GeV GRBs

1. GeV GRBs Gabriele Ghisellini With the collaboration of: Giancarlo Ghirlanda, Lara Nava, Annalisa Celotti

2. EGRET – GRB 940217 It starts during the prompt at lower energies It lasts much longer The most energetic photon arrives late 18 GeV Prompt or afterglow? Compactness argument?? 1.5 hours

3. GRB 090510 Fermi-LAT • Short • Very hard • z=0.903 • Detected by the LAT up to 31 GeV!! • Well defined timing

4. precursor 8-260 keV 0.26-5 MeV Delay between GBM and LAT. Due to Lorentz invariance violation? LAT all Abdo et al 2009 >100 MeV >1 GeV 31 GeV 0.6s 0.5s Time since trigger (precursor)

5. 0.1 GeV 30 GeV Average 090510 Different component Time resolved 0.5-1s 2 3 nF(n) [erg/cm2/s] If LAT and GBM radiation are cospatial: G>1000 to avoid photon-photon absorption 3 4 Abdo et al 2009 1 Energy [keV]

6. 1/3 (Ek53 /n) tdec~ 0.4 (1+z) seconds 8/3 G3

7. 0.1 GeV 30 GeV Average 090510 Different component Time resolved 0.5-1s 2 3 nF(n) [erg/cm2/s] If LAT and GBM radiation are cospatial: G>1000 to avoid photon-photon absorption 3 4 Abdo et al 2009 1 If G>1000: deceleration of the fireball occurs early  early afterglow! (see also Kumar & Barniol Duran 2009) Energy [keV]

8. No matter the origin of the GeV emission, the bulk Lorentz factor must be large

9. Fermi-LAT t2 t-1.5 background level Ghirlanda+ 2010 T*=0.6s

10. ~MeV and ~GeV emission are NOT cospatial. But the ~GeV emission is… No measurable 0.1-10 GeV delay in arrival time: tdelay<0.2 s  Strong limit to quantum gravity  MQG > 4.7 MPlanck 0.1-1 GeV Ghirlanda+ 2010 >1 GeV T-T* [s]

11. LAT GRBs GG+ 2010

12. background background LAT GRBs GG+ 2010

14. z no z short

15. Time integrated spectra a b G LognFn GBM LAT Band PL Log n

16. a G b Log nFn Log n LAT GBM b vs G a vs G G-values consistent with Zhang+ 2011 G

17. The 8 brightest LAT GRBs z=2, assumed z=1, assumed z=2, assumed

18. The 4 brightest LAT GRBs t-10/7 Spectrum and decay: afterglow; LGeV~Lbol Radiative!

19. The 4 brightest LAT GRBs t-10/7 Radiative?

20. From Beloborodov (2002) e

21. e

22. e e- e+

23. e e- e+

24. LAT GBM Opt

25. 0.1 1 10 102 103 1 10 102 103 1 10 102 103 1 10 102 103 Time [s] Time [s]

26. Problems Fast variability of the GeV emission (Abdo+ 2009)

27. FERMI observations of GRB 090902B: a distinct spectral component in the prompt and delayed emission Abdo+ 2009, ApJ, 706, L138 ”…the observed large amplitude variability on short timescales (≈90 ms) in the LAT data, which is usually attributed to prompt emission, argues against such models.” 090902B

28. 5s Counts per bin

29. Problems Fast variability of the GeV emission (Abdo+ 2009). No evidence Simultaneous GBM-LAT spikes (Ackermann+ 2011; Zhang+ 2011

30. 090926A Ackermann+ 2011

31. LEC Lg,iso,54 e ~ Lsyn R17 G3eB,-1n 2 4 e- e+

32. Problems Fast variability of the GeV emission (Abdo+ 2009). No evidence Simultaneous GBM-LAT spikes (Ackermann+ 2011; Zhang+ 2011 EC scattering of prompt photons? Numbers are ok LAT spectra on the extrapolation of GBM spectra (Zhang+ 2011; with exceptions) if fitted together (but LAT emission lasts longer…) Highest energy photons that arrive after the peak of the LAT light curve are too energetic to be synchro(Piran & Nakar 2010).

33. 13 GeV 33 GeV LAT GRBs GG+ 2010

34. Problems Fast variability of the GeV emission (Abdo+ 2009). No evidence Simultaneous GBM-LAT spikes (Ackermann+ 2011; Zhang+ 2011 EC scattering of prompt photons? Numbers are ok LAT spectra on the extrapolation of GBM spectra (Zhang+ 2011; with exceptions) if fitted together (but LAT emission lasts longer…) Highest energy photons that arrive after the peak of the LAT light curve are too energetic to be synchro (Piran & Nakar 2010). Very few, possible additional component (SSC)?

35. Bulk Lorentz factors G= 900 G=2000 G= 630 G= 670

36. GeV detected GRBS could be the ones with the largest Lorentz factors… For smaller G… 1/3 (Ek54 /n) tdec~ 420 (1+z) seconds 8/3 G2 A factor ~103 dimmer in luminosity, but if nearby… If pair enrichment is required, GeV detected GRBs could be the ones with Epeak(1+z)>mec2 If Epeak < 511 keV and t-1: adiabatic because of no pairs

37. 090510 511 keV Ghirlanda 2009

38. Conclusions • GeV preferentially in Epeak>511 keV GRBs • GeV when Gis large  early onset of the afterglow  very bright • Large EAft: helps to understand Eprompt/EAft

40. Internal shocks: relative kinetic energy of the shells External shocks: entire kinetic energy of the fireball Afterglows should be more energetic than the prompt

41. 0.1 GeV 30 GeV Average Different component Time resolved 0.5-1s 2 3 nF(n) [erg/cm2/s] If LAT and GBM radiation are cospatial: G>1000 to avoid photon-photon absorption 3 4 Abdo et al 2009 1 If G>1000: deceleration of the fireball occurs early  early afterglow! If G>1000: large electron energies  synchrotron afterglow! Energy [keV]

42. Eafterglow < Eprompt Eafterglow ~ 0.1 Eprompt Willingale+ 2007

43. X-ray and optical often behave differently optical X-ray Late prompt?

44. We expected the opposite, if the efficiency of prompt is ~ 0.1. Why is the afterglow so faint? Can it be hidden in some “unexplored” frequency range, i.e. GeV-TeV?

47. Eaft ~ Eprompt/10 Willingale+ 2007

48. Interpretations In GRB 080916C (Abdo et al. 2009a), there is evidence that the spectrum from 8 keV to 10 GeV can be described by the same Band function (i.e. two smoothly connected power laws), suggesting that the LAT flux has the same origin of the low energy flux. On the other hand, the flux level of the LAT emission, its spectrum and its long lasting nature match the expectations from a forward shock, leading Kumar & Barniol–Duran (2009) to prefer the “standard afterglow” interpretation (see also Razzaque, Dermer & Finke 2009 for an hadronic model; Zhang & Peer 2009 for a magnetically dominated fireball model and Zou et al. 2009 for a synchrotron self–Compton origin). In the short bursts GRB 090510 the spectrum in the LAT energy range is not the extrapolation of the flux from lower energies, but is harder, leading Abdo et al. (2009b) to propose a synchrotron self–Compton interpretation for its origin. Instead we (Ghirlanda, Ghisellini & Nava 2009) proposed that the LAT flux is afterglow synchrotron emission, on the basis of its time profile and spectrum (see also Gao et al. 2009; De Pasquale et al. 2009). Finally, the LAT flux of GRB 090902B decays as t−1.5 (Abdo et al. 2009c), it lasts longer than the flux detected by the GBM, and its spectrum is harder than the extrapolation from lower frequen- cies, making it a good candidate for an afterglow interpretation, despite the arguments against put forward by Abdo et al. (2009c), that we will discuss in this paper. Moreover, in GRB 090902B there is evidence of a soft excess (observed in the GBM spectrum below 50 keV) which is spectrally consistent with the extrapolation at these energies of the LAT spectrum.

49. Ackermann 2010: 090510 coincidental peaks in GBM and LAT. SSC code to explain LAT: disfavored, afterglow has less problems. Confusing. Too many indices. De Pasquale 2010 : 090510 Curva di luce e confronto con Swift Ackermann 2011: 090926A: break a 1.4 GeV. Confusione sugli alpha del solo LAT: ripido nel time integrated (come noi) e piattozzo nel time resolved. The delay timescale of the extra spectral component would correspond to the time needed for the forward shock to sweep up material and brighten (Kumar & Barniol Duran 2009; Ghisellini et al. 2010; Razzaque 2010). The rapid variability observed in GRB 090926A is contrary to expectations from an external shock model, unless it is produced by emission from a small portion of the blast wave within the Doppler beaming cone. This could occur, for instance, if the external medium is clumpy on length scale ≈Γf cΔT /(1 + z) ≃ 1012 (Γf /103 )(ΔT /0.2 s) cm, where Γf is the Lorentz factor of the forward shock and ΔT is the pulse duration (Dermer & Mitman 1999; Dermer 2008). Cenko 2010: analysis of afterglows of a few LAT bursts. Ioka 2010 fa tutto, ma non ho capito nente… Kumar- Barniol Duran 2010: fanno LAT e resto dell’afterglow, con closure relations… + calcolo del flusso external shock a 100 MeV + confronto 100 MeV / X-ray e ottico. Tutto adiabatico. B molto molto piccolo (1e-5). Dicono che se fosse radiative si sballerebbe l’X early.

50. Kumar Barniol Duran 2009: I primi a dire external shock. Il lavoro e’ complicato. B~2e-5 Gauss, non ho capito perche’. Larsson+ 2011: There have been many suggestions for the origin of the extra component, including external shocks (Ghisellini et al. 2010; Kumar & Barniol Duran 2010), hadronic processes (Asano et al. 2009; Razzaque 2010), Compton upscattering of a photospheric component (Toma et al. 2010) as well as a combination of different emission mechanisms (Pe’er et al. 2010). Liu 2010: A partially radiative blast wave model, which though is able to produce a sufficiently steep decay slope, can not explain the broadband data of GRB 090902B. The two-component jet model can. Maxham, Zhang 2011: Detailed modelling adia/radia: Fit is good only after the peak. I think they do not include pairs. In any case fit is reasonably good, even if not perfect. McBreen+ 2010: GROND data for 4 LAT: they go on Amati, but not on Ghirlanda (no jet break or too late) Toma+ 2010: photospheric emission scattered by relat. e- in internal shocks (ma come fanno a farla durare piu’ del prompt? E poi anche loo dicno che ci sono problemi nella parte a bassa energia, piu’soft di un BB ma piu’ hard di un sincro coolato).