Radiation from Poynting Jets and Collisionless Shocks

1
Radiation from Poynting Jets and Collisionless
Shocks Edison Liang, Koichi Noguchi Shinya
Sugiyama, Rice University Acknowledgements
Scott Wilks, Bruce Langdon Bruce Remington Talk
given at AAS Seattle Meeting January 2007
2
Two Paradigms for the Energetics
of Relativistic Jets in Blazars GRBs
What is primary energy source? How are the ee-
accelerated? How do they radiate?
B
ee- ions
ee-
shock
g-rays SSC, EC
g-rays
Synchrotron Radiation is usually assumed in All
paradigms
Internal shocks Hydrodynamic Outflow
Poynting flux Electro-magnetic -dominated
outflow
3
We have developed a Particle-In-Cell code that
simultaneously computes radiation output from
each superparticle self-consistently. We find
that in-situ radiation output from electrons
accelerated by Poynting flux and relativistic
collisionless shocks are much below that
predicted by the classical synchrotron formula.
This is because the highest energy electrons
are preferentially accelerated by Lorentz forces
that are parallel, not perpendicular, to the
particle momentum.
4
From PIC runs, we compute the
radiation power directly from the force terms
Prad 2e2(F 2 g2F2) /3cm3 where
F is force along v and F is force orthogonal
to v (algorithm is carefully calibrated against
analytic formula using monoenergetic electrons
radiating in a uniform static B-field)
For Poynting flux acceleration, we find that
Prad We2g2sin4a ltltlt Psyn We2g2 where altlt1 is
angle between v and Poynting vector k. Also
critical frequency wcr Weg2sin2a ltlt wcrsyn
5
This result Radiation efficiency ltlt classical
synchrotron can be understood as follows The
most efficient accelerations to higher ? are
produced by Lorentz forces parallel to
the particle momentum. But only forces
perpendicular to particle momentum can produce
synchrotron radiation.
6
ee-Poynting jet running into cool e-ion
ambient plasma
B
(movie by Noguchi)
7
Poynting Flux Particle acceleration by induced j
x B(ponderomotive) force
EM pulse
Entering
By
Plasma
JxB force snowplows all surface particles
upstream ltggt max(B2/4pnmec2, ao) Leading
Ponderomotive Accelerator (LPA)
Ez
Jz
x
Exiting
Plasma
JxB force pulls out surface particles. Loaded EM
pulse (speed lt c) stays in-phase with the fastest
particles, but gets lighter as slower particles
fall behind. It accelerates indefinitely over
time ltggt gtgt B2 /4pnmec2, ao Trailing
Ponderomotive Accelerator (TPA). (Liang et al.
PRL 90, 085001, 2003)
x
8
Electrons snowplowed by Poynting flux radiates at
a level 10-4 of synchrotron formula using
local B and g. This is due to sina pz/px lt 0.1.
Prad
px
By
Prad
pz
x
Panalytic We2g2sin4a
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Details of TPA expansion
Momentum gets more and more Anisotropic with
time pz/pxltlt1
10
The power-law index seems remarkably robust
independent of initial plasma size or
temperature and only weakly dependent on B
Lo105rce
Lo 104rce
f(g)
-3.5
g
11
In TPA, we also see good correlation between
Prad and Panalytic
Prad
Panalytic We2g2sin4a
12
Poynting Jet Prad asymptotes to constant level
at late times as increase in g is compensated by
decrease in a and B
Prad
Prad
10By
x
x
x
Lo120c/We
Lo105c/We
po10
13
Asymptotic Prad scales as (We/wpe)n with n
2 - 3
Prad
Prad
x
x
x
We/wpe102
103
104
14
Inverse Compton scattering against ambient
photons can slow or stop acceleration
(Sugiyama et al 2005)
ngne
ng10-4ne
ng10-2ne
1 eV BB ambient photon field We/wpe100 jet
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• Next we study radiation from Collisionless Shocks
• 3 Examples
• ee-/ee- Magnetized Shock (B2 bulk KE)
• ee- /e-ion Magnetized Shock (B2 bulk KE)
• ee- Nonmagnetized Shock (B2 ltlt bulk KE)

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Magnetic Shock of ee- sweeping up cold
ambient ee- shows broad (gtgt c/We, c/wpe)
transition region with 3-phases (nej40no)
By100
ejecta
px
ambient
x
x
f(g)
decelerated ejecta spectral evolution
swept-up ambient spectral evolution
g
g
17
Both ejecta and swept-up electrons are highly
anisotropic pzltltpx
ejecta
pz
swept-up
px
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Prad of swept-up electron is lower than Prad of
decelerating ejecta electron. The radiative layer
is very thin
Prad
x
x
Swept-up ambient e-
ejecta e-
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Magnetic shock in ee-/e-ion plasma is very
complex with 5 phases and broad transition
region(gtgt c/Wi, c/wpe). Swept-up electrons
are accelerated by ponderomotive force. Swept-up
ions are accelerated by charge separation
electric fields.
100pxi
100By
Prad
100Ex
f(g)
ejecta e
-10pxe
-10pxej
ambient ion
ejecta e-
ambient e-
g
x
20
Prad of shocked swept-up electrons is comparable
to the ee-/ee- case. But the radiation layer is
much thicker
Prad
x
x
ejecta e-
swept-up e-
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Comparison of collisionless shocks ee- shocking
B0 ee- cold plasma ejecta hi-B, hi-g
weak-B, moderate g B0, low g
100By
ejecta px
100By
100By
100Ex
swept-up px
100Ex
-px swept-up
-pxswrpt-up
-3 power-law
no power-law
-3 power-law
swept-up
f(g)
ejecta
f(g)
f(g)
ejecta
swept-up
swept-up
g
g
g
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Nonmagnetic ee-/ee- shock Radiation
not Dominated By Weibel turbulence
Ex2
swept-up px
ejecta -px
By2
nejecta
ejecta
nswept-up
energy evolution
ejecta energy
swept-up energy
Prad swept-up
Ex energy
Prad ejecta
By energy
x
x
time
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• SUMMARY
• Radiation power of Poynting jets are many orders
  of magnitude below classical synchrotron formula
  due to anisotropy of acceleration (Forces
  parallel to velocity are most efficient in
  acceleration).
• Structure and radiation power of collisionless
  shocks are highly sensitive to ejecta (upstream)
  B field strength and Lorentz factor.
• 3. Radiation power of collisionless shocks are
  also much lower than classical synchrotron
  formula for a given ejecta magnetic field and
  Lorentz factor. The highest energy particles are
  accelerated mainly by (electrostatic or
  ponderomotive) forces parallel to the velocity.
• 4. Swept-up electrons are accelerated by
  ponderomotive and/or electrostatic forces.
  Swept-up ions are accelerated by electrostatic
  force caused by charge separation.