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Great Debate on GRB Composition: A Case for Poynting Flux Dominated GRB Jets

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).

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Great Debate on GRB Composition: A Case for Poynting Flux Dominated GRB Jets

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  1. 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)

  2. Sherlock Holmes

  3. Fingerprint/ footprint/ Smoking gun: GRB 080916C(Abdo et al. 2009, Science)

  4. Standard Fireball Shock Model central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission: from internal shocks and photosphere Afterglow: from external shocks

  5. Predicted spectra Meszaros & Rees (00) Zhang & Meszaros (02, unpublished) Daigne & Mochkovitch (02)

  6. 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.

  7. The simplest fireball model does not work! Modified fireball models

  8. Modified Fireball Model (1) central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission: from internal shocks and photosphere Afterglow: from external shocks

  9. 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 …

  10. Difficulties/issues of the internal shock model • Missing photosphere problem • Low efficiency • Fast cooling problem • Electron number excess problem • Ep – Eiso (Liso) correlation inconsistency

  11. Modified Fireball Model (2) central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission: from internal shocks and photosphere Afterglow: from external shocks

  12. 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)

  13. 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) ? ?

  14. 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)

  15. Sherlock Holmes

  16. Fingerprint/ footprint/ Smoking gun: Preece’s talk

  17. Lu, Hou & Liang 2010

  18. 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”!

  19. 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

  20. Back-up slides

  21. 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

  22. Method One: X-Rays • If the steep-decay phase of the X-ray tail is defined by the high-latitude emission, one has: ttail R tail j Kumar et al. 07 Lyutikov, 06 R,X> 1015 cm GRB Rj2/2

  23. Method Two: Optical “Tracking” optical band detection constraints the self-absorption frequency and, hence, the emission radius GRB 050820A Shen & Zhang (08): R,opt > several 1014 cm Vestrand et al. 2006a,b

  24. GRB 080319B: naked-eye GRB(Racusin et al. 2008) Spectrum: two distinct spectral components Lightcurve: optical roughly traces gamma-rays

  25. Syn + SSC model for GRB 080319B (Racusin et al. 2008; Kumar & Panaitescu 2008) E2 N(E) Y ~ 10 Y2 ~100 Klein-Nishina cut-off Y ~ 10 R,opt ~ 1016 cm E Esyn~20 eV ESSC2st ~25 GeV ESSC1st ~650 keV

  26. 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 1000 Gupta & Zhang 2008

  27. Radius constraints(Zhang & Pe’er 09) Emission must come from a large radius far above the photosphere.

  28. 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

  29. 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)

  30. 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).

  31. 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)

  32. Distance Scales in the ICMART Model Emission suppressed GRB At most 1/(1+σ) energy released At most 1/(1+σ) energy released 1/(1+σend) energy released photosphere R ~ 1011 - 1012 cm   0 early collisions R ~ 1013 - 1014 cm  ~ 1- 100 ICMART region R ~ 1015 - 1016 cm ini ~ 1- 100 end  1 External shock R ~ 1017 cm   1 central engine R ~ 107 cm  = 0 >> 1

  33. GRB ejecta is turbulent in nature Reynold’s number: Magnetic Reynold’s number: Magnetic fields can be highly distorted and turbulent if turbulent condition is satisfied

  34. λ L

  35. Turbulent Reconnection is needed to power GRBs In order to reach GRB luminosity, the effective global reconnection rate has to be close to c . Relativistic Sweet-Parker reconnection speed is << c (Lyubarsky 2005). Turbulent reconnection (Lazarian & Vishniac 1999) can increase reconnection speed by a factor L/λ .

  36. Multiple collisions can distort field lines and eventually trigger turbulence in a high-σ flow Required condition from the observations (reach GRB luminosity): Condition for relativistic turbulence (I): relativistic shock Condition for relativistic turbulence (II): relativistic reconnection outflow

  37. 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

  38. 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

  39. 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

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