1. 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

2. 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.

3. 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.

4. General picture: the “fireball” model
Paczynski (1986); Goodman (1986); Rees & Meszaros (1992, 1994);
High optical depth: >1
Low optical depth: <1
EG  Ek  E (EB)
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)
Pros: In qualitative agreement with all obs;
Obtain AG as a prediction

5. General picture: the “fireball” model
Dynamical part: Jet acceleration, Collisionless / nal shock waves ? Energy transfer from B-field ? External shock
Radiative part: 2 stages: 1. Particle acceleration 2. Emission processes: Leptonic / Hadronic (?)

6. Prompt GRB spectra: the “Band” curse
Log n
Log nFn
a(+2) b
GBM
10keV
100MeV
“Band” function: Broken power law (4 free parameters)
-- good fit to (narrow band) spectra; NO PHYSICAL MEANING !!!
David Tierney, Michael Briggs talks

7. Fermi - GBM bursts Most are similar to BATSE bursts: <a>~-1
BATSE data: Kaneko+06
Nava+11;
Goldstein+12 (picture taken from Ghisellini)
Log n
Log nFn
a(+2) b
GBM
Violate ‘synchrotron line of death’ (Preece98);
Emission mechanism cannot be (only) synchrotron

8. Fermi - GBM bursts Most GRBs have similar properties to BATSE bursts
BATSE data: Kaneko+06
Inconsistent with sync. origin
Nava+11;
Goldstein+12 (picture taken from Ghisellini)
Log n
Log nFn b
GBM
a(+2)
Photon spectral index
Violate ‘synchrotron line of death’ (Preece98);
Emission mechanism cannot be (only) synchrotron
Main (observational) motivation to study photospheric emission
Synchrotron line of death >>

9. Spectral analysis latest news: abandoning the “Band” fits
Fit to GRB110721A: “Band” + BB
The Fermi team + AP, in prep.; see Magnus Axelsson, Briggs talks

10. 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.

11. General picture: the “fireball” model
High optical depth: >1
Low optical depth: <1
EG  Ek  E (EB)
Variability -> several emission zones;
NOTHING tells what is the emission radius !!

12. How can we explain the observed spectrum ?
GRB080916C (Abdo+09)
Synchrotron – too flat
Planck – too steep
Idea: Broaden “Planck” ! “Geometrical broadening”: “Physical broadening”: Tob = S D(q)T’(r,q) Sub photospheric energy dissipation

13. I. “Physical broadening” of the photospheric signal
Basic idea:
Energy dissipated (heating plasma) at r<=rpht Key point: ng >> ne
Definition: at r=rpht, tge=dRnesT = 1
at r<=rpht, teg=dRngsT >> 1 Every electron undergoes many scattering !!
tcool,elec << tdyn
Pe’er, Meszaros & Rees (2005, 2006)
Beloborodov (2010); Vurm+ (2011)
Lazatti & Begelman (2010) Giannios (2012)
Electrons rapidly cools !!

14. I. “Physical broadening” of the photospheric signal
Basic idea:
Energy dissipated (heating plasma) at r<=rpht.
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.
Pe’er, Meszaros & Rees (2005, 2006)
Beloborodov (2010); Vurm+ (2011)
Lazatti & Begelman (2010) Giannios (2012)

15. I. “Physical broadening” of the photospheric signal
Basic idea:
Energy dissipated (heating plasma) at r<=rpht.
Plasma characterized by 2 temperatures:
Tel(steady state) >Tph.
The resulting spectrum: Above the thermal peak -> depends (mainly) on: 1. tge (# scatterings)
2. ue/uth
Below the thermal peak: Synchrotron (from COLD particles)…. Comptonized.
Pe’er, Meszaros & Rees (2005, 2006)
Beloborodov (2010); Vurm+ (2011)
Lazatti & Begelman (2010) Giannios (2012)
Conclusion:
Multiple IC scattering broadens the thermal peak

16. Examples of possible spectral shapes: sub photospheric energy dissipation
High B
Low B
High B
Low B
tge= 1
tge= 10
Pe’er, Meszaros & Rees (2006) See talk by Giannios

17. Complex relation between thermal and n.t. emission
Pe’er, Meszaros
& Rees 2006
See also
Giannios 2006, 2012
Giannios & Spruit 2007
Ioka
Pe’er
Beloborodov 2010
Lazatti & Begelman 2010
“Quasi steady state”: Electrons distribution is not power law Real life spectra is not easy to model !! (NOT simple broken Power law)

18. 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.

19. How can we explain the observed spectrum ?
GRB080916C (Abdo+09)
Synchrotron – too flat
Planck – too steep
Idea: Broaden “Planck” ! “Geometrical broadening”: “Physical broadening”: Tob = S D(q)T’(r,q) Sub photospheric energy dissipation

20. II. “Geometrical broadening” photosphere in relativistically expanding plasma
Photon emission radius
Relativistic wind
Pe’er (2008)
High lat. >>

21. 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,)
Pe’er (2008) ; see also Beloborodov (2011)

22. Observed photospheric spectrum: multicolor black body
Pe’er & Ryde (2011)
“Limb darkening” in rel. expanding plasma !!
At early times: multicolor BB. At late times, Fn~n0 -> Identical to “Band” a

23. More ambitious goal: maybe photospheric emission is not “just a component” “reality”: G=G(q)
(Lundman, AP & Ryde, in prep)
(Zhang, Woosley & MacFadyen, 03)

24. Photospheric emission: ‘realistic’ velocity profileG qj qv q G0 qj qv p
4 free parameters:
(Lundman, AP & Ryde, in prep)

25. Extended emission from high anglesq G G q
(Lundman, AP & Ryde, 12)
Relativistic Limb darkening effect

26. G0=100; G0qj = 3; qv =0 ; p=4

27. Flat spectra for different viewing angles
G0=100; G0qj = 1; p=1 ; qv = {0,1,2} qj (red, green, magenta)

28. Photospheric emission: flat spectrum !!
(Nava+11; Goldstein+12)
a+1 = 0 -> a=-1
Not conclusive yet… but very promising
(Lundman, AP & Ryde, in prep)

29. 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.

30. Example: numerical fit to GRB090902B
‘Two zones’ model
Dissipation radius
Magnetic field strength
Rg = 1017, 1016, , 1015 cm
eB=0.33, 0.1, 0.01
Self consistent physical picture of both emission zones ; Full determination of parameters values. Natural explanation to delayed H.E. emission
Pe’er et. al.,
2012

31. Combined sub- and super- photospheric emission: numerical results
Ryde + 11: spectral broadening by sub-photospheric dissipation
Pe’er + 12: GRB090902B - thermal + dissipation above the photosphere
synchrotron
IC of photosphere and sync.
Thermal Comptonization
Requirements: uel ~ uth; strong B (eB ~ tens %); t ~ few
No time - skip to summary >>

32. 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.

33. Key spectral features:
Geometric broadening
Sub photospheric dissipation, multiple regions.
1. a~-1
2. E_pk ~ sub-MeV
3. Separated* & delayed GeV component

34. 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.