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Hot Electromagnetic Outflows and Prompt GRB Emission. Chris Thompson CITA, University of Toronto. Venice - June 2006. ApJ v. 647; astro-ph/0507387. OUTLINE: Constraints on B-field dissipation at large radius from dynamo mechanism operating in the engine - PowerPoint PPT Presentation
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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