Radiation from Poynting Jets and Collisionless Shocks

Edison Liang, Koichi NoguchiShinya Sugiyama, Rice University

Acknowledgements: Scott Wilks, Bruce Langdon

Bruce Remington

Talk given at AAS Seattle Meeting January 2007

Internal shocksHydrodynamic Outflow

Poynting fluxElectro-magnetic-dominated outflow

Two Paradigms for the Energetics of Relativistic Jets in Blazars & GRBs

e+e-ions

e+e-

What is primary energy source?How are the e+e- accelerated? How do they radiate?

shock-raysSSC, EC… -rays

B

Synchrotron Radiationis usually assumed in

All paradigms

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.

From PIC runs, we compute the radiation power directly from the force terms

Prad = 2e2(F|| 2+ 2F+2) /3cm3

where F|| is force along vand F+ is force orthogonal to v

(algorithm is carefully calibrated against analyticformula using monoenergetic electrons radiating

in a uniform static B-field)

For Poynting flux acceleration, we find that Prad ~ e

22sin4<< Psyn ~e22

where is angle between v and Poynting vector k.Also critical frequencycr ~ e2sin2crsyn

This resultRadiation efficiency << 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.

QuickTime™ and a decompressor

are needed to see this picture.

B

e+e-Poynting jet running into cool e-ion ambient plasma

(movie by Noguchi)

By

Ez

Jz

PlasmaJxB force snowplows all surface particles upstream:<> ~ max(B2/4nmec2, ao)“Leading PonderomotiveAccelerator” (LPA)Plasma

JxB force pulls out surface particles. Loaded EM pulse (speed < c) stays in-phase with the fastest particles, but gets “lighter” as slower particles fall behind. It accelerates indefinitely over time: <> >> B2 /4nmec2, ao “Trailing Ponderomotive Accelerator” (TPA).(Liang et al. PRL 90, 085001, 2003)

Entering

Exiting

Poynting Flux: Particle accelerationby induced j x

B(ponderomotive) force

x

x

EM pulse

By

x

y

z

Ez

Jz JxB

k

Prad

Panalytic =e22sin4x

px

pz

ByPrad

Electrons snowplowed by Poynting flux radiates at a level ~ 10-4

of synchrotron formula using local B and . This is due to

sin ~ pz/px < 0.1.

Momentum gets more and moreAnisotropic with time: pz/px<<1

Details of TPA expansion

The power-law index seems remarkably robust independent of initial plasma size or temperature

and only weakly dependent on B

f()

-3.5

Lo=105rce

Lo= 104rce

QuickTime™ and aGraphics decompressor

are needed to see this picture.

Prad

Panalytic =e22sin4

In TPA, we also see good correlation between Prad and Panalytic

Poynting Jet Prad asymptotes to ~ constant level at late times

as increase in is compensated by decrease in and B

Lo=120c/e Lo=105c/e

po=10

Prad

10*By

Prad

x x x

Asymptotic Prad scales as (e/pe)n with n ~ 2 - 3

e/pe=102 103 104

x x x

PradPrad

Inverse Compton scattering against ambient photons can slow or stop acceleration (Sugiyama et al 2005)

n=10-4ne n=10-2ne n=ne

1 eV BB ambient photon field epe=100 jet

Next we study radiation from Collisionless Shocks

3 Examples:

1.e+e-/e+e- Magnetized Shock (B2 ~ bulk KE)

2.e+e- /e-ion Magnetized Shock (B2 ~ bulk KE)

3.e+e- Nonmagnetized Shock (B2 << bulk KE)

px

By*100

f()

Magnetic Shock of e+e- sweeping up cold ambient e+e- shows broad (>> c/e, c/pe) transition region with 3-phases (nej=40no)

ejecta

ambient

deceleratedejectaspectralevolution

swept-upambientspectral evolution

xx

pz

px

Both ejecta and swept-up electrons are highly anisotropic: pz<<px

ejecta

swept-up

ejecta e- Swept-up ambient e-

Prad of swept-up electron is lower than Prad of decelerating ejecta electron.The radiative layer is very thin

Prad

xx

ejecta e+

ejecta e-ambient ion

ambient e-

f()-10pxe-

10pxej

100pxi

100Ex

100By

Prad

Magnetic shock in e+e-/e-ion plasma is very complex with 5 phases and broad transition region(>> c/i, c/pe). Swept-up electrons areaccelerated by ponderomotive force. Swept-up ions are accelerated

by charge separation electric fields.

x

ejecta e- swept-up e-

Prad of shocked swept-up electrons is comparable to the e+e-/e+e- case.But the radiation layer is much thicker

xx

Prad

Comparison of collisionless shocks: e+e- shocking B=0 e+e- cold plasma ejecta: hi-B, hi- weak-B, moderate B=0, low

swept-up

swept-up

swept-up

100By

ejecta px

swept-up px

100By

100Ex

100By100Ex

-px swept-up

-pxswrpt-up

ejecta ejecta

no power-law~ -3 power-law~ -3 power-law

f() f() f()

ejecta

Nonmagnetic

e+e-/e+e-shock:

Radiation not

DominatedBy Weibelturbulence

ejecta -px

swept-up px

nswept-up

Ex2

nejectaBy2

time

ejecta energyswept-up energy

Ex energy

By energy

energy evolution

Prad swept-up

Prad ejecta

x x

SUMMARY

1. 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).

2. 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.