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Fermi Observations of Gamma-ray Bursts

Fermi Observations of Gamma-ray Bursts. Masanori Ohno(ISAS/JAXA) on behalf of Fermi LAT/GBM collaborations. GRB940217(Hurley et al. 94). GRB941017 (Gonzaletz et al. 03). -18 to 14 sec 14 to 47 sec 47 to 80 sec. 80-113 sec 113-211 sec. HE emission from GRBs : Pre-Fermi Era.

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Fermi Observations of Gamma-ray Bursts

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

  2. GRB940217(Hurley et al. 94) GRB941017 (Gonzaletz et al. 03) -18 to 14 sec 14 to 47 sec 47 to 80 sec 80-113 sec 113-211 sec HE emission from GRBs : Pre-Fermi Era GeV photons up to 90min after the trigger - Many observations in keV-MeV - Little is known about HE (>100 MeV) emission from GRBs Temporary distinct HE spectral component 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 Deciphering the Ancient Universe with Gamma-Ray Bursts

  3. Fermi Gamma-ray Space Telescope LAT Gamma-ray BurstMonitor (GBM) • 12 NaI detectors (8keV-1MeV) • - onboard trigger , localization • - spectroscopy • 2 BGO detectors (150keV-40MeV) • - spectroscopy (overlapping LAT band) 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 Deciphering the Ancient Universe with Gamma-Ray Bursts

  4. Fermi GRBs 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 ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? GRB941017 shows the sign of extra component • 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 ? Suggests another emission mechanism A few GRBs show delayed high energy emission (GRB940217, GRB080714) (Quantum gravity model, EBL…) Deciphering the Ancient Universe with Gamma-Ray Bursts

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

  7. What we have seen from Fermi GRB observations • 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 ? Suggests another emission mechanism A few GRBs show delayed high energy emission (GRB940217, GRB080714) (Quantum gravity model, EBL…) Deciphering the Ancient Universe with Gamma-Ray Bursts

  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 Γmin Γmin can be derived using observed highest energy photon 090510 E=31 GeV 080916C E=3 GeV 090902B E=33 GeV z Γmin~1000 for short and long GRBs Deciphering the Ancient Universe with Gamma-Ray Bursts

  9. GRB 090926A: the first HE spectral cutoff Preliminary ! 8-14.3keV Time-integrated photon spectrum(3.3-21.6s) 14.3-260 keV νFν(erg/cm2/s) 0.26-5 MeV LAT all event 10 102 103 104 105 106 Energy (keV) - 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-γabsorption) >100 MeV >1GeV (See Uehara’s poster #095) Deciphering the Ancient Universe with Gamma-Ray Bursts

  10. What we have seen from Fermi GRB observations • 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 ? 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) Deciphering the Ancient Universe with Gamma-Ray Bursts

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

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

  13. Constraint on QG and EBL models GRB 090510 GRB 090902B Abdo et al. 2009, Nature 462, 331 Abdo, A. A. et al., ApJL 706, 138 (2009) GBM NaI GBM BGO LAT (>1MeV) 31 GeV 0.83 s Constraints on the quantum gravity mass (MQG) by direct measurement of photon arrival time 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 MQG,1/Mplank > 1.19 Disfavors quantum gravity models which linearly alters the speed of light (n=1) Deciphering the Ancient Universe with Gamma-Ray Bursts

  14. Models for HE delayed onset and extra-PL • 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 Deciphering the Ancient Universe with Gamma-Ray Bursts

  15. Summary of LAT GRBs Detections as of 090904 Detections as of 090904 Deciphering the Ancient Universe with Gamma-Ray Bursts

  16. Summary of LAT GRBs Detections as of 090904 Detections as of 090904 Delayed onset and long-lived HE emission is common feature of LAT GRBs ? Deciphering the Ancient Universe with Gamma-Ray Bursts

  17. Summary of LAT GRBs Detections as of 090904 Detections as of 090904 Deciphering the Ancient Universe with Gamma-Ray Bursts

  18. Summary of LAT GRBs Detections as of 090904 Detections as of 090904 Deciphering the Ancient Universe with Gamma-Ray Bursts

  19. Long vs Short GRBs Abdo, A. A. et al., ApJ 712, 558 (2010) Preliminary ! short short short • 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 Deciphering the Ancient Universe with Gamma-Ray Bursts

  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 • Extra component of the prompt emission ? -Clear evidence of extra PL component from 3 LAT GRBs -Low-energy excess is also seen • What is the maximum energy of HE emission ? -Constraint lower limit of bulk Lorentz factor: Γmin ~1000 -GRB 090926A, first detection of HE spectral cutoff : Γ ~ 630 • HE emission is delayed and/or long-lived ? -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 ?) • Constraint on QG and EBL models Deciphering the Ancient Universe with Gamma-Ray Bursts

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