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Understanding GRBs at LAT Energies
Understanding GRBs at LAT Energies
Robert D. Preece
Dept. of Physics
UAH
Robert D. Preece
Dept. of Physics
UAH
Mar. 3, 2006 Data Challenge II 2
Example Spectrum: GRB990123Example Spectrum: GRB990123
10-4
10-3
10-2
10-1
100
101
102
103
BATSE SD0 BATSE SD1 BATSE LAD0 BATSE SD4 OSSE COMPTEL Telescope COMPTEL Burst Mode EGRET TASC
GRB 990123
Low-Energy Indexα = -0.6 ± 0.07
- High Energy Indexβ = -3.11 ± 0.07
νFν Peak EnergyEp = 720 ± 10 keV
10-8
10-7
10-6
0.01 0.1 1 10 100
Photon Energy (MeV)
10-4
10-3
10-2
10-1
100
101
102
103
BATSE SD0 BATSE SD1 BATSE LAD0 BATSE SD4 OSSE COMPTEL Telescope COMPTEL Burst Mode EGRET TASC
GRB 990123
Low-Energy Indexα = -0.6 ± 0.07
- High Energy Indexβ = -3.11 ± 0.07
νFν Peak EnergyEp = 720 ± 10 keV
10-8
10-7
10-6
0.01 0.1 1 10 100
Photon Energy (MeV)
Mar. 3, 2006 Data Challenge II 3
OT Synchrotron ‘Line of Death’
Cooling ‘Line of Death’
Kaneko et al. 2006
~8900 spectral fits from 350 bright BATSE GRBs
Spectral Observations by BATSE: αSpectral Observations by BATSE: α
‘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, 2006 Data Challenge II 4
~ 6 Decades of full energy coverage
Precise determinationof high-energy power
law index
Good photon countingstatistics at highest
energies
LAT will be very good at localization; all it needs is one high-energy photon!
Expected Spectral Performance of GLAST
Expected Spectral Performance of GLAST
GBM NaI
GBM BGO
LAT
Mar. 3, 2006 Data Challenge II 5
GRB 990123 Simulation: LAT + GBMGRB 990123 Simulation: LAT + GBM40
30
20
10
BATSE24–120 keV
GRB 990123
-4
-3
-2
100806040200
( )Time Since BATSE Trigger s
-1
-0.5 ( )Assumed BATSE LAT Fit + GBM LAT Joint Fit
1
0.5
Mar. 3, 2006 Data Challenge II 6
GLAST GRB Science: EPeakGLAST 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)
Kaneko et al. 2006
GMB + LAT Coverage
BATSE
Mar. 3, 2006 Data Challenge II 7
GLAST GRB Science: βGLAST GRB Science: β• β > –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, 2006 Data Challenge II 8
Spectral Observations by BATSE: βSpectral Observations by BATSE: β 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: 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, 2006 Data Challenge II 9
EGRET Observation of 940217EGRET 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)!
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, 2006 Data Challenge II 10
GRB 941017: Gonzalez et al. (2003)GRB 941017: Gonzalez et al. (2003)
BATSE
Continuum only
EGRET-TASC:Continuum+PL
Hard Gamma-ray excess
Mar. 3, 2006 Data Challenge II 11
GLAST and NHE BurstsGLAST 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, 2006 Data Challenge II 12
GLAST and Quantum GravityGLAST 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
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