Klein-Nishina effect on high-energy gamma-ray emission of GRBs

1. Deciphering the Ancient Universe with Gamma-Ray Bursts 19-23 April 2010, Kyoto, Japan Klein-Nishina effect on high-energy gamma-ray emission of GRBs Xiang-Yu Wang (王祥玉） Nanjing University, China （南京大學） Co-authors: Hao-Ning He (NJU), Zhuo Li (PKU), Zi-Gao Dai (NJU), Xue-Feng Wu (PennState), Peter Meszaros (PennState),

2. Fermi observations of GRB080916c Abdo et al. 09 Xiang-Yu Wang Nanjing Univ.

3. Extended high-energy emission of short GRB 090510 Short GRB: T_90=~1 s Significant high-energy emission up to T0+200s GRB090510 De Pasquale et al. 09

4. GRB090902B—a long GRB t^-1.5 Abdo et al. 09

5. Klein-Nishina (KN) effect may be important in Prompt high-energy gamma-ray emission ( if it is due to synchrotron emission) Temporal Extended High Energy Emission

6. Klein-Nishina IC scattering Thomson IC scattering KN IC scattering Xiang-Yu Wang Nanjing Univ.

7. KN effect may be important in Prompt high-energy gamma-ray emission Temporal Extended High Energy Emission

8. Prompt spectrum of GRB080916C a b • Band function fits the KeV-GeV data • 2. No bump is seen at high energies d c Xiang-Yu Wang Nanjing Univ.

9. Some other bursts: high-energy emission consistent with the extrapolation GRB090217 GRB080825C, GRB090217

10. The synchrotron scenario ( Wang, Li, Dai & Meszaros 2009 ) • 1) Can the maximum syn. energy reach 70 GeV ? ---a parameter describing the efficiency of the shock acceleration (cf Ioka’talk) Yes, only when Bohm diffusive shock acceleration ( ) Xiang-Yu Wang Nanjing Univ.

11. The synchrotron scenario • 2) Why no visible IC bump? • two assumptions: • equipartition magnetic field: • 2) causality constraint: Inverse Compton must be in the Klein-Nishinaregime, which leads naturally to a low, invisible IC component Xiang-Yu Wang Nanjing Univ.

12. The synchrotron scenarioKN effect on the low-energy spectrum • Low-energy spectral index ? The ratio of IC cooling efficiency to syn cooling efficiency is not a constant anymore, but depends on γ Can makes the low-energy spectrum harder (α= -1.02±0.02) in GRB080916C (also see Derishev et al. 2003; Nakar et al. 09) Xiang-Yu Wang Nanjing Univ.

13. KN effect may be important in Prompt high-energy gamma-ray emission (some bursts Band function fit well, some deviate from Band function) Temporal Extended High Energy Emission

14. Models for the extended emission • Hadronic cascade process (Dermer & Atoyan 04) • Forward shock—long lived • synchrotron: slow decay (Kumar & Barniol Duran09) • IC: rise initially and slow decay (Zhang & Meszaros 01; Fan et al. 08) • Reverse shock --- short lived, fast decay (Wang et al. 2001) Xiang-Yu Wang Nanjing Univ.

15. Forward shock IC emission Zhang & Meszaros 01 First rise, then decay Xiang-Yu Wang Nanjing Univ.

16. Forward shock synchrotron scenario • Can easily explain the simple decay • The flux level matches the observations (Kumar &Barniol Duran 09) • Possible problems: 1) maximum photon energy (Abdo et al. arXiv:0909.2470; Piran & Nakar 10) 2) too steep (Ghisellini et al. 09), see also Poster #95 (T. Uehara) on 090926A Barniol Duran & Kumar 09

17. If afterglow emission, KN effect should be taken into account • For afterglow electrons in the Thomson scattering, Y • For high-energy afterglow emission, ( ) is large, inverse Compton scattering with synchrotron peak photons should be in Klein-Nishina regime Sari & Esin 2001: Compton Y parameter depends on γ, therefore depends on ν ! <

18. One example: the slow-cooling case

19. i) Values of comptonY parameters (t=1 s) Wang, He, et al. 2010, ApJ

20. Compton Y parameters (t=10 s)

21. KN effects -summary (1) Kumar & Barniol Duran 09: Y here is dependent of ν Xiang-Yu Wang Nanjing Univ. For a wide range of parameters, Y(100MeV) is initially small, typically smaller than 1 Leading to a high synchrotron luminosity at early time

22. ii) KN effect on the spectrum Slow-cooling case Fast-cooling case

23. The short GRB case (He & Wang 09) • Spectra and Light curves under typical parameters for short GRBs Xiang-Yu Wang Nanjing Univ.

24. KN effects –summary (2) Xiang-Yu Wang Nanjing Univ. At very early time, synchrotron emission is usually dominant in the LAT energy band (30MeV to tens of GeV) SSC dominates only above the maximum synchrotron energy

25. iii) Evolution of Y parameters with time • Slow-cooling case Wang, He, et al. 2010

26. The fast-cooling case

27. KN effects –summary (3) Xiang-Yu Wang Nanjing Univ. For certain parameter space, Y(100MeV) increases with time--- the KN suppression effect of high-energy electrons weakens with time, so that the IC loss increases with time. If Y(100MeV) >1 as well, leading to a steeper decay than predicted by the standard synchrotron theory A testable prediction for this scenario: the spectrum becomes harder meanwhile

28. GRB090510 De Pasquale et al. 2009 Ghirlanda et al. 2009 Standard syn model: LAT should decay as

29. GRB090902B t^-1.5

30. Conclusions • If the prompt high-energy emission is of synchrotron origin, KN is important and may affect the prompt low-energy spectrum • KN effect is important in estimating the afterglow synchrotron emission, which leads to • Early high synchrotron luminosity • Faster temporal decay in some parameter space • When modeling the high-energy afterglow emission, treat carefully the KN scattering effect on the electron radiation Xiang-Yu Wang Nanjing Univ.

31. Backup slide KN effect on electron distribution • How the synchrotron luminosity is affected depends on the electron distribution (Nakar et al. 09; Wang et al. 10) 1) slow cooling 2) fast cooling