1. Yizhong Fan (Niels Bohr International Academy, Denmark Purple Mountain Observatory, China) Fan (2009, MNRAS) and Fan & Piran (2008, Phys. Fron. China)

2. MeV-GeV emission from GRBs (EAGRET) GRB 940217 GRB 940217 (Hurley et al. 1994) GRB afterglow detection for the first time!

3. MeV-GeV emission from GRBs (EGRET) Quick evolution Almost constant Almost constant Much longer high energy emission GRB 941017 (Gonzalez et al. 2003)

4. Theoretical predictions—before Fermi (see Fan & Piran 2008 for a review) ( Pe’er & Waxman 2004; Pilla & Loeb 1998; Gupta & Zhang 2007 ) ( Fan, Piran, Narayan & Wei 2008 )

6. Fermi GRBs with GeV emission GRB 080916C (Abdo et al. 2009) GRB 081024B (Omodei et al. 2008) GRB 090217 (Ohno et al. 2009a) GRB 090323 (Ohno et al. 2009b) GRB 090328 (Cutini et al. 2009) GRB 090510 (Ohno & Pelassa 2009c) One detection once a month, as expected (assuming Band spectrum of GRBs, i.e., no GeV excess in most GRBs )

7. The delayed onset of the >100 MeV emission (Abdo et al. 2009)

8. Extended high energy emission (Abdo et al. 2009)

9. Main properties of the Fermi GRBs No GeV spectrum excess detected in almost all GRBs The delay of arrival of the >100 MeV photons The extended high energy emission from both short and long bursts

10. Interpreting the non-detection of the GeV spectrum excess in most GRBs (Fan 2009) Poynting-flux dominated outflow model Gradual magnetic energy dissipation (e.g., Giannios 2007): The strong magnetic field in the emitting region suppresses the SSC and the electrons are only mildly-relativistic Sudden energy dissipation at R~1E16 cm (Lyutikov & Blandford 2003): the SSC is in the extreme Klein-Nishina regime

11. Standard internal-shock model (The SSC in the extreme Klein-Nishina regime?) Interpreting the non-detection of the GeV spectrum excess in most GRBs (Fan 2009)

12. Mildly (0.1<sigma<1) magnetized internal shocks The strong magnetic field in the emitting region suppresses the SSC, and the synchrotron spectrum may be very soft Interpreting the non-detection of the GeV spectrum excess in most GRBs (Fan 2009)

13. Photosphere-internal shock model The electrons are assumed to be only mildly-relativistic (Thompson et al. 2007) Interpreting the non-detection of the GeV spectrum excess in most GRBs (Fan 2009)

14. Interpreting the delayed onset of the > 100 MeV emission (Fan 2009) In both collapsar and compact star merger models, the early outflow likely suffers more serious baryon pollution and thus has a smaller bulk Lorentz factor than the late ejecta. The GeV photons can not escape from the early outflow In the collapsar scenario, before the breakout, the initial outflow is choked by the envelope material of the progenitor (Zhang, W. et al. 2003). The emission of the breaking out material may be dominated by the quasi-thermal radiation from the photosphere and may last a few seconds

15. The extended high energy emission from both short and long bursts Synchrotron and SSC radiation of the forward/reverse shocks (e.g., Meszaros & Rees 1994; Dermer et al. 2000; Sari & Esin 2001; Zhang et al. 2001; Wang et al. 2001a, b; Wei & Fan 2007; Gou et al. 2007; Yu et al. 2008; Fan et al. 2008; Galli & Piro 2008; Zou et al. 2009) External inverse Compton in reverse/forward shock regions (e.g., Beloborodov 2005; Fan et al. 2005; Wang et al. 2006; Fan & Piran 2006; Wang & Meszaros 2006; Fan et al. 2008; Zou et al. 2009) SSC radiation of the extended prompt emission (e.g., Wei et al. 2006; Wang et al. 2006; Fan et al. 2008; Galli & Guetta 2008; Yu & Dai 2009; Zou et al. 2009) (see Fan & Piran 2008 for a review) Predicted high energy emission from the naked-eye burst GRB 080319B (Zou, Fan & Piran 2009)

16. Are the GRB outflows magnetic rather than baryonic?

17. The non-detection of GeV spectrum excess by Fermi in almost all GRBs: magnetic fireball? (Fan & Piran 2008; Fan 2009: arXiv:0905.0908) synchrotron SSC (magnetized fireball)

18. phasephenomena ImplicationReferences Prompt emission Highly polarized gamma-rays? Ordered magnetic field in the emitting region? Lyutikov et al. 2003; Granot 2003 Prompt emission Absence of thermal emission component Poynting flux dominated outflow? Daigne & Mochkovitch 2002 ; Zhang & Pe’er 2009 Prompt emission Non-detection of the GeV Spectrum excess A strong magnetic field component? Fan 2009 Very early afterglow Bright flash outshining the forward shock optical emission Weakly magnetized reverse shock region? Fan et al. 2002; Zhang et al. 2003; Kumar & Panaitescu 2003 Very early afterglow Absence of bright optical flashes Mildly magnetized reverse shock region? Fan et al. 2004, A&A; Zhang & Kobayashi 2005; Mimica et al. 2009 =magnetic energy density/particle energy density Prompt emission Reverse shock emission? Possible evidence for the magnetized outflow model

19. The low energy spectrum crisis in the case of a baryonic fireball (Cohen et al. 1997; Preece et al. 1998) The magnetic field generated in the shocks is very, very low or decays quickly?

20. A possible solution in the case of baryonic fireball? (Derishev et al. 2001; Derishev 2007) Pro : For typical GRB parameters, within the synchrotron radiation model, the SSC of electrons emitting X-rays is very likely in Klein-Nishina regime Cons: 1. Fine tuning of microphysical parameters 2. Only for hard GRBs (not for X-ray flashes and X-ray flares)

21. Magnetic fireball: spectrum problem (Giannios 2007: gradual magnetic dissipation) GRB 080916C

22. (Lyutikov & Blandford 2003: sudden magnetic dissipation) Poynting flux dominated outflow model

23. Synchrotron radiation: sudden magnetic dissipation (in preparation)

24. Summary The non-detection of GeV spectrum excess in almost all GRBs can be well understood in a number of scenarios. The simplest interpretation may be the magnetized outflow model. The delayed onset of the >100 MeV photons may reflect the physical condition of the early outflow (in particular the breaking out material in the collapsar scenario) For the magnetic fireball, there is a serious low-energy spectrum problem. For the baryonic fireball, there might be more freedom (for example, a hard low energy spectrum can be obtained if the magnetic field generated in the shocks is very, very low or decays quickly).