cf stellar physics where neutrino masses were “discovered” – Worth keeping in mind

• Auger, GLAST, AGILE, HESS, VERITAS, MAGIC, IceCube, LIGO… can all make major discoveries

Fundamental Issues in High Energy Astrophysics Roger Blandford KIPAC Stanford.

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Transcript of Fundamental Issues in High Energy Astrophysics Roger Blandford KIPAC Stanford.