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

Documents

prepphotospheric emission

prepextended emission

emission zonesnothing

emission radius

thermal emission component

low energy

observed photospheric

ap ryde