Hot Electromagnetic Outflows and Prompt GRB Emission Chris Thompson CITA, University of Toronto Venice - June 2006

OUTLINE: 1.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’ 4.MHD/electron turbulence: anisotropy, electrostatic heating, and cooling 5. Beamed inverse-Compton emission and Distributed heating ApJ v. 647; astro-ph/

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 E peak - E iso 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)?

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 B poloidal varies stochastically t dyn ~ s << t dynamo << t GRB ~ 10 s

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

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

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

h peak - E isotopic Relation Amati et al Lamb et al E pk ~ E iso 1/4 (Blackbody emission from a fixed radius) E pk ~ E iso 1/2 (OBSERVED) GRB / SN 1998bw GRB / SN 2003lw LONG (Type I) GRBs

Radiative Acceleration 1.Photon field collimates  ~ r (outside Wolf-Rayet photosphere) Limiting Lorentz factor:  Reverse Shock is mildly relativistic

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

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

Bulk relativistic motion: (Beloborodov 2002)

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

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

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

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

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

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

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

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

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

EXAMPLE: photon spectrum Continuous Heating Flash Heating + Passive Cooling

Beaming of inverse-Compton photons Observed! (Synchrotron: normalization is too small)

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

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

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

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