Post on 19-Dec-2015
Fundamental Issues in High Energy Astrophysics
Roger BlandfordKIPAC
Stanford
5 v 07 INPAC 2
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
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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.
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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
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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?
5 v 07 INPAC 20
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!
5 v 07 INPAC 25
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
5 v 07 INPAC 35
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
5 v 07 INPAC 36
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
5 v 07 INPAC 37
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
5 v 07 INPAC 38
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
5 v 07 INPAC 39
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
5 v 07 INPAC 40
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
5 v 07 INPAC 41
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