[PPT] - AFTERGLOW PHYSICS Alin Panaitescu Los Alamos National Laboratory PowerPoint Presentation

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AFTERGLOW PHYSICS Alin Panaitescu

Los Alamos National Laboratory

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Relativistic blast-wave model for GRB afterglows

Rel. outflow Γ0~100-500

Interaction with CBM

Reverse shock into ejecta (Sari&Piran 99, Meszaros&Rees 99, Kobayashi 00) Forward shock into ambient medium (Rhoads&Paczynski 93, Meszaros & Rees 97) RS FS

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Γα R-1/2, wind CBM nαR−

2

R-3/2, homogeneous CBM

+ Fν α ν−β Fνα t−

α(β;s,e,...)

Afterglow radiation mechanism: synchrotron (inverse-Compton much less) Even if EB ~ Eγ initially, B is too weak at 1017 cm Origin of magnetic field and relativistic electrons:

1. magnetic dissipation in Poynting outflow (Rees & Meszaros 97, Lyutikov & Blandford 2000)
2. dissipation of ejecta energy by interaction with CBM (Rees & Meszaros 1994, 1997)

+ plasma instability (e.g. Weibel – Medvedev 2001) or Fermi acceleration (electrons)

Afterglow light-curve depends on

1. dynamics of shocked gas (Ne, γe for FS; B, Lorentz boost)
2. distribution with energy of radiating electrons (sets synchrotron spectrum)

& 3. distribution of incoming ejecta (sets Ne, γe for RS)

{

Power-law spectrum Power-law light-curve

{

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Power-law decay indices Fν α t−α for RS and FS light-curves

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Multi-wavelength afterglow observations

RADIO OPTICAL X-RAY

ls - ls -l

ATCA VLA OVRO MDM VLT HST BSAX CXO SWIFT

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Parameters of forward-shock emission up to 4 constraints (νa,νp,νc,Fp) 4 parameters blast-wave kinetic energy E~1053 erg/sr medium density n~0.1-1 cm-3 micro parameters εB~10-3

& εe~10-2

970805-Wijers & Galama 98

030329

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Collimated outflow

if Γ> θ-1 (spherical) → α= 1.5β+(-.5,0,.5)

if Γ< θ−1 → α= 2β, 2β+1

Flux dimming is faster after Γ = θ−1 because

1. lack of emitting fluid at angle > θ

∆αcoll= 1/2(wind), 3/4 (homogeneous)

2. jet lateral spreading :faster deceleration

Γα t-1/4(-3/8) → Γα t-1/2 , ∆αspread<1/2 θ 1/Γ O B S

emitting surface Kulkarni et al 99: GRB 90123

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Optical light-curve breaks in pre-Swift afterglows

Zeh, Klose & Kann 06

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Jet dynamics -Γ(r),θ(r) - and emission - Fν(t) - calculated numerically + data fit determine jet parameters and medium

comparable fits – Jet model

homog. better fit than wind – Jet model
homog. better fit than wind – SO model

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Best fit parameters for uniform jets from fits to multiwavelength afterglow data

Results: - high GRB efficiency (10-80%)

narrow jets (2-3 deg)
initial jet kinetic energy comparable with that of SNe
wind density parameter consistent with Galactic WRs
non-universal microphysical parameters

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Numerical modeling of broadband emission of 10 GRB afterglows: Jet/Struct.outflow model – uniform CBM fits better than wind: 7-1(2)/6-1(3) → why is ambient medium homogeneous if progenitor is Wolf-Rayet star ?

Chevalier, Li & Fransson 04

1. termination shock of free WR wind

with radius smaller than Raglow = 0.4 (E53tday/no)1/4pc

2. peculiar motion (~vshock~50 km/s)
f WR star → smaller Rshock
3. faster & tenuous wind (expelled in

the last < 1000 yrs before core-collapse)

interacting with WR wind

Rshock

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Swift X-ray afterglows: three phases

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Jet-breaks in X-ray light-curves (Swift)

1/3 of Swift X-ray afterglows display breaks

0.5 < αx < 1.5 to 1.5 < αx < 2.5 at 0.5-10 d

1/3 may also have a break at 1-10 d 1/3 do not have a break until > 10 d while ~75% of pre-Swift optical afterglows display a break at 0.3-3 d

Reason: Swift “sees” dimmer afterglows

from wider jets, whose lc breaks occur later

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red = light-curves with breaks purple = lcs without breaks until ~10 d

Flux & jet-break time dep on θj

if jet energy were universal

Fν α dE/dΩ α θj

2

tbreak α (dE/dΩ)θj

4 α θj 2

Fν α 1/tbreak means that

afterglows with earlier jet-breaks

are brighter b

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Jet-breaks = achromatic (late and followed by steep decay)

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X-ray plateaus (100-600s → 1-10 ks)

no spectral evolution at plateau end

βx1=βx2 → no spectral break crosses X-ray

Plateaus require a departure from assumptions of standard blast-wave model:

1. constant kinetic energy

(but variable before deceleration or if ejecta are anisotropic)

2. constant micro-physical parameters

(contrived)

plateau

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1. energy injection (Nousek et al 06, Zhang et al 06, AP et al 06)

Plateaus from increasing average dE/dΩ over visible 1/Γ area

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2. structured outflow (e.g. Eichler & Granot 06)

Plateaus from increasing average dE/dΩ over visible 1/Γ area

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d(dE/dΩ)/dt>0 model

decoupled optical & X-ray light-curves

cannot be explained by energy injection alone because EI alters dynamics of forward-shock, hence resulting light-curve features should be achromatic

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Possible reasons for X-ray and optical decoupled light-curves

1. X-ray = reprocessed synchrotron forward-shock emission

scattered in another part of rel. outflow = bulk-scattering

(to be continued)

2. X-ray (& optical ?) emission is (are) from
2a. a long-lived reverse shock (Uhm & Beloborodov 07 )
2b. long-lived internal shocks (Ghisellini et al 07)

Note: All require long-lived engine, producing rel. outflow for tsource~ taglow >106 s

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rel. boost of specific flux and photon energy by γ & Γ/γ

Bulk-scattering - relativistic effects

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Unifying forward-shock model for X-ray plateaus Plateaus require existence of an outflow behind forward-shock (FS)

either for energy injection or for scattering Scattering negligible rel to FS Scattering dominant rel to FS achromatic light-curve breaks chromatic light-curve breaks O=Synchrotron, X=Sy O=Sy, X=inverse-Compton

O&X decays well correlated O&X decays correlated O&X decays decoupled

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Early optical emission from reverse shock (RS)

excess emission from RS during early afterglow

sy-FS

Ejecta energized by RS, followed by adiabatic cooling: Fνα ν−

β t− α(β) with α= (0.67/0.80) + (1.19/1.47)β

α= 2.5 for β=1.5 (homogeneous CBM) or β=1.2 (wind) (but optical β at 100-1000s not known for 990123 & 021211)

first optical flash (1999)

AP & Meszaros (1998)

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Measurements of optical spectral slope β during early afterglow enable test of RS expectation : α = 3/4 + 4β/3

α2=0.76 β2=0.50? α1=1.96 β1=0.89 ? α2=1.23(.02) β2=0.0+0.2*log(t/1ks)

080319B: Wozniak et al 08, Bloom et al 08

hardening hardening softening

061126: Perley et al 07

α1=2.53 β1=0.63

α1& β1 consistent with RS model

early decay too fast for RS model Note: fast-decaying early emission is softer than slower-decaying late emission, indicating different origins (early=RS, late=FS)

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CONCLUSIONS

1. Confirmed predictions of relativistic blast-wave model (RS or FS)
a. power-law afterglow spectra Fνα ν−

β (seen in optical and X-ray)

b. power-law flux decay Fνα t−

α(β) (seen in radio, optical and X-ray)

c. optical flashes from RS (very rare, RS emission present until 1ks, afterwards FS)
d. light-curve jet breaks (achromatic breaks, very few cases)
2. Early (<10 ks) X-ray LC breaks at end of plateau due to
a. achromatic breaks: long-lived injection of energy into FS
b. chromatic X-ray LC breaks:

“external” scattering in outflow inner to FS or

“central engine” (e.g. internal shocks) emission dominant over FS