Hot Electromagnetic Outflowsand 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/0507387

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)?

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

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 ~ RWolf-Rayet ~ 21010 cm

Rapid thermalization by multiple e- scattering

if

Thermalization in a relativistically-moving fluid

Regulation of Gamma-Ray Spectral Peakby Prompt Thermalization

Jet emission opening angle Total energy constrained by afterglow observations:

Causal contact across jet axis:

(Frail et al. 2001)

hpeak - Eisotopic Relation

Amati et al. 2002Lamb et al. 2004

Epk ~ Eiso1/4

Epk ~ Eiso1/4

(Blackbody emission

from a fixed radius)

Epk ~ Eiso1/2

(OBSERVED)

GRB 980425 / SN 1998bwGRB 031203 / 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 B2/8 > c2

Poynting flux

Momentum flux

Change in S, P at fixed Bvr :

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 2000Beloborodov 2002

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)

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 forcingat 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-balancedcascade:

EXAMPLE: Black Hole Corona

Dilute plasmas

(e.g. magnetosphere

of PSR 0737-3039B)

Compton Heating/Coolingvs 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 Epk - Eiso 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 andContinuous Pair Creation

Pair density builds up linearly with time:

Assume:

Continuous balancebetween 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 bydeceleration off W-R wind and breakout shell

0. Two inevitable (sufficient) ingredients of GRB outflow: non-radial magnetic field + thermal seed photons