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

Mar. 3, 2006Data Challenge II2 Example Spectrum: GRB990123

Mar. 3, 2006Data Challenge II3 OT Synchrotron ‘Line of Death’ Cooling ‘Line of Death’ Kaneko et al ~8900 spectral fits from 350 bright BATSE GRBs 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  ‘Band’ Function:  Synchrotron emission constrains alpha < –2/3  Significant fraction of spectra fail  If cooling is taken into account, there is a second limit

Mar. 3, 2006Data Challenge II4 ~ 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! Expected Spectral Performance of GLAST GBM NaI GBM BGO LAT

Mar. 3, 2006Data Challenge II5 GRB Simulation: LAT + GBM

Mar. 3, 2006Data Challenge II6 GLAST GRB Science: E Peak Narrow distribution: GLAST will determine upper limit: esp. for COMP model Some fits unbounded: (beta > –2) E peak in LAT range Red-shift? Cosmological + intrinsic GLAST will verify Ghirlanda relation (Swift has limited bandpass) Kaneko et al GMB + LAT Coverage BATSE

Mar. 3, 2006Data Challenge II7 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: 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

Mar. 3, 2006Data Challenge II8 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

Mar. 3, 2006Data Challenge II9 EGRET Observation of  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)!  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

Mar. 3, 2006Data Challenge II10 GRB : Gonzalez et al. (2003) BATSE Continuum only EGRET-TASC: Continuum+PL Hard Gamma- ray excess

Mar. 3, 2006Data Challenge II11 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: 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

Mar. 3, 2006Data Challenge II12 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  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