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 and spacibm.rice.edu/~knoguchi)

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

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.

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: P rad = 2e 2 (F || 2 +  2 F + 2 ) /3m 3 c where F || is force along v and F + is force orthogonal to v (we have carefully calibrated our procedure against analytic results)

 p ByBy EzEz k In Poynting flux acceleration, most energetic particles ~ comoving with local EM field P rad ~  e 2  2 sin 4  < P syn ~  e 2  2 where  is angle between v and Poynting vector k. critical frequency  cr ~  e  2 sin 2  crsyn ~  e  2  << 1 in the limit E z ~ B y and  ~  wave

By Ez Jz Plasma JxB force pushes all surface particles upstream: ~ max(B 2 /4  nm e c 2, a o ) “Leading Poynting Accelerator” (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: >> B 2 /4  nm e c 2, a o “Trailing Poynting Accelerator”(TPA). (Liang et al. PRL 90, , 2003) Entering Exiting Two different kinds of Poynting Flux Acceleration via induced j x B(ponderomotive) force x x EM pulse By x y z Ez Jz JxB k

P rad P analytic ~  e 2  2 sin 4  x pxpx pzpz ByBy P rad Electrons accelerated by LPA radiate at a level ~ of classical synchrotron formula, due to sin  ~ p z /p x ≤ 0.1  2  e 2 ~10 5

50 pxpx P rad ByBy Evolution of e+e- plasma accelerated by Poynting flux (LPA) shows decline of radiative power output P rad despite increase of  x

By Ez Jz Plasma JxB force pushes all surface particles upstream: ~ max(B 2 /4  nm e c 2, a o ) “Leading Ponderomotive Accelerator” (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: >> B 2 /4  nm e c 2, a o “Trailing Ponderomotive Accelerator” (TPA). (Liang et al. PRL 90, , 2003) Entering Exiting Relativistic Poynting Flux Acceleration via induced j x B(ponderomotive) force x x EM pulse By x y z Ez Jz JxB k

t.  e =800 t.  e =10000 magnify  e /  pe =10 TPA Occurs whenever EM- dominated plasma is rapidly unconfined (Liang & Nishimura PRL 91, )

t  e = Fourier peak wavelength scales as ~ c.  m /  pe hard-to-soft GRB spectral evolution diverse and complex BATSE light curves

TPA produces Power-Law spectra with low-energy cut-off. Peak(bulk) Lorentz factor  m corresponds roughly to the profile/group velocity of the EM pulse mm the maximum  max ~ e  E(t)  z dt /mc where E(t) is the comoving electric field Typical GRB spectrum  =(n+1)/2

The power-law index (p ~ 3 - 4) is remarkably robust independent of initial plasma size or temperature and only weakly dependent on B f(  )  -3.5 L o =10 5 r ce L o = 10 4 r ce Photon Index  =(p+1)/2 ~

x 300 pxpx By*100 P rad  2  e 2 ~3x10 6 P rad from TPA << P syn (~  2  e 2 )

P rad P analytic ~  e 2  2 sin 4  In TPA, we also find P rad ~ P analytic for the highest energy particles

In TPA jets, P rad asymptotes to ~ constant level at late times as increase in  is compensated by decrease in  and B L o =120c/  e L o =10 5 c/  e p o =10 P rad x x

Inverse Compton scattering against ambient photons can slow or stop PF acceleration (Sugiyama et al 2005) n  =10 -4 n e n  =10 -2 n e n  =n e 1 eV photon field  e  pe =100

We have studied radiation from Collisionless Shocks 3 Examples: 1.e+e-/e+e- Magnetic Shock (B 2 ~ bulk KE) 2.e+e- /e-ion Magnetic Shock (B 2 ~ bulk KE) 3.e+e- Nonmagnetic Shocks (B 2 << bulk KE)

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

ejecta e+ ejecta e- ambient ion ambient e-  f(  ) -10p xe -10p xej 100p xi 100E x 100B y Magnetized collisionless shock produced by collision of e+e- Poynting Jet with cold e-ion plasma. radiative shock layer x

swept-up e- radiation snapshots The radiative shock layer gets thicker and bifurcates with time due to ion drag, but max P rad stays ~ constant x P rad

SUMMARY 1.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.