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Understanding GRBs at LAT Energies

Understanding GRBs at LAT Energies. Robert D. Preece Dept. of Physics UAH. Example Spectrum: GRB990123. Based on Briggs et al. 1999. ~8900 spectral fits from 350 bright BATSE GRBs. OT Synchrotron ‘Line of Death’. Cooling ‘Line of Death’. Kaneko et al. 2006.

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Understanding GRBs at LAT Energies

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  1. Understanding GRBs at LAT Energies Robert D. Preece Dept. of Physics UAH

  2. Example Spectrum: GRB990123 Based on Briggs et al. 1999 Data Challenge II

  3. ~8900 spectral fits from 350 bright BATSE GRBs OT Synchrotron ‘Line of Death’ Cooling ‘Line of Death’ Kaneko et al. 2006 Spectral Observations by BATSE:  • ‘Band’ Function: • Synchrotron emission constrains alpha < –2/3 • Significant fraction of spectra fail • If cooling is taken into account, there is a second limit Data Challenge II

  4. Expected Spectral Performance of GLAST ~ 6 Decades of full energy coverage Precise determination of high-energy power law index Good photon counting statistics at highest energies LAT will be very good at localization; all it needs is one high- energy photon! GBM BGO LAT GBM NaI Data Challenge II

  5. GRB 990123 Simulation: LAT + GBM Data Challenge II

  6. GLAST GRB Science: EPeak • Narrow distribution: GLAST will determine upper limit: esp. for COMP model • Some fits unbounded: (beta > –2) Epeak in LAT range • Red-shift? Cosmological + intrinsic • GLAST will verify Ghirlanda relation (Swift has limited bandpass) BATSE GMB + LAT Coverage Kaneko et al. 2006 Data Challenge II

  7. GLAST GRB Science:  •  > –2 can not continue forever: infinite energy! • No high-energy spectral cut-off has been observed • GLAST will be able to observe 10 keV to ~300 GeV: very long baseline • Low deadtime allows good photon statistics (c.f. Hurley ‘94) • No High-Energy (NHE) bursts exist (no emission > 300 keV) Kaneko et al. 2006 Data Challenge II

  8. Spectral Observations by BATSE:  • 1st order Fermi: Electrons are accelerated by successively reflecting off of 2 converging fluids; magnetic field conveys them across the boundary • PIC simulations of relativistic shocks unanimously predict a constant electron power-law index ~ –2.4, or equivalent photon spectral index ~ –2.2 • BATSE observations of high-energy photon power-law indices clearly contradicts this • However, if there were no acceleration, cooling would take place much faster than observed 1st order Fermi Power Law Decay Data Challenge II

  9. EGRET Observation of 940217 • Persistent hard emission lasted nearly 92 minutes after the BATSE emission ended. • A single 18 GeV photon is observed at ~T+80 min: hardest confirmed event from any GRB. • We have no idea what the spectrum was, nor how it evolved with time (given EGRET’s deadtime)! Hurley et al. 1994, Nature Data Challenge II

  10. GRB 941017: Gonzalez et al. (2003) BATSE EGRET-TASC: Continuum+PL Hard Gamma-ray excess Continuum only Data Challenge II

  11. GLAST and NHE Bursts GRB970111: no-high-energy GRB • Initial, very hard, (alpha ~ +1) portion smoothly transitions to classical GRB • First 6 s spectra consistent with BB • BB kT falling with increasing flux: fading fireball • May be best example of initial fireball becoming optically thin • LAT can determine HE emission with good statistics • LAT upper limits on normal bursts will still provide good science GRB970111 Data Challenge II

  12. GLAST and Quantum Gravity • If certain QG theories are correct, very high energy (VHE) photons will be delayed: • If Spacetime is ‘corrugated’, photon travels farther • Lower energy limit depends somewhat upon theory • Observation is quite tricky: • VHE photon count rate must be actually observable • Must assume a particular relation between energy and time within a GRB: • A relation has already been observed: spectral lag - Norris, et al. • Lag is somewhat correlated with luminousity • Chance coincidence: bright, very hard GRB with very sharp leading edge pulse - increases with mission lifetime Data Challenge II

  13. Challenging Theory: • All Synchrotron emission models predict a maximum energy for accelerated electrons: • No cut-off energy has ever been observed! • Temporal behavior? • Photon starvation? • Extra spectral components: • Hadronic synchrotron? • Compton upscattering? • High-energy physics (e.g. 0 decay)? • Related to Prompt or Afterglow emission? • Origin of spectral lag (Norris)? • Temporal-spectral correlation (Liang, Ryde)? Data Challenge II

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