Radiation from Poynting Jets and Collisionless Shocks Edison Liang, Koichi Noguchi

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 Glast Symposium, Feb 2007 (see http://spacibm.rice.edu/~liang/picsim and spacibm.rice.edu/~knoguchi)

2. Popular Paradigms for the radiation of relativistic outflows in GRBs & Blazars B PIC sims can address difficult microphysics G >>1 e+e- ions e+e- shock g-rays SSC, EC… g-rays What is energy source? How are the e+e/ion accelerated? How do they radiate? Internal shocks Hydrodynamic Outflow Poynting flux Electro-magnetic -dominated outflow

3. 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 muchbelow that predicted by the classical synchrotron formula. This may solve the problem of too rapid synchrotron cooling in many internal shock models of GRBs.

4. 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+ g2F+2) /3m3c where F|| is force along v and F+ is force orthogonal to v (we have carefully calibrated our procedure against analytic results)

5. In Poynting flux acceleration, most energetic particles ~ comoving with local EM field Prad ~ We2g2sin4a << Psyn ~We2g2 where a is angle between v and Poynting vector k. By p a k a << 1 in the limit Ez ~ By and g ~ gwave Ez critical frequency wcr ~ Weg2sin2a << wcrsyn~Weg2

6. Two different kinds of Poynting Flux Acceleration via induced j x B(ponderomotive) force y By Ez JxB Jz x z k EM pulse Entering By Plasma JxB force pushes all surface particles upstream: <g> ~ max(B2/4pnmec2, ao) “Leading Poynting Accelerator” (LPA) Ez Jz x Exiting 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: <g> >> B2/4pnmec2, ao“Trailing Poynting Accelerator”(TPA). (Liang et al. PRL 90, 085001, 2003) x

7. Electrons accelerated by LPA radiate at a level ~ 10- 4 of classical synchrotron formula, due to sina ~ pz/px ≤ 0.1 g2We2~105 Prad px By Prad pz x Panalytic ~We2g2sin4a

8. Evolution of e+e- plasma accelerated by Poynting flux (LPA) shows decline of radiative power output Prad despite increase of g Prad By px 50 x

9. Relativistic Poynting Flux Acceleration via induced j x B(ponderomotive) force y By Ez JxB Jz x z k EM pulse Entering By Plasma JxB force pushes all surface particles upstream: <g> ~ max(B2/4pnmec2, ao) “Leading Ponderomotive Accelerator” (LPA) Ez Jz x Exiting 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: <g> >> B2/4pnmec2, ao“Trailing Ponderomotive Accelerator” (TPA). (Liang et al. PRL 90, 085001, 2003) x

10. t.We=800 t.We=10000 TPA Occurs whenever EM- dominated plasma is rapidly unconfined (Liang & Nishimura PRL 91, 175005 2004) magnify We/wpe =10

11. tWe=1000 hard-to-soft GRB spectral evolution 5000 10000 diverse and complex BATSE light curves 18000 Fourier peak wavelength scales as ~ c.gm/ wpe

12. 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=(n+1)/2 the maximum gmax ~ eE(t)bzdt /mc where E(t) is the comoving electric field

13. The power-law index (p ~ 3 - 4) is remarkably robust independent of initial plasma size or temperature and only weakly dependent on B Lo=105rce Photon Index b=(p+1)/2 ~ 2 -2.5 Lo= 104rce f(g) -3.5 g

14. Prad from TPA << Psyn (~ g2We2) g2We2~3x106 Prad By*100 px 300 x

15. In TPA, we also find Prad ~ Panalytic for the highest energy particles Prad Panalytic ~We2g2sin4a

16. 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 Lo=105c/We Lo=120c/We po=10

17. Inverse Compton scattering against ambient photons can slow or stop PF acceleration (Sugiyama et al 2005) ng=ne ng=10-4ne ng=10-2ne 1 eV photon field We/wpe=100

18. We have studied radiation from Collisionless Shocks 3 Examples: e+e-/e+e- Magnetic Shock (B2 ~ bulk KE) e+e- /e-ion Magnetic Shock (B2 ~ bulk KE) e+e- Nonmagnetic Shocks (B2 << bulk KE)

19. Poynting jet running into cool e-ion ambient plasma B (movie by Noguchi)

20. Magnetized collisionless shock produced by collision of e+e- 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

21. 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

22. 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.