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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 ), Sin é ad McGlynn (MPE) . June 2012. Outline.

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Radiative processes during GRB prompt emission

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  1. Radiative processes during GRB prompt emission Based on works by AsafPe’er(ITC / Harvard University) in collaboration with Peter Meszaros(PSU), Martin Rees (IoA) ChristofferLundman, 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. High optical depth: >1 Low optical depth: <1 General picture: the “fireball” model • Paczynski (1986); Goodman (1986); Rees & Meszaros (1992, 1994); 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 acceleration2. Emission processes:Leptonic / Hadronic (?)

  6. Prompt GRB spectra: the “Band” curse a(+2) b LognFn GBM Logn 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 burstsMost are similar to BATSE bursts: <a>~-1 a(+2) BATSE data:Kaneko+06 Nava+11; Goldstein+12(picture taken from Ghisellini) b LognFn GBM Logn • Violate ‘synchrotron line of death’ (Preece98); • Emission mechanism cannot be (only) synchrotron

  8. Fermi - GBM burstsMost GRBs have similar properties to BATSE bursts BATSE data:Kaneko+06 Inconsistent with sync. origin Nava+11; Goldstein+12(picture taken from Ghisellini) b LognFn GBM Logn 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. High optical depth: >1 Low optical depth: <1 General picture: the “fireball” model 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 = SD(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 • atr<=rpht, teg=dRngsT >> 1Every 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 + 2007 • Pe’er + 2010 • Beloborodov 2010 • Lazatti & Begelman 2010 “Quasi steady state”: Electrons distribution is not power lawReal 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 = SD(q)T’(r,q) Sub photospheric energy dissipation

  20. Relativistic wind Photon emission radius II. “Geometrical broadening”photosphere in relativistically expanding plasma 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 profile G qj qv q G0 qj qv p 4 freeparameters: (Lundman, AP & Ryde, in prep)

  25. Extended emission from high angles G G q 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 = 1017,1016,1015.5, 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 Pe’er + 12: GRB090902B -thermal + dissipation above the photosphere Ryde + 11:spectral broadening by sub-photospheric dissipation 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* & delayedGeV 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.

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