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Photosphere Emission in Gamma-Ray Bursts

4 th Fermi Asian Network Workshop, HKU, July 8-12, 2013. Photosphere Emission in Gamma-Ray Bursts. Xuefeng Wu Purple Mountain Observatory Chinese Center for Antarctic Astronomy Chinese Academy of Sciences Collaborators: Shujin Hou, Zigao Dai,

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Photosphere Emission in Gamma-Ray Bursts

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  1. 4th Fermi Asian Network Workshop, HKU, July 8-12, 2013 Photosphere Emission in Gamma-Ray Bursts Xuefeng Wu Purple Mountain Observatory Chinese Center for Antarctic Astronomy Chinese Academy of Sciences Collaborators: Shujin Hou, Zigao Dai, Bing Zhang, Enwei Liang, Tan Lu et al.

  2. Temporal Characteristics • light curve profiles complicated • durations ～ ms - 1000 s • variabilities ～ 1ms , even ～ 0.1ms

  3. Spectral Characteristics • photon energies： 10keV – 10GeV • non-thermal GRB090510 GRB090902B multi-color blackbody

  4. GRBs：stellar explosions δT～ms  Ri ≤ cδT = 300 km (Ri: emission size) Blackhole： R = 2GM/c2 M≤ 100 M⊙  GRBs:stellar objects (compact stars)

  5. GRBs：energy bugget Fγ~ 10-6 erg/cm2 DL ~ 3 Gpc Eisotropic = 4DL２Fγ ~ 1051 erg EGRB970228~ 1051 erg EGRB990123~ 1054 erg unisotropic Jet?

  6. Expanding Fireball The fireball will expand and accelerate to be ultra-relativistic driven by the high radiation temperature and pressure, while the optical depth decreases from extremely thick to thin and produce non-thermal emission. Ri ≤ cδTnon-thermal spectrum optically thicksolutionopticalthin  ultra-relativistic Lorentz factor:  >>1

  7. Seminal papers on GRB fireball models

  8. Acceleration of GRB baryonic fireball • Ideal hydrodynamic assumption： • outside is vacuum (environmental density is low) • Photons are coupled (optical depth > 1) • Baryons and photons are coupled • (lepton-photon scattering depth > 1) Conservations of energy, momentum and particle number： ( energy ) ( momentum ) ( particle number )

  9. Scaling laws of accelerating fireball radiation-dominated epoch matter-dominated epoch

  10. Characteristic radii of GRB fireball-photosphere-internal shocks

  11. Long Way in Discovery of GRB Fireball Emission • Since 1997，cosmological GRB internal-external shocks models have been confirmed by many observations； • No thermal emission was detected from the energetic GRB 080916C (Fermi GBM/LAT) – evidence of highly magnetization of the initial fireball of this burst! Zhang & Pe’er 2009

  12. Long Way in Discovery of GRB Fireball Emission • Thermal emission from GRB fireball photosphere was first discovered (with high confidence level) in GRB 090902B by Fermi • Thermal emission have been found in a few GRBs, such as 970828、 081221、090510、090618 GRB 081221 GRB 090902B Ryde et al. 2009 Hou et al. 2013

  13. Static Photosphere (un-relativistic)

  14. Relativistic Photosphere

  15. Relativistic Photosphere Assumptions： （1）do not consider the Equal Arrival Time Surface Effect； （2）impulsive photosphere; （3）uniform fireball

  16. Relativistic Photosphere

  17. Relativistic Photosphere Approximation：

  18. Relativistic Photosphere

  19. Thermal Spectrum from a Relativistic Photosphere wider than Planck function！ we call it“relativistic Planckfunction”

  20. Realistic Relativistic Photosphere （1）fireball is not isotropic （2）there are many fireballs in a GRB （3）equal arrival time surface effect  multi-color black body (mBB)

  21. Model of multi-color black body (mBB) Single black body see Ryde et al. (2009) A(>Tmin) =1, normalization multi-color black body

  22. Analytical Approach of mBB Model For m<-1

  23. mBB Model: Analytical vs. Accurate

  24. Light Curve of GRB081221

  25. Time-Resolved Spectra in 081221

  26. Summary of Time-Resolved Spectral Fit

  27. Time-Integrated Spectrum of 081221 Time-resolved spectral models are not self-consistent with time-integrated spectrum！

  28. Moments of temperature of mBB For 081221: See Hou Shujin’s Poster ~ (9.9 keV)^4 ~ 7.1 keV

  29. Comparison with 090902B(time-integrated spectrum) m ~ -2 Rayleigh – Jeans part not observed m ~ -4 Rayleigh – Jeans part observed ! GRB 090902B GRB 081221 Ryde et al. 2009 Hou et al. 2013

  30. Relativistic Photosphere

  31. High efficiency photosphere

  32. High efficiency photosphere

  33. High efficiency photosphere

  34. Low efficiency photosphere

  35. Low efficiency photosphere

  36. Low efficiency photosphere Constraint-1

  37. Low efficiency photosphere Constraint-2

  38. Low efficiency photosphere Constraint-3

  39. Low efficiency photosphere Constraint-1,2 & 3

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