Hot Electromagnetic Outflows and Prompt GRB Emission

1. Hot Electromagnetic Outflowsand Prompt GRB Emission Chris Thompson CITA, University of Toronto Venice - June 2006

2. ApJ v. 647; astro-ph/0507387 OUTLINE: • Constraints on B-field dissipation at large radius from dynamo mechanism operating in the engine 2. Hot electromagnetic outflows: acceleration and spectral regulation 3. Deceleration: effect of pair-loading of the ambient medium and of the `breakout shell’ • MHD/electron turbulence: anisotropy, electrostatic heating, and cooling 5. Beamed inverse-Compton emission and Distributed heating

3. Puzzles • Variability: why relatively constant within each burst, in spite of strong burst-to-burst differences? • What are key components of the inner outflow needed to produce prompt GRB emission? Choose from … I. Baryons; II. Thermal radiation; III. Magnetic Field (Answer: II. and III.) • Spectrum: why Epeak - Eiso correlation(s)? why low-energy spectrum often harder than F ~ 4/3 (synchrotron emission)? • Is the same radiative mechanism shared by long, short GRBs (+ magnetar flares)?

4. Main Constituents of Outflow I. Non-radial magnetic field (Poynting-dominated jet from BH horizon/ergosphere; millisecond magnetar) Dynamo in BH torus / magnetar  Sign of Bpoloidal varies stochastically tdyn~ 10-3 s << tdynamo << tGRB ~ 10 s

5. Constraintson the Dissipation of a Non-radial B-field 1. Flux conservation: (Compression enhanced by conversion of toroidal to radial field: Thompson 1994; Lyubarsky & Kirk 2001) 2. Strong compression at reverse `shock’:  3. Causality:

6. Thermalization in a relativistically-moving fluid II. Nearly black-body radiation field Long Bursts: Strong internal shocks / KH instabilities out to R ~ RWolf-Rayet ~ 21010 cm Rapid thermalization by multiple e-scattering if

7. Regulation of Gamma-Ray Spectral Peakby Prompt Thermalization Jet emission opening angle  Total energy constrained by afterglow observations: (Frail et al. 2001) Causal contact across jet axis:

8. hpeak - Eisotopic Relation LONG (Type I) GRBs Epk ~ Eiso1/4 (Blackbody emission from a fixed radius) GRB 980425 / SN 1998bw GRB 031203 / SN 2003lw Epk ~ Eiso1/2 Epk ~ Eiso1/4 (OBSERVED) Amati et al. 2002 Lamb et al. 2004

9. Radiative Acceleration Photon field collimates  ~ r (outside Wolf-Rayet photosphere) Limiting Lorentz factor:  Reverse Shock is mildly relativistic

10. 2. Radiative Acceleration B2/8 > c2 Poynting flux Momentum flux Change in S, P at fixed Bvr : if Can be neglected compared with (c.f. Drenkhahn & Spruit: acceleration by dP/dr)

11. Pre-acceleration Thompson & Madau 2000 Beloborodov 2002 • Gamma rays side scatter off ambient electrons  +   e+ e-, exponentiation of pair density Compactness of radiation Streaming ahead of (forward) shock ____ • Strong radiation force on pair-loaded medium - • relativistic motion inside ~ 1016 cm of engine • relevant for deceleration in Wolf-Rayet wind (long GRBs)

12. Bulk relativistic motion: (Beloborodov 2002)

13. Deceleration of the Contact Wolf-Rayet Wind Mass-loss rate: Velocity: Magnetized relativistic outflow, luminosity

14. Equilibrium Lorentz factor of the contact discontinuity No pre-acceleration: Pre-acceleration to :

15. Deceleration begins (ambient medium is slower than contact): Deceleration ends (reverse shock passes through ejecta shell): Compactness (in frame of contact):

16. Breakout Shell Mass limited by sideways spreading: Faster deceleration of Relativistic ejecta:

17. Damping of Alfvenic Turbulence:Compton effects 1. Bulk compton drag: compactness in photons and magnetic field Magnetization parameter: 2. Torsional wave-dominated cascade: Anisotropic cascade (Goldreich & Sridhar) (anisotropic forcing at outer scale, e.g. Cho)

18. Alfven modes (ions and electrons coupled): Alfven wave slows down when Electron-Supported Modes (R and L-handed): + Strong Shear:

19. Electrostatic heating of e+ e- Critically balanced cascade: Wave displacement Strong longitudinal excitation of electrons/positrons: at cold ions

20. Dilute plasmas (e.g. magnetosphere of PSR 0737-3039B) Charge Starvation: EXAMPLE: Black Hole Corona Critically- balanced cascade:

21. Compton Heating/Coolingvs Synchrotron Emission Perpendicular temperature is excited by multiple Compton scatterings: Single scattering: Relative emissivities: Continuous Heating Flashing Heating + Passive Cooling

22. EXAMPLE: photon spectrum Continuous Heating Flash Heating + Passive Cooling

23. Beaming ofinverse-Compton photons Observed! (Synchrotron: normalization is too small)

24. Quasi-thermal Comptonization in `Patchy’ Jet (Thompson 1996; c.f. Giannios 2006) Homogeneous heating of soft seed photons (Kompane’ets): Discrete hotspots

25. Problems: • t ~ E-1/2 ; Lag of Soft Photons  Power law e-/e+ energy distribution; and/or variable  due to gradual pair loading • Low energy photon index no harder than ~ -1 • (if seed photons soft) • Seed photons not adiabatically cooled (Ghisellini & Celotti 1999; Stern & Poutanen 2005) Alternative: Synchrotron Self-Compton Emission • Why strong Epk - Eiso correlation?

26. Distributed Heating andContinuous Pair Creation Pair density builds up linearly with time: Assume: then Continuous balance between heating/cooling Flash heating followed by cooling

27. 0. Two inevitable (sufficient) ingredients of GRB outflow: non-radial magnetic field + thermal seed photons 1. GRB emission mechanism is intrinsically anisotropic because: electrostatic acceleration of e+/e- Rayleigh-Taylor instability of breakout shell  angular variations in  2. Non-thermal emission can then be triggered by deceleration off W-R wind and breakout shell 3. Observed spectrum is then a convolution of a thermal seed spectrum, but with strong angular `bias’ 4. Distributed heating of e+/e- allows soft-hard lags and non-thermal X-ray spectra even without non-thermal particle spectra [smooth bursts!]