Radiation from Poynting Jets and Collisionless Shocks

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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 Glast Symposium, Feb 2007 (see
http//spacibm.rice.edu/liang/picsim and
spacibm.rice.edu/knoguchi)
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Popular Paradigms for the radiation of
relativistic outflows in GRBs Blazars
B
PIC sims can address difficult microphysics
G gtgt1
ee- ions
ee-
shock
g-rays SSC, EC
g-rays
What is energy source? How are the ee/ion
accelerated? How do they radiate?
Internal shocks Hydrodynamic Outflow
Poynting flux Electro-magnetic -dominated
outflow
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Highlight We have developed a
Particle-In-Cell code that simultaneously
computes total radiation output from each
superparticle. We find that in-situ radiation
output of highest energy electrons accelerated by
Poynting Flux (and some Collisionless Shocks) are
much below that predicted by the classical
synchrotron formula. This may solve the
problem of too rapid synchrotron cooling in many
internal shock models of GRBs.
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Question How do particles radiate while they
are being accelerated to high energies? We
compute the power radiated simultaneously from
the force terms used in the particle movers of
the PIC code Prad 2e2(F 2
g2F2) /3m3c where F is force along v and F
is force orthogonal to v (we have carefully
calibrated our procedure against analytic
results)
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In Poynting flux acceleration, most
energetic particles comoving with local EM
field Prad We2g2sin4a ltlt Psyn We2g2
where a is angle between v and Poynting vector k.

By
p
a
k
a ltlt 1 in the limit Ez By and g gwave
Ez
critical frequency wcr Weg2sin2a ltlt
wcrsyn Weg2
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Two different kinds of Poynting Flux Acceleration
via induced j x B(ponderomotive) force
EM pulse
Entering
By
Plasma
JxB force pushes all surface particles
upstream ltggt max(B2/4pnmec2, ao) Leading
Poynting 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 Poynting
Accelerator(TPA). (Liang et al. PRL 90, 085001,
2003)
x
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Electrons accelerated by LPA radiate at a level
10- 4 of classical synchrotron formula, due to
sina pz/px 0.1
g2We2105
Prad
px
By
Prad
pz
x
Panalytic We2g2sin4a
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Evolution of ee- plasma accelerated by Poynting
flux (LPA) shows decline of radiative power
output Prad despite increase of g
Prad
By
px
50
x
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Relativistic Poynting Flux Acceleration via
induced j x B(ponderomotive) force
EM pulse
Entering
By
Plasma
JxB force pushes 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
10
t.We800
t.We10000
TPA Occurs whenever EM- dominated plasma is
rapidly unconfined (Liang Nishimura PRL
91, 175005 2004)
magnify
We/wpe 10
11
tWe1000
hard-to-soft GRB spectral evolution
5000
10000
diverse and complex BATSE light curves
18000
Fourier peak wavelength scales as c.gm/ wpe
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TPA produces Power-Law spectra with low-energy
cut-off. Peak(bulk) Lorentz factor gm corresponds
roughly to the profile/group velocity of the EM
pulse
Typical GRB spectrum
gm
b(n1)/2
the maximum gmax e E(t)bzdt /mc where E(t)
is the comoving electric field
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The power-law index (p 3 - 4) is remarkably
robust independent of initial plasma size or
temperature and only weakly dependent on B
Lo105rce
Photon Index b(p1)/2 2 -2.5
Lo 104rce
f(g)
-3.5
g
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Prad from TPA ltlt Psyn ( g2We2)
g2We23x106
Prad
By100
px
300
x
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In TPA, we also find Prad Panalytic for
the highest energy particles
Prad
Panalytic We2g2sin4a
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In TPA jets, Prad asymptotes to constant
level at late times as increase in g is
compensated by decrease in a and B
Prad
Prad
x
x
Lo105c/We
Lo120c/We
po10
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Inverse Compton scattering against ambient
photons can slow or stop PF acceleration
(Sugiyama et al 2005)
ngne
ng10-4ne
ng10-2ne
1 eV photon field We/wpe100
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• We have studied radiation from Collisionless
  Shocks
• 3 Examples
• ee-/ee- Magnetic Shock (B2 bulk KE)
• ee- /e-ion Magnetic Shock (B2 bulk KE)
• ee- Nonmagnetic Shocks (B2 ltlt bulk KE)

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Poynting jet running into cool e-ion ambient
plasma
B
(movie by Noguchi)
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Magnetized collisionless shock produced by
collision of ee- Poynting Jet with cold e-ion
plasma .
100pxi
100By
100Ex
f(g)
ejecta e
-10pxe
-10pxej
ambient ion
ejecta e-
radiative shock layer
ambient e-
g
x
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The radiative shock layer gets thicker and
bifurcates with time due to ion drag, but max
Prad stays constant
Prad
x
swept-up e- radiation snapshots
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• SUMMARY
• Radiation power of Poynting Flux accelerated
  electrons are orders of magnitude below classical
  synchrotron formula due to Force parallel to
  velocity. This result may be generic and also
  applies to
• some Collisionless Shocks.
• 2. Structure and radiation power of collisionless
  shocks are highly sensitive to magnetization and
  ion loading. Shocked radiative layer
• in e-ion shocks is much thicker and bifurcates.
• 3. Inverse Compton of external photons may
  dominate synchrotron and SSC.
• 4. Critical frequency of PF acceleration
  radiation is much lower than the classical
  synchrotron critical frequency.