Radiative processes during GRB prompt emission

Radiative processes during GRB prompt emission

Based on works by

Asaf Pe’er (ITC / Harvard University)

in collaboration with

Peter Meszaros (PSU), Martin Rees (IoA) Christoffer Lundman, Felix Ryde (Stockholm),

Sinéad McGlynn (MPE)

June 2012

Outline The problem: understanding what we see

Emission from optically thick regions

Broadening mechanisms of Planck spectrum: A theory of photospheric emission from collimated outflows

Success: separation of high energy emission from low energy part .

Failure: still, no natural explanation to observed spectra.

General picture: the “fireball” model

High optical depth: >1 Low optical depth: <1

EG Ek E

(EB)

•Paczynski (1986); Goodman (1986); Rees & Meszaros (1992, 1994) ;

Pros: In qualitative agreement with all obs ;

Obtain AG as a prediction

Cons: No quantitative explanation of obs. (Emission ?)Some parts are not explained at all (e.g., particle acc.)Some parts are ‘problematic’ (e.g., Internal shocks)


Dynamical part:

Jet acceleration,Collisionless / nal shock waves ?Energy transfer from B-field ?External shock

Radiative part:

2 stages:1. Particle acceleration2. Emission processes:

Leptonic / Hadronic(?)

Prompt GRB spectra: the “Band” curse

“Band” function: Broken power law (4 free parameters) -- good fit to (narrow band) spectra;

NO PHYSICAL MEANING!!!

10keV 100MeV

Log n

Log

nFn

a(+2

)b

GBM

David Tierney,Michael Briggs talks

Fermi - GBM burstsMost are similar to BATSE bursts: <a>~-1

Violate ‘synchrotron line of death’ (Preece98);Emission mechanism cannot be (only) synchrotron

Nava+11;Goldstein+12

(picture taken from Ghisellini)

BATSE data:Kaneko+06

Log n

Log

nFn

a(+2) b

GBM

Fermi - GBM burstsMost GRBs have similar properties to BATSE bursts

Violate ‘synchrotron line of death’ (Preece98);Emission mechanism cannot be (only) synchrotron

Nava+11;Goldstein+12

(picture taken from Ghisellini)

Inconsistent with sync.

origin

Photon spectral index

BATSE data:Kaneko+06

Log n

Log

nFn

b

GBM

a(+2)

Synchrotron line of death>> Main (observational) motivation to study photospheric emission

Spectral analysis latest news: abandoning the “Band” fits

The Fermi team + AP, in prep.; see Magnus Axelsson, Briggs talks

Fit to GRB110721A: “Band” + BB


High optical depth: >1 Low optical depth: <1

EG Ek E

(EB)

Variability -> several emission zones;NOTHING tells what is the emission radius!!

GRB080916C (Abdo+09)

How can we explain the observed spectrum ?

Synchrotron – too flat

Planck – too steep

Idea: Broaden “Planck” !

“Geometrical broadening”: “Physical broadening”:Tob = S D(q)T’(r,q) Sub photospheric energy dissipation

I. “Physical broadening” of the photospheric signal

Pe’er, Meszaros & Rees (2005, 2006)Beloborodov (2010); Vurm+ (2011)Lazatti & Begelman (2010)Giannios (2012)

Electrons rapidly cools!!

Basic idea: Energy dissipated (heating plasma)at r<=rpht.

Key point: n >> ne

Definition: at r=rpht, te=dRnesT = 1 at r<=rpht, te=dRnsT >> 1

Every electron undergoes many scattering!!

tcool,elec << tdyn

tcool,elec << tdyn

Electrons rapidly cool..but are also heated!

System in ‘quasi steady state’: external heating & IC cooling

Plasma characterized by 2 temperatures:Tel(steady state) >Tph.




Plasma characterized by 2 temperatures:Tel(steady state) >Tph.

Conclusion:



Multiple IC scattering broadens the thermal peak


The resulting spectrum:Above the thermal peak -> depends (mainly) on:1. e (# scatterings)

2 .ue/uth

Below the thermal peak:Synchrotron (from COLD particles)…. Comptonized.

e= 1 e= 10

High BLow B

High BLow B

Examples of possible spectral shapes:sub photospheric energy dissipation

Pe’er, Meszaros & Rees (2006)See talk by Giannios

Complex relation between thermal and n.t. emission

Pe’er, Meszaros &Rees 2006

“Quasi steady state”: Electrons distribution is not power law

Real life spectra is not easy to model !! (NOT simple broken Power law)

See also• Giannios 2006, 2012

• Giannios & Spruit 2007

• Ioka + 2007• Pe’er + 2010

•Beloborodov 2010•Lazatti & Begelman

2010

GRB080916C (Abdo+09)

How can we explain the observed spectrum ?

Synchrotron – too flat

Planck – too steep

Idea: Broaden “Planck” !

“Geometrical broadening”: “Physical broadening”:Tob = S D(q)T’(r,q) Sub photospheric energy dissipation

II. “Geometrical broadening”photosphere in relativistically expanding plasma

€

for θ <<1;Γ >>1 →

rph (θ) ≈ Rd2π

1Γ 2 + θ

2

3

⎛ ⎝ ⎜

⎞ ⎠ ⎟

Photon emission radius

Relativistic wind

cm1034

1252

17 −Γ×=≡ Lcm

MRp

Td b

σ&

€

rph (θ) = Rdπ

θsin(θ)

−β ⎡ ⎣ ⎢

⎤ ⎦ ⎥

Pe’er (2008) High lat>> .

Extending the definition of a photosphere

Thermal photons escape from the entire space !Photons escape radii and angles - described by probability density function

P(r,q)

Pe’er (2008) ; see also Beloborodov (2011)

€

Fν (t)∝ P(r)dr P(θ)dθT obδ t ob. = r(1−β cosθ)βc

⎛ ⎝ ⎜

⎞ ⎠ ⎟∫∫ δ T ob = T 'D( )∝ t−2e

−tNtν max

ν

Pe’er & Ryde (2011)

Observed photospheric spectrum: multicolor black body

At early times: multicolor BB.At late times, Fn~n0 -> Identical to “Band” a

“Limb darkening” in rel. expanding plasma!!

More ambitious goal: maybe photospheric emission is not “just a

component” “reality”: Γ=Γ(q)

(Zhang, Woosley & MacFadyen, 03)

(Lundman, AP & Ryde, in prep)

Photospheric emission: ‘realistic’ velocity profile

Γ

q

qv

qj

€

Γ(θ ) −1[ ]2 =Γ0 −1[ ]

2

1+θθ j

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟2 p

Γ0qjqvp

4 freeparameters:


Extended emission from high angles

q

Γ

q

Γ(Lundman, AP & Ryde, 12) Relativistic Limb darkening effect

Γ0=100; Γ0qj = 3; qv =0 ; p=4

Flat spectra for different viewing angles

Γ0=100; Γ0qj = 1; p=1 ; qv = {0,1,2} qj (red, green, magenta)


Photospheric emission: flat spectrum !!

(Nava+11; Goldstein+12)a+1 = 0 -> a=-1

Not conclusive yet… but very promising

Pe’er et. al,.

2012

Example: numerical fit to GRB090902B ‘Two zones’ model

Dissipation radius Magnetic field strength

eB=0.33, 0.1, 0.01R = 1017, 1016, 1015.5, 1015 cm

Self consistent physical picture of both emission zones ;

Full determination of parameters values.Natural explanation to delayed H.E. emission

Combined sub- and super- photospheric emission:

numerical results

synchrotronThermal Comptonization

Requirements:uel ~ uth; strong B (eB ~ tens %); ~ few

Pe’er + 12: GRB090902B -

thermal + dissipation above the photosphere

Ryde + 11:spectral broadening by

sub-photospheric dissipation

IC of photosphere and sync.

No time - skip to summary>>

Key spectral features:

1 .a~-1

2 .E_pk ~ sub-MeV

3 .Separated* & delayed GeV component

Geometric broadening

Sub photospheric dissipation, multiple regions.

Bottom lines & summary Major efforts in understanding the

physical origin of prompt emission

Failure of optically thin models, raise interest in photospheric emission.

Sub-photospheric heating leads to broadening of Planck spectrum.

Photospheric emission from collimated outflow may hold the key to the observed spectra.

Radiative processes during GRB prompt emission

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