Transcript Bing Zhang

Great Debate on GRB Composition:
A Case for Poynting Flux
Dominated GRB Jets
Bing Zhang
Department of Physics and Astronomy
University of Nevada, Las Vegas
March 6, 2011
In “Prompt Activity of Gamma-Ray Bursts”
Raleigh, North Carolina
Reference: Zhang & Yan (2011, ApJ, 726, 90)
Sherlock Holmes
Fingerprint/
footprint/
Smoking gun:
GRB 080916C
(Abdo et al. 2009, Science)
Standard Fireball Shock Model
central
engine
photosphere
internal shocks
external shocks
(reverse)
(forward)
GRB prompt emission: from internal shocks and photosphere
Afterglow: from external shocks
Predicted spectra
Meszaros & Rees (00)
Zhang & Meszaros (02, unpublished)
Daigne & Mochkovitch (02)
Expected photosphere emission from a fireball
(Zhang & Pe’er 09)
Sigma: ratio between Poynting flux and baryonic flux:
 = LP/Lb: at least ~ 20, 15 for GRB 080916C
Confirmed by
Fan (2010) with a wider
parameter space study.
The simplest fireball model does
not work!
Modified fireball models
Modified Fireball Model (1)
central
engine
photosphere
internal shocks
external shocks
(reverse)
(forward)
GRB prompt emission: from internal shocks and photosphere
Afterglow: 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
engine
photosphere
internal shocks
external shocks
(reverse)
(forward)
GRB prompt emission: from internal shocks and photosphere
Afterglow: 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
α ~ -1
?
?
α ~ (+0.4 - +1)
Beloborodov (2010); Mizuta et al. (2010);
Deng & Zhang (poster)
The band function is emission
from the photosphere
• 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)
Giannios (2008)
Sherlock Holmes
Fingerprint/
footprint/
Smoking gun:
Preece’s talk
Lu, Hou & Liang 2010
Diverse Ejecta Composition:
Thermal emission in GRB 090902B!
Ryde et al. (2010); B.-B. Zhang et al. (2011) - Bin-Bin’s talk
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
If the steep-decay phase of the
X-ray tail is defined by the highlatitude emission, one has:
R
j
tail
GRB
Rj2/2
ttail
Kumar et al. 07
Lyutikov, 06
R,X > 1015 cm
Method Two: Optical
“Tracking” optical band
detection constraints the selfabsorption frequency and,
hence, the emission radius
GRB 050820A
Shen & Zhang (08):
R,opt > several 1014 cm
Vestrand et al. 2006a,b
GRB 080319B: naked-eye GRB
(Racusin et al. 2008)
Spectrum: two distinct spectral
components
Lightcurve: optical roughly traces
gamma-rays
Syn + SSC model for GRB 080319B
E2 N(E)
(Racusin et al. 2008; Kumar & Panaitescu 2008)
Y ~ 10
Y ~ 10
Y2 ~100
Klein-Nishina cut-off
R,opt ~ 1016 cm
Esyn~20 eV
ESSC2st
ESSC1st ~650 keV
E
~25 GeV
Method Three: GeV
Pair cutoff feature
depends on both
bulk Lorentz factor
(Baring & Harding
1997; Lithwick &
Sari 2001) and the
unknown emission
radius (Gupta &
Zhang 2008)

100
200
400
600
800
100
0
Gupta & Zhang
2008
Radius constraints
(Zhang & Pe’er 09)
Emission must come from a large radius far above the 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 ~ 3109 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).
ICMART Model
Zhang & Yan (2010)
(a) Initial collisions only distort magnetic fields
(b) Finally a collision triggers fast turbulent reconnection
- An ICMART event (a broad pulse in GRB lightcurve)
Distance Scales in the ICMART Model
Emission suppressed
At most
1/(1+σ)
energy released
central engine
R ~ 107 cm
 = 0 >> 1
photosphere
R ~ 1011 - 1012 cm
  0
At most
1/(1+σ)
energy released
early collisions
R ~ 1013 - 1014 cm
 ~ 1- 100
GRB
1/(1+σend)
energy released
ICMART region
R ~ 1015 - 1016 cm
ini ~ 1- 100
end  1
External shock
R ~ 1017 cm
1
GRB ejecta is turbulent in nature
Reynold’s number:
Re 
LV
~ 10 28  1

Magnetic Reynold’s number:


Rm,Bohm 
LV
~ 3.4  1012 1

Magnetic fields can be highly distorted and
turbulent if turbulent condition is satisfied
λ
L
Turbulent Reconnection is needed
to power GRBs
In order to reach GRB luminosity, the effective global
reconnection rate has to be close to c .
'2
' L 1 
B
'
2
2 '
Vrec,global 
~
c~c

4R
~ L
8
t'
t' Lw 
Relativistic Sweet-Parker reconnection speed is << c
(Lyubarsky 2005).

'
rec,local
V
1/ 2
 VA s   c
VA
s
 1

Turbulent reconnection (Lazarian & Vishniac 1999) can
increase reconnection speed by a factor L/λ .

'
rec,global
V

V
'
rec,local
L
( )

Multiple collisions can distort field lines and
eventually trigger turbulence in a high-σ flow
Required condition from the observations (reach GRB luminosity):
  2  10 cm
9
Condition for relativistic turbulence (I): relativistic shock
1 1' 1/ 2
21    ' 
2  2 
 Condition for relativistic turbulence (II): relativistic reconnection outflow

s  ,
 10 cm
4
Features of the ICMART model
Zhang & Yan (2011)
• 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
Two-Component Variability
in the ICMART Model
Consistent with data:
Shen & Song (03)
Vetere et al. (06)
Poster:
H. Gao, B.-B. Zhang
& B. Zhang
slow variability component
related to central engine
fast variability component
related to turbulence
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