1. Fermi Observations of Gamma-ray Bursts Masanori Ohno(ISAS/JAXA) on behalf of Fermi LAT/GBM collaborations April 19, 20101 Deciphering the Ancient Universe with Gamma-Ray Bursts

2. HE emission from GRBs : Pre-Fermi Era GRB940217(Hurley et al. 94) -18 to 14 sec 14 to 47 sec 47 to 80 sec 80-113 sec 113-211 sec GRB941017 (Gonzaletz et al. 03) GeV photons up to 90min after the trigger Temporary distinct HE spectral component April 19, 20102 Deciphering the Ancient Universe with Gamma-Ray Bursts - Many observations in keV-MeV - Little is known about HE (>100 MeV) emission from GRBs 1) Distinct HE spectral component ? 2) Maximum photon energy (cut-off ?) 3) Long-lived HE emission ? Important key for emission mechanism and environment of GRBs Need more sensitivity, larger FoV

3. Fermi Gamma-ray Space Telescope Gamma-ray Burst Monitor ( GBM ) 12 NaI detectors (8keV-1MeV) - onboard trigger, localization - spectroscopy 2 BGO detectors (150keV-40MeV) - spectroscopy (overlapping LAT band) LAT Silicon-Strip detectors - Identification &direction measurement of γ-rays CsI calolimetor - Energy measurement ACD (plastic scintillators) - background rejection -Efficient observing mode -Wide FoV -Low deadtime -Large effective area -Good angular resolution -Energy coverage More photons from Many GRBs April 19, 20103 Deciphering the Ancient Universe with Gamma-Ray Bursts

4. Fermi GRBs April 19, 20104 Detections as of 090904 The GBM detects ~250 GRBs/year (~400 total) – ~18% short – ~50% in the LAT FoV The LAT detects ~10 GRBs/year – 17 total as of today (recent detection :100225A, 100325A, and 100414A) – ~10% of GBM GRBs observed Deciphering the Ancient Universe with Gamma-Ray Bursts

5. What we have seen from Fermi GRB observations 1. Extra component of the prompt emission ? 1. Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? GRB941017 shows the sign of extra component What is the maximum energy of HE emission ? What is the maximum energy of HE emission ? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. HE emission is delayed and/or long-lived ? HE emission is delayed and/or long-lived ? Suggests another emission mechanism A few GRBs show delayed high energy emission (GRB940217, GRB080714) (Quantum gravity model, EBL…) April 19, 20105 Deciphering the Ancient Universe with Gamma-Ray Bursts

6. Extra PL component in short and long GRBs Abdo, A. A. et al., ApJL 706, 138 (2009) Abdo, A. A. et al., ApJ submitted GRB 090902B (long)GRB 090510 (short) First time a low-energy extension of the PL component has been seen April 19, 20106 3 LAT GRBs shows extra PL component (090510, 090902B, 090926A) First extra component by Fermi At > 5 sigma level Deciphering the Ancient Universe with Gamma-Ray Bursts T0+4.6s to T0+9.6s

7. Extra component of the prompt emission ? Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? Only GRB941017 shows the sign of extra component 2. What is the maximum energy of HE emission ? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. HE emission is delayed and/or long-lived ? HE emission is delayed and/or long-lived ? Suggests another emission mechanism A few GRBs show delayed high energy emission (GRB940217, GRB080714) (Quantum gravity model, EBL…) April 19, 20107 Deciphering the Ancient Universe with Gamma-Ray Bursts What we have seen from Fermi GRB observations

8. Limit on bulk Lorentz factor Due to large luminosity and small emitting region, optical depth for the γ-γ -> e+e- pair production is too large to observe the non-thermal emission from GRB  compactness problem. Relativistic motion (Γ>>1) could avoid this compactness problem Γ min can be derived using observed highest energy photon April 19, 20108 Γ min ~1000 for short and long GRBs z Γ min Deciphering the Ancient Universe with Gamma-Ray Bursts 090510 E=31 GeV 090902B E=33 GeV 080916C E=3 GeV

9. GRB 090926A: the first HE spectral cutoff Preliminary ! - Delay in HE onset: ~3 s - The extra component shows at >5 σ spectral break at ~1.4 GeV - First direct measurement of Γ ~ 630 (if cutoff due to γ-γ absorption) April 19, 20109 8-14.3keV 14.3-260 keV 0.26-5 MeV LAT all event >100 MeV >1GeV Deciphering the Ancient Universe with Gamma-Ray Bursts Time-integrated photon spectrum(3.3-21.6s) νF ν (erg/cm 2 /s) Energy (keV) 10 10 2 10 3 10 4 10 5 10 6 (See Uehara’s poster #095)

10. Extra component of the prompt emission ? Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? Only GRB941017 shows the sign of extra component What is the maximum energy of HE emission ? What is the maximum energy of HE emission ? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. 3. HE emission is delayed and/or long-lived ? Suggests another emission mechanism A few GRBs show delayed high energy emission (GRB940217, GRB080714) April 19, 201010 Deciphering the Ancient Universe with Gamma-Ray Bursts What we have seen from Fermi GRB observations

11. Long-lived GeV emission ~ Swift and Fermi view of GRB 090510 ~ De Pasquale et al., ApJL 709, 146 (2010) Forward shock model can reproduce the spectrum from the optical up to GeV energies Extensions needed to arrange the temporal properties t  1.38  0.07 Simultaneous fit of the SED at 5 different times LAT emission until 200 s No spectral evolution (photon index -2.1 ± 0.1) April 19, 201011 GRB 090510 (short GRB) UVOT XRT Fermi/LAT Deciphering the Ancient Universe with Gamma-Ray Bursts

12. HE delayed onset in short and long GRBs The first few GBM peaks are missing in the LAT but later peaks coincide Delay in HE onset: 0.1-0.2 s Abdo et al. 2009, Science 323, 1688 The first LAT peak coincides with the second GBM peak Delay in HE onset: ~4-5 s Abdo et al. 2009, Nature 462, 331 GRB 080916C (long) GRB 090510 (short) HE delayed onset can be seen from almost all LAT GRBs April 19, 201012 8-260keV 0.26-5MeV LAT all events >100 MeV >1GeV Deciphering the Ancient Universe with Gamma-Ray Bursts

13. Constraint on QG and EBL models April 19, 201013 Deciphering the Ancient Universe with Gamma-Ray Bursts Constraints on the quantum gravity mass (M QG ) by direct measurement of photon arrival time M QG,1 /M plank > 1.19 Disfavors quantum gravity models which linearly alters the speed of light (n=1) Most models are optically thin for 33 GeV photon from GRB 090902B (z=1.822) “baseline” and “fast evolution” models are rejected at 3.6 σ level Abdo et al. 2009, Nature 462, 331 GRB 090510 GRB 090902B Abdo, A. A. et al., ApJL 706, 138 (2009) 31 GeV GBM NaI GBM BGO LAT (>1MeV) 0.83 s

14. Leptonic models (inverse-Compton or SSC) (Toma et al., 2009) –Hard to produce a delayed onset longer than spike widths –Hard to produce a low-energy (<50 keV) power-law excess – Hard to account for the different photon index values of the Band spectrum at low energie (but photospheric models can) and of the HE component – But, photospheric models could explain these properties (Toma et al. 2010) Hadronic models (pair cascades, proton synchrotron) (Asano et al., 2009) – GRBs as possible sources of Ultra-High Energy Cosmic Rays – Late onset: time to accelerate protons & develop cascades? – Proton synchrotron radiation (requires large B-fields) – Synchrotron emission from secondary e± pairs produced via photo-hadron interactions can naturally explain the power-law at low energies require substantially more energy than observed (GRB 090510: Etotal / Eiso ~ 100-1000) – Hard to produce correlated variability at low- and high-energies (e.g. spikes of GRB 090926A) ? Early Afterglow (e+e- synchrotron from external shock) (Kumar et al, 2009) – Can account for possible delayed (~9 s) onset of power-law component in GRB 090902B – Short variability time scales in LAT data (e.g. GRB 090926A) argues against external shock – Requires larger bulk Lorentz factor than measured for GRB 090926A Models for HE delayed onset and extra-PL April 19, 201014 Deciphering the Ancient Universe with Gamma-Ray Bursts

15. Detections as of 090904 Summary of LAT GRBs Detections as of 090904 GRB Angle from LAT Duration (or class) # of events > 100 MeV # of events > 1 GeV Delayed HE onset Long-lived HE emission Extra spectral comp. Highest photon Energy Redshift 080825C ~ 60° long~ 100 ? ✔ X~ 600 MeV 080916C 49° long14514 ✔✔ ?~ 13.2 GeV~ 4.35 081024B 21° short~ 102 ✔✔ ?3 GeV 081215A ~ 86° long————--— 090217 ~ 34° long~ 100XXX~ 1 GeV 090323 ~ 55° long~ 20> 0? ✔ ?3.57 090328 ~ 64° long~ 20> 0? ✔ ?0.736 090510 ~ 14° short> 150> 20 ✔ ✔ ✔ ~ 31 GeV0.903 090626 ~ 15° long~ 20> 0? ✔ ? 090902B 51° long> 200> 30 ✔ ✔ ✔ ~ 33 GeV1.822 090926 ~ 52° long> 150> 50 ✔ ✔ ✔ ~ 20 GeV2.1062 091003A ~ 13° long~ 20> 0? ? ?0.8969 091031 ~ 22° long~ 20> 0? ? ?~ 1.2 GeV 100116A ~ 29° long~ 103? ? ?~ 2.2 GeV April 19, 201015 Deciphering the Ancient Universe with Gamma-Ray Bursts

16. Detections as of 090904 Summary of LAT GRBs Detections as of 090904 GRB Angle from LAT Duration (or class) # of events > 100 MeV # of events > 1 GeV Delayed HE onset Long-lived HE emission Extra spectral comp. Highest photon Energy Redshift 080825C ~ 60° long~ 100 ? ✔ X~ 600 MeV 080916C 49° long14514 ✔✔ ?~ 13.2 GeV~ 4.35 081024B 21° short~ 102 ✔✔ ?3 GeV 081215A ~ 86° long————--— 090217 ~ 34° long~ 100XXX~ 1 GeV 090323 ~ 55° long~ 20> 0? ✔ ?3.57 090328 ~ 64° long~ 20> 0? ✔ ?0.736 090510 ~ 14° short> 150> 20 ✔ ✔ ✔ ~ 31 GeV0.903 090626 ~ 15° long~ 20> 0? ✔ ? 090902B 51° long> 200> 30 ✔ ✔ ✔ ~ 33 GeV1.822 090926 ~ 52° long> 150> 50 ✔ ✔ ✔ ~ 20 GeV2.1062 091003A ~ 13° long~ 20> 0? ? ?0.8969 091031 ~ 22° long~ 20> 0? ? ?~ 1.2 GeV 100116A ~ 29° long~ 103? ? ?~ 2.2 GeV April 19, 201016 Deciphering the Ancient Universe with Gamma-Ray Bursts Delayed onset and long-lived HE emission is common feature of LAT GRBs ?

17. Detections as of 090904 Summary of LAT GRBs Detections as of 090904 GRB Angle from LAT Duration (or class) # of events > 100 MeV # of events > 1 GeV Delayed HE onset Long-lived HE emission Extra spectral comp. Highest photon Energy Redshift 080825C ~ 60° long~ 100 ? ✔ X~ 600 MeV 080916C 49° long14514 ✔✔ ?~ 13.2 GeV~ 4.35 081024B 21° short~ 102 ✔✔ ?3 GeV 081215A ~ 86° long————--— 090217 ~ 34° long~ 100XXX~ 1 GeV 090323 ~ 55° long~ 20> 0? ✔ ?3.57 090328 ~ 64° long~ 20> 0? ✔ ?0.736 090510 ~ 14° short> 150> 20 ✔ ✔ ✔ ~ 31 GeV0.903 090626 ~ 15° long~ 20> 0? ✔ ? 090902B 51° long> 200> 30 ✔ ✔ ✔ ~ 33 GeV1.822 090926 ~ 52° long> 150> 50 ✔ ✔ ✔ ~ 20 GeV2.1062 091003A ~ 13° long~ 20> 0? ? ?0.8969 091031 ~ 22° long~ 20> 0? ? ?~ 1.2 GeV 100116A ~ 29° long~ 103? ? ?~ 2.2 GeV April 19, 201017 Deciphering the Ancient Universe with Gamma-Ray Bursts

18. Detections as of 090904 Summary of LAT GRBs Detections as of 090904 GRB Angle from LAT Duration (or class) # of events > 100 MeV # of events > 1 GeV Delayed HE onset Long-lived HE emission Extra spectral comp. Highest photon Energy Redshift 080825C ~ 60° long~ 100 ? ✔ X~ 600 MeV 080916C 49° long14514 ✔✔ ?~ 13.2 GeV~ 4.35 081024B 21° short~ 102 ✔✔ ?3 GeV 081215A ~ 86° long————--— 090217 ~ 34° long~ 100XXX~ 1 GeV 090323 ~ 55° long~ 20> 0? ✔ ?3.57 090328 ~ 64° long~ 20> 0? ✔ ?0.736 090510 ~ 14° short> 150> 20 ✔ ✔ ✔ ~ 31 GeV0.903 090626 ~ 15° long~ 20> 0? ✔ ? 090902B 51° long> 200> 30 ✔ ✔ ✔ ~ 33 GeV1.822 090926 ~ 52° long> 150> 50 ✔ ✔ ✔ ~ 20 GeV2.1062 091003A ~ 13° long~ 20> 0? ? ?0.8969 091031 ~ 22° long~ 20> 0? ? ?~ 1.2 GeV 100116A ~ 29° long~ 103? ? ?~ 2.2 GeV April 19, 201018 Deciphering the Ancient Universe with Gamma-Ray Bursts

19. Long vs Short GRBs April 19, 201019 Comparable LE and HE gamma-ray outputs for short GRBs Long GRBs seem to emit ~5-20 times less at HE than at LE w.r.t. short GRBs short Abdo, A. A. et al., ApJ 712, 558 (2010) Preliminary ! Deciphering the Ancient Universe with Gamma-Ray Bursts short

20. Summary  Fermi detected ~400 GRBs including 17 LAT GRBs in ~1.5 years => 250 GRBs/year for GBM and ~10 GRBs/year for LAT April 19, 201020 Extra component of the prompt emission ? Extra component of the prompt emission ? What is the maximum energy of HE emission ? What is the maximum energy of HE emission ? HE emission is delayed and/or long-lived ? HE emission is delayed and/or long-lived ? -Clear evidence of extra PL component from 3 LAT GRBs -Low-energy excess is also seen -Constraint lower limit of bulk Lorentz factor: Γ min ~1000 -GRB 090926A, first detection of HE spectral cutoff : Γ ~ 630 -Many LAT GRBs show delayed and long-lived high energy emission Many leptonic or hadronic models are proposed for LAT high energy emission No difference of high energy properties between short and long GRBs (but lower energy in high energy for long GRBs ?) Deciphering the Ancient Universe with Gamma-Ray Bursts Constraint on QG and EBL models