Great Debate on GRB Composition:A Case for Poynting Flux

Dominated GRB Jets

Bing ZhangDepartment of Physics and Astronomy

University of Nevada, Las Vegas

March 6, 2011In “Prompt Activity of Gamma-Ray Bursts”

Raleigh, North Carolina

Reference: Zhang & Yan (2011, ApJ, 726, 90) Reference: Zhang & Yan (2011, ApJ, 726, 90)

Sherlock HolmesSherlock Holmes

GRB 080916C(Abdo et al. 2009, Science)

Fingerprint/Fingerprint/footprint/footprint/Smoking gun:Smoking gun:

Standard Fireball Shock Model

central photosphere internal shocks external shocksengine (reverse) (forward)

GRB prompt emission:GRB prompt emission: from internal shocks and photosphere from internal shocks and photosphereAfterglow:Afterglow: from external shocks from external shocks

Predicted spectra

Meszaros & Rees (00)Meszaros & Rees (00)

Daigne & Mochkovitch (02)Daigne & Mochkovitch (02)

Zhang & Meszaros (02, unpublished)Zhang & Meszaros (02, unpublished)

Expected photosphere emission from a fireball (Zhang & Pe’er 09)

Sigma: ratio between Poynting flux and Sigma: ratio between Poynting flux and baryonic flux:baryonic flux:

= L= LPP/L/Lbb: at least ~ 20, 15 for GRB 080916C: at least ~ 20, 15 for GRB 080916C

Confirmed by Confirmed by Fan (2010) with a Fan (2010) with a widerwiderparameter space parameter space study.study.

The simplest fireball model does not work!

Modified fireball models

Modified Fireball Model (1)

central photosphere internal shocks external shocksengine (reverse) (forward)

GRB prompt emission:GRB prompt emission: from internal shocks and photosphere from internal shocks and photosphereAfterglow:Afterglow: from external shocks from external shocks

Magnetic acceleration?

High initially, but quickly low ?– MHD model, magnetic pressure gradient accelerate ejecta,

Poynting flux can be (partially) converted to kinetic energy (Vlahakis & Konigl 2003; Komissarov et al. 2009; Tchekhovskoy, Narayan & McKinney 2010; Granot, Komissarov & Spitkovsky 2011; Lyubarsky 2010)

– The conversion efficiency is low– With external confinement (e.g. stellar envelope), the

efficiency can be higher, but the flow can still have a moderately high σ in the emission region.

Talks by Narayan, Tchekhovsky, McKinney, Giannios …

Difficulties/issues of the internal shock model

• Missing photosphere problem• Low efficiency• Fast cooling problem• Electron number excess problem• Ep – Eiso (Liso) correlation inconsistency

Modified Fireball Model (2)

central photosphere internal shocks external shocksengine (reverse) (forward)

GRB prompt emission:GRB prompt emission: from internal shocks and photosphere from internal shocks and photosphereAfterglow:Afterglow: from external shocks from external shocks

The band function is emission from the photosphere

• Dissipated photosphere with upscattering (Thompson 1994; Rees & Meszaros 2005; Ghisellini et al. 2007; Pe’er et al. 2006; Giannios 2008; Beloborodov 2010; Lazzati & Begelman 2010; Pe’er & Ryde 2010; Ioka 2010; Metzger et al. 2011)

• Two problems:– Cannot reach > 1 GeV– Low energy spectral index is

too hard

??

??

αα ~ -1 ~ -1

αα ~ (+0.4 - +1) ~ (+0.4 - +1)

Beloborodov (2010); Mizuta et al. (2010);Beloborodov (2010); Mizuta et al. (2010);Deng & Zhang (poster)Deng & Zhang (poster)

• Superposition (Toma et al. 2010; Li 2009)?– Contrived fine-tuning– Seems not supported by

data (Binbin’s talk)

??

??

The band function is emission from the photosphere

• Synchrotron + photosphere (Giannios 2008; Vurm’s talk; Beloborodov’s talk)?

• Predict bright optical emission

• Prompt optical data (Yost et al.) do not support this possibility (Shen & Zhang 09)

The band function is emission from the photosphere

Giannios (2008)Giannios (2008)

Sherlock HolmesSherlock Holmes

Preece’s talkPreece’s talk

Fingerprint/Fingerprint/footprint/footprint/Smoking gun:Smoking gun:

Lu, Hou & Liang 2010Lu, Hou & Liang 2010

Diverse Ejecta Composition:Thermal emission in GRB 090902B!

Ryde et al. (2010); B.-B. Zhang et al. (2011) - Bin-Ryde et al. (2010); B.-B. Zhang et al. (2011) - Bin-Bin’s talk Bin’s talk

This is a Paczynski-Goodman “fireball”!This is a Paczynski-Goodman “fireball”!

GRB 090902B (probably also 090510)

• At least GRB 090902B is a fireball (probably also 090510)

• Rare: 2 of 17 LAT GRBs (B.-B. Zhang’s talk)• Photosphere emission looks quasi-thermal.

Band function is not superposition of photosphere emission

• Photosphere model must address diversity of Comptonization

Back-up slides

Constrain Emission Site• It is very difficult to constrain the sub-MeV/MeV

emission radius R directly• There are three ways to constrain R

– The emission radius of X-ray steep decay phase can be estimated. If Rx= R, then R can be constrained

– The emission radius of prompt optical emission Ropt can be constrained by the self-absorption limit. If Ropt= R, then R can be constrained

– The emission radius of the GeV photons RGeV can be constrained by the pair-production limit. If RGeV = R, then R can be constrained

Method One: X-Rays

tttailtail

Kumar et al. 07Lyutikov, 06

R,X > > 101015 15 cmcm

If the steep-decay phase of the X-ray tail is defined by the high-latitude emission, one has:

GRB

tail

jj

RR

RRjj22//

22

Method Two: Optical

Vestrand et al. 2006a,bVestrand et al. 2006a,b

“Tracking” optical band detection constraints the self-absorption frequency and, hence, the emission radius

GRB 050820AGRB 050820A

R,opt > several 1014 cm

Shen & Zhang (08):Shen & Zhang (08):

GRB 080319B: naked-eye GRB(Racusin et al. 2008)

Lightcurve: optical roughly Lightcurve: optical roughly traces gamma-raystraces gamma-rays

Spectrum: two distinct Spectrum: two distinct spectral componentsspectral components

Syn + SSC model for GRB 080319B

(Racusin et al. 2008; Kumar & Panaitescu 2008)

Esyn~20 eVESSC

1st ~650 keVESSC

2st ~25 GeVE

E2

N(E

)

Klein-Nishina cut-off Y ~ 10

Y ~ 10

Y2 ~100

R,opt ~ 1016 cm

Method Three: GeVPair cutoff feature depends on both bulk Lorentz factor (Baring & Harding 1997; Lithwick & Sari 2001) and the unknown emission radius (Gupta & Zhang 2008)

10010020020040040060060080080010010000

Gupta & ZhangGupta & Zhang20082008

Radius constraints(Zhang & Pe’er 09)

Emission must come from a large radius far above the Emission must come from a large radius far above the photosphere.photosphere.

A Poyting-Flux-Dominated Flow:Kill Three Birds with One Stone

• Invoking a Poynting flux dominated flow can explain the lack of the three expected features– Non-detection of the pair cutoff feature is consistent

with a large energy dissipation radius– Non-detection of the SSC feature is naturally expected,

since in a Poynting flux dominated flow, the SSC power is expected to be much less that the synchrotron power

– Non-detection of the photosphere thermal component is consistent with the picture, since most energy can be retained in the form of Poynting flux energy rather than thermal energy

Counter-arguments: Hide the thermal component

Change R0?– R0 = c t ~ 3109 cm (based on the observed minimum

variability, and the collapsar scenario)– If R0 is smaller (106 cm - not observed, not favored for a

massive star progenitor), the thermal temperature is higher, may be hidden below the non-thermal component.

– But it does not work - a Poynting flux dominated flow is still needed to hide the thermal component (Fan 2010)

A Model in the High-σ Regime:The ICMART Model

(Internal Collision-induced MAgnetic Reconnection & Turbulence)(Zhang & Yan 2011, ApJ, 726, 90)

Basic Assumptions:

• The central engine launches a high-σ flow. The σ is still ~ (10-100) at R ~ 1015 cm.

• The central engine is intermittent, launching an outflow with variable Lorentz factors (less variable in σ).

(a) Initial collisions only distort magnetic fields

(b) Finally a collision triggers fast turbulent reconnection- An ICMART event (a broad pulse in GRB lightcurve)

ICMART ModelZhang & Yan (2010)Zhang & Yan (2010)

central enginecentral engineR ~ 10R ~ 1077 cm cm = = 00 >> 1 >> 1

photospherephotosphereR ~ 10R ~ 1011 11 - 10- 101212 cm cm 00

early collisionsearly collisionsR ~ 10R ~ 1013 13 - 10- 101414 cm cm ~ 1- 100~ 1- 100

ICMART regionICMART regionR ~ 10R ~ 1015 15 - 10- 101616 cm cminiini ~ 1- 100 ~ 1- 100 endend 1 1

External shockExternal shockR ~ 10R ~ 101717 cm cm 11

GRBGRB

Distance Scales in the ICMART Model

Emission suppressedEmission suppressed

At mostAt most1/(1+1/(1+σσ))energy releasedenergy released

At mostAt most1/(1+1/(1+σσ))energy releasedenergy released

11/(1+/(1+σσendend))energy releasedenergy released

GRB ejecta is turbulent in nature

Reynold’s number:

Magnetic Reynold’s number:

Magnetic fields can be highly distorted and turbulent if turbulent condition is satisfied

€

Re ≡LδV

ν~ 1028 >>1

€

Rm,Bohm ≡LδV

η~ 3.4 ×1012 >>1

λλ LL

Turbulent Reconnection is needed to power GRBs

In order to reach GRB luminosity, the effective global reconnection rate has to be close to c .

Relativistic Sweet-Parker reconnection speed is << c (Lyubarsky 2005).

Turbulent reconnection (Lazarian & Vishniac 1999) can increase reconnection speed by a factor L/λ .

€

Vrec ,global' =

Δ'

Δt'~LγLw

1+σ

σc ~ c

€

Vrec ,local' =VAs

−1/ 2 << c

€

s ≡λVAη

>>1

€

Vrec ,global' =Vrec ,local

' (L

λ)

€

2 B' 2

8π4πR2 Δ'

Δt '~ Lγ

Multiple collisions can distort field lines and eventually trigger turbulence in a high-σ flow

Required condition from the observations (reach GRB luminosity):Required condition from the observations (reach GRB luminosity):

Condition for relativistic turbulence (I): relativistic shockCondition for relativistic turbulence (I): relativistic shock

Condition for relativistic turbulence (II): relativistic reconnection outflowCondition for relativistic turbulence (II): relativistic reconnection outflow€

λ ≤2 ×109cm

€

s <σ ,

λ ≤104 cm€

21 ≥1

2σρ1

'

ρ 2'

⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 2

Features of the ICMART model

• Carries the merits of the internal shock model (variability related to central engine)

• Overcomes the difficulties of the internal shock model (carries the merits of the EM model)– High efficiency ~ 50%– Electron number problem naturally solved (electron number is

intrinsically small)– Turbulent heating may overcome fast cooling problem– Amati relation more naturally interpreted (larger R, smaller ,

easier to have reconnection “avalanche”)– No missing photosphere problem

Zhang & Yan (2011)Zhang & Yan (2011)

Two-Component Variability in the ICMART Model

slow variability component slow variability component related to central engine related to central engine

fast variability component fast variability component related to turbulence related to turbulence

Consistent with data:Consistent with data:Shen & Song (03)Shen & Song (03)Vetere et al. (06)Vetere et al. (06)

Poster:Poster:H. Gao, B.-B. Zhang H. Gao, B.-B. Zhang & B. Zhang& B. Zhang

General picture

• 15/17 LAT GRBs are Band only• 2/17 with extra PL component

• Applicability of ICMART: at least GRB 080916C, probably most Band-only GRBs