1. High energy (20MeV-TeV) photon emission from Gamma-ray Bursts Yi-Zhong Fan (Niels Bohr International Academy, Denmark; Purple Mountain Observatory, China ) Collaborators: Tsvi Piran, Ramesh Narayan, Da-Ming Wei, Bing Zhang ( Fan & Piran 2008, arXiv:0805.2221 )

2. GRB internal-external shock model central photosphere internal external shocks engine (shocks) (reverse) (forward) 1E17cm for ISM 1E6cm 1E9cm 1E12-1E14cm 1E15cm for wind gamma-ray UV/opt/IR/radio gamma-ray X-ray UV/optical IR mm radio (see Piran 1999, 2004; Meszaros 2002; Zhang & Meszaros 2004 for reviews) prompt promptemission afterglow

3. MeV-GeV observations (EGRET) GRB 940217 GRB 940217 (Hurley et al. 1994) GRB 930131 (Superbowl Burst) The first afterglow detection, but no redshift information afterglow

4. MeV-GeV observations (EGRET): GRB 941017: Gonzalez et al. 2003 Quick evolution Almost constant Almost constant Much longer high energy emission

5. VHE (>50 GeV) Observations Milagrito observation of GRB 970417 at energies above ∼ 0.1 TeV ( 3 σ? Atkins et al. 2000 ) Upper limits from Magic for several Swift bursts (Albert et al., 06) Claims of detection GRAND at 2.7 σ (Poirier et al 03, but see Fragile et al 03 ) Tibet array: 7σ coincidence ? (Amenomori et al 01) ARGO-YBJ array find only upper limits (Di Sciascio, et al., 06) Only upper limits!

6. The optical depth of universe to VHE gamma-rays (Stecker et al. 2006)z=5 z=3 z=1 z=2 z=0.5 z=0.03 z=0.2 tau~6

7. Physical processes producing high energy gamma-rays Synchrotron radiation (Syn-Rad) of electrons/protons Inverse Compton processes Pion production Electromagnetic cascade of TeV gamma-rays see Fan & Piran (2008 ) for a review see Fan & Piran (2008 ) for a review

8. Inverse Compton (IC) processes electronsSeed photonstimescale Synch. Self- Compton (SSC) Shock-acceleratedSynch. radiation of the electrons Simultaneous with Synch. Rad External IC (EIC) Shock-acceleratedIrrelevant to the electrons and along the direction of outflow material Much Longer than that of the seed photons Bulk ComptonMoving with a high bulk Lorentz factor (cold in their comoving frame) Irrelevant to electrons and arbitrary distributed Determined by that of the ejecta carrying the electrons see Fan & Piran (2008 ) for a review see Fan & Piran (2008 ) for a review

9. One novel feature of EIC The EIC emission lasts much longer than the seed photons because the duration is affected by (1) the spherical curvature of the blast wave (Beloborodov 05) and by (2) the highly anisotropic radiation of the up-scattered photons (Fan & Piran 06) Fan, Piran, Narayan & Wei (2008 ) Fan, Piran, Narayan & Wei (2008 )

10. High energy photons from Pion production

11. Electromagnetic cascade of TeV photons TeV source Infrared background Seed photon MCB GeV photons (Nikoshov 1962; Gould & Schreder 1967)

12. High energy processes in GRBs and afterglows

13. SSC in GRBs and their afterglows Prompt SSC SSC ForwardShock  e ≈1000  e ≈tens  e ≈ tens  e ≈10 4 -10 Reverse Shock Shock SSC SSC From Piran (2003)

14. Standard forward shock (Fan et al. 08) SSC Synch Energy electron’s Lorentz Factor SSC energy Duration Prompt (GRB) 100 keV1000100GeVPrompt Prompt (XRF) 10 keV3001 GeVPrompt X-ray flares0.2 keV5000.2GeVShort Reverse Shock 1 eV10010keVShort Forward Shock 10keV-1eV10 4 -10TeV-keVLong (M’esz’aros & Rees 94; Pilla & Leob 98; Pe’er & Waxman 04; Gupta & Zhang 07, 08 Guetta & Granot 03; Wei et al. 06; Wang et al. 06; Fan et al. 08; Galli & Perna 08 Wang et al. 01a,b; Granot & Guetta 03; Pe’er & Waxman 04b;Kobayashi et al. 07 Wang et al. 01a,b; Granot & Guetta 03; Pe’er & Waxman 04b; Kobayashi et al. 07 Dermer et al. 00; Sari & Esin 01; Zhang & M´esz´aros 01; Wei & Fan 07; Galli & Piro 07; Gou & Meszaros 07; Yu et al. 07) Internal shocks (Pe’er & Waxman 04) External shock model: GeV-TeV Standard internal shocks (Pe’er & Waxman 04) Standard forward shock (Fan et al. 08)

15. IC of very early afterglow IC: RS emission + FS electrons IC: FS emission + RS electrons (Wang et al. 2001a,b; Granot & Guetta 2003; Piran et al. 2004) SSC of FS SSC of RS (Both reverse shock (RS) and forward shock (FS) exist)

16. IC of very early afterglow The energy of RS electrons to total energy of RS+Fs electrons Importance of RS IC emission timescale Weak RS ~0.1 (Nakar & Piran 04) UnimportantLonger than the prompt emission Relativistic RS~0.5 Important but dominated by EIC Longer than the prompt emission Prompt photon flow The prompt photon flow overlaps RS/FS shock regions and the cooling of RS/FS electrons may be dominated by EIC, and GeV-TeV EIC plateaus are produced ( Beloborodov 05: EIC in RS; Fan, Zhang & Wei 05, ApJ629 : EIC in RS + FS ) (in GRB 080319B: prompt optical photons cool the FS electrons+prompt gamma-rays cool the RS electrons)

17. Is strong reverse shock popular? Bright optical flashes, predicted in RS model, are detected only in a few bursts (Akerlof et al. 1999; Fox et al. 2003; Li, W. et al. 2003; Boer et al. 2006; Klotz et al. 2006; Roming et al. 2006) Even for these limited detections, the afterglow modeling usually suggests a weakly magnetized RS region (Fan et al. 2002; Zhang et al. 2003; Kumar & Panaitescu 2003; Wei et al. 2006; Klotz et al. 2006). A stronger magnetization may account for the non-detection in other events (Fan, Wei & Wang 2004; Zhang & Kobayashi 2005; Giannios et al. 2008). The IC emission of reverse shock is expected to be weak in most cases (cf. Kobayashi et al. 2007)

18. EIC in early afterglow (Wang et al. 06; Fan & Piran 06; Fan et al. 2008) Fan & Piran (2006 ) Fan & Piran (2006 ) Any central engine afterglow photons

19. EIC in early afterglow (Fan, Piran, Narayan & Wei 2008)

20. Bulk Compton in GRBs and their afterglows Shemi (94) and Shaviv & Dar (95a,b) suggested that the ultra-relativistic GRB ejecta was moving into a dense soft photon background and the electrons in the ejecta Compton scattered on the photons and boosted them to MeV-GeV (producing GRB prompt emission) Bulk Compton in GRB internal shocks (Takagi & Kobayashi 05), producing GeV-TeV emission (efficiency ~1E-3) Bulk Compton in GRB afterglows (Panaitescu 08a, b), producing flares, plateaus followed by a sharp drop, some X-ray flattening and GeV emission The late outflow launched by the re-activity of the central engine has to have a Gamma~10 4 and is electron/positron dominated

21. Electromagnetic cascade of TeV photons In the presence of intergalactic magnetic field (B_IGM), the magnetic deflection angle of the electron/positron at a radius R_IC they lose most of their energy through IC scattering the CMB The time-delay caused by the magnetic deflection is B IGM <10 -18 G is needed to get detectable GeV emission signatures. It is not clear that whether such a small value is realistic within a radius ~10 Mpc to the GRB host galaxy. (Plaga 95; Cheng & Cheng 96; Dai & Lu 02; Guetta & Granot 03; Wang et al. 04; Razzaque et al. 04; Murase et al. 07; Ichiki et al. 07) z~0.1 and B IGM ~10 -20 Gauss (Murase et al. 07)

22. pion production The collision of relativistic nucleons (Gamma~300) with a dense cloud to produce \pi^0 ( Katz 94; to account for the 18 GeV photon detected in afterglow of GRB 940217 ) Pions produced in standard GRB internal shocks ( Waxman & Bahcall 97; Gupta & Zhang 07 ) Neutron rich GRB outflow: inelastic n, p collision produces \pi^0 ( Bahcall & Meszaros 01; Meszaros & Rees 01 ) The neutral beam model ( Dermer & Atoyan 04; >10 18 eV neutrons created in p+\gamma process escaped from internal shocks and were subjected to further photopion processes with photons……; see also Ioka et al. 04 )

23. Interpretation of the EGRET data

24. We need to reproduce the spectrum and the nearly constant count rate. The SSC of an X-ray plateau followed by a sharp decline? High energy afterglow of GRB 940217: the SSC component of a slowly decaying X-ray light curve? (Wei & Fan 2007) GRB 940217 (Hurley et al. 1994) Afterglow emission 30MeV- 30GeV

25. The ~0.2 GeV hard component of GRB 941017: the EIC of the RS+FS shocks in wind medium? (Fan & Piran 08 based on Beloborodov 05 + Fan, Zhang & Wei 05) 3. The MeV-GeV emission has a very hard spectrum F v ~v 0 1. The MeV-GeV plateau has a duration about 3 times that of the sub-MeV emission 2.The MeV-GeV emission energy is at least ~3 times that of the sub-MeV emission Prompt emission overlaps FS+RS (Sari & Piran 99; Fan et al. 05): EIC works The timescale favors a process relevant to the very early afterglow (Granot & Guetta 04) Suppose that a significant part of MeV-GeV emission is powered by the RS, the reverse shock has to be relativistic Fan et al. 08

26. What can GLAST tell us?

27. Constrain the physical composition of the GRB outflow ( If magnetized, no GeV-TeV excess: Giannios 08 ), the particle acceleration models and the radiation mechanisms Probe the initial Lorentz factor of the GRB ejecta ( Lithwick & Sari 01; Dai & Lu 02; Fan & Wei 04 ) and the radius of the prompt energy dissipation ( Gupta & Zhang 08 ) Test current various modifications of forward shock model that introduced to account for the peculiar Swift X-ray data ( Fan et al. 08 ) Do something with GLAST detections

28. Is there a canonical high energy afterglow light curve? (Fan, Piran, Narayan & Wei 08) (Fan, Piran, Narayan & Wei 08) ? ? ? Given the small number of high energy photons, these novel features are not expected to be identified as frequently as in X-ray band. But in some extremely bright bursts, like GRBs 940217, 941017 and 030329, more than 1000 sub-GeV photons may be collected.

29. MAGIC-II, HESS-II: ~30 GeV photons from GRBs? z=1

30. Thank you!

31. EIC in early afterglow (something to be clarify) The total energy to be emitted into GeV energies is that of the blast wave and does not strongly depend on the brightness of the central engine afterglow (e.g., X-ray flares). The SSC of the early forward shock also peaks at GeV energies. In the absence of EIC, the SSC will convert a significant of blast wave energy into high energy emission. So the EIC can not enhance the GeV detection significantly. After taking into account the SSC of the forward shock before and after the X-ray flare, the detected high energy lightcurve should be a plateau rather than a GeV flare. The SSC of the X-ray flares can be distinguished as sub-GeV (Wei et al. 2006; Wang et al. 2006; Fan et al. 2008)or even GeV-TeV flashes (Galli & Piro 2007; Fan et al. 2008) The SSC of the X-ray flares can be distinguished as sub-GeV (Wei et al. 2006; Wang et al. 2006; Fan et al. 2008)or even GeV-TeV flashes (Galli & Piro 2007; Fan et al. 2008)

32. Stern & (2004) Stern & Poutanen (2004) Naked-eye GRB 080319B: energetic GeV source (Zou et al. 08)? Syn SSC 2nd IC SSC and 2nd IC in GRBs? SSC and 2nd IC in GRBs?