Brief introduction to the history and science of Gamma Ray Bursts and a report of the latest observational results of the Fermi-Large Area Telescope on GRBs.

1. Observations of Gamma-Ray
Bursts with the Fermi-Large
Area Telescope
Vlasios Vasileiou
Laboratoire Univers et Particules de Montpellier
(CNRS/IN2P3 & Universite Montpellier 2)
for the Fermi LAT and GBM collaborations

2. 2
Overview
●
Gamma-Ray Bursts
●
The Fermi Gamma Ray Space Telescope
●
Scientific highlights from Fermi-LAT GRB observations
●
Systematic Studies
●
Of LAT-detected GRBs: “The first Fermi-LAT GRB catalog”
●
Of non LAT-detected GRBs
●
Non-GRB Science performed with GRBs and Fermi
●
Constraining Lorentz-Invariance Violation
●
Constraining the opacity of the Universe to high-energy
gamma rays.

3. Gamma Ray Bursts

4. Gamma-Ray Bursts
● Bright flashes of gamma rays ~ 1/day
– Brightest events in the gamma-ray sky.
● Discovered accidentally by the U.S. Military
Vela Satellites in 1967 – announced in 1973.
● Have cosmological distances (detected up to
redshift of 8.2).
● Are distributed uniformly in the sky.
2704 BATSE Bursts
First detected GRB
Klebesadel et al. 1973

5. Prompt and Extended Emission
➢Prompt spiky emission
➢Primarily observed in kev — MeV energies
➢No preferred pattern in the lightcurves
➢Non-thermal spectra
➢Typically follow empirical “Band” function
GRB 090916c, Science, Volume
323, Is, 5922, pp. 1688 - (2009)
➢Emission followed by a smooth afterglow
➢Observed in X-rays, visible, IR, optical, and MeV/GeV
➢Exponential decrease in intensity t-1
, t-2
GRB090510 – De. Pasquale et al.
ApJL 709 (2010) L146-L151
• Measured fluences in the 10-8
– 10-3
erg/cm2
range.
• Imply an isotropic energy release Eiso
~1051
- 1054
erg
• Comparable to the rest mass of the Sun (2x1054
erg)
• Jetted (collimated) emission relaxes the energy output down
to ~ 1051
erg

6. Horvath 2002
Hardness Ratio vs Duration
Fluence vs Duration
Duration Distribution
Credit: Pete Woods, UAH & NASA/Marshall.
GRB Progenitors
●
Plots imply the existence of two distinct populations
●
Separation supported by other observables, e.g. galaxy types,
redshift distributions.
●
Short-hard GRBs → believed to be created by mergers of compact
binary systems such as NS-NS or BS-BH.
●
Long-soft GRBs → believed to be created by explosion of massive
rapidly-rotating stars.
●
Both systems end up in a massive rapidly-rotating black hole – torus
system that emits radiation from two relativistic jets.

7. The Fermi
Gamma­Ray Space Telescope

8. LAT Collaboration
~400 members
France
 IN2P3/LUPM Montpellier
 IN2P3/LLR Ecole Polytechnique
 IN2P3/CENBG Bordeaux
 CEA/Saclay
 CESR Toulouse
Germany
 MPI fuer extraterrestr. Physik
Italy
 INFN Bari, Perugia, Pisa, Rome, Trieste, Udine
 ASI
 INAF-IASF
Japan
 Hiroshima University
 ISAS/JAXA
 Tokyo Institute of Technology
Spain
 IEEC-CISC, Barcelona
Sweden
 Royal Institute of Technology (KTH)
 Stockholm University
United States
 Stanford University (SLAC and HEPL/Physics)
 UC Santa Cruz
 Goddard Space Flight Center
 Naval Research Laboratory
 Sonoma State University
 Ohio State University
 University of Washington
 University of Denver
 Purdue University – Calumet
 Yahoo Inc.
The Fermi Gamma-Ray Space Telescope
GBM Collaboration
USA
University of Alabama in Huntsville
NASA Marshall Space Flight Center
Los Alamos National Laboratory
Germany
 MPI fuer extraterrestrische Physik
● Launched August 2008
● Multi-national Collaboration
● Carries two instruments on board
– Large Area Telescope (LAT)
– Gamma-Ray Burst Monitor (GBM)

9. The Gamma-Ray Burst Monitor (GBM)
●
12 x Sodium Iodide (NaI)
−
8keV – 1MeV
−
Used for burst triggering and localization
−
Source direction inferred from the
relative rates in each of the hit NaI
detectors.
●
2 x Bismuth Germanate (BGO)
−
150keV – 40MeV
−
Provide spectral overlap with the LAT
●
Detector arrangement provides coverage to
the whole unocculted by the earth sky (8sr).
●
Together the GBM detectors provide broad 5-
decade-in-energy spectral coverage of
GRBs.

10. The Large Area Telescope
Pair-conversion gamma-ray detector
● Tracker
➢ Measures the direction (and energy), primary particle ID.
● Imaging Calorimeter
➢ Measures the primary's energy
➢ Images the shower (helps with energy/direction rec.)
● Segmented anti-Coincidence Shield
➢ Identifies background of charged Cosmic Rays
● Performance
✔ 20MeV – >300GeV
✔ Wide field of view (2.4sr at 1GeV)
✔ Full sky coverage every ~3h (2 orbits)
✔ Large effective area (8000 cm2 at 1GeV on axis)
✔ Good angular resolution (~0.2o
at 1GeV)

11. GBM + LAT
● GBM+LAT=Wide field-of-view coverage of the sky in
a broad 7-decade-in-energy spectral range
● Usual method of operation:
– GBM triggers
• Gives few-degree-accurate localization for
LAT to observe
• If event bright or hard → also requests
repoint of Fermi → allows for on-axis
sensitive observations by the LAT
– If LAT detects at E>100MeV
• publishes more accurate ~0.3-1o
(sys+stat)
localization through GCN
• Swift & ground-based telescopes observe
→ hopefully detect an afterglow →
hopefully a redshift
• In addition, the LAT continuously performs blind
searches for GRBs both on board and on ground.

12. Automatic Repointing Towards GRB090902B
● Red cross → GRB090902B
● Red/Green lines → The LAT
● White points → Detected events
● White circle → LAT Field of View
● Dark gray region → Earth's shadow
● Yellow dot →Sun
The Sun
GRB090902B
LAT's FOV
The Earth's
Shadow
The LAT
Events

13. Automatic Repointing Towards GRB090902B
● Red cross → GRB090902B
● Red/Green lines → The LAT
● White points → Detected events
● White circle → LAT Field of View
● Dark gray region → Earth's shadow
● Yellow dot →Sun

14. Detections as of 090904
Fermi GRB detection statistics
• The GBM detects ~250 GRBs/year
– ~half in the LAT FoV
• The LAT detects ~10 GRBs/year (35 total)
– ~8% of GBM GRB in LAT's FOV observed
– 26 bursts above 100 MeV →
• ~1/2 with more accurate followup localisations by Swift and ground-based observatories
– Swift XRT/UVOT, GROND, Gemini-S, Gemini-N, VLT
• most of the bursts in this half resulting to a redshift measurement
– Largest z: 4.35 for GRB 080916C, smallest z: 0.74 for GRB090328
• 3 joint Swift/GBM/LAT prompt detections to date
Map as of April 6th
2010

15. Scientific Highlights from
Fermi­LAT GRB
Observations

16. ?GBM NaI
LAT – decay as t-1.2+-0.2
GRB 080916C – our first bright GRB
GBM ← | → LAT
GBM BGO 260keV-5MeV
LAT E>100MeV
GBM BGO 260keV-5MeV
GBM NaI 8keV-260keV
LAT E>1GeV
Delayed emission
appears as a spectral
hardening
Abdo et al. Science
2009, 323, 1688
LAT emission starts delayed and persists longer (up to
1.4ks) with respect to GBM emission.
Highest-energy photon detected: 13GeV at 16.5s
~70GeV in the GRB frame (z=4.35)
➔Constrains dependence of c on Eγ
Minimum bulk Lorentz Factor (γγ opacity arguments):
 Γmin
=887+-21 and 608+-15 for bins b and d
Minimum emission radius:
Rmin
≈Γmin
2
cΔt/(1+z)=9x1055
cm for bin b.
Brightest Eiso
ever measured:
4.3x1054
erg (20keV–2MeV) and 9x1055
(10keV-10GeV)
➔implies a very narrowly-collimated jet.
?
?

17. Additional power-law component
 First time detected in a Short GRB
 Starts delayed in the prompt and persists up to ~200s
 Dominates at E>100MeV and E<20keV
Highest Epeak
=~4MeV ever for a time-integrated spectrum.
Highest-E photon detected ~31GeV at 0.83s post-trigger
 Sets Γmin
=1200 – 1000 interval c
 Sets strongest constraints on dependence of c on Eγ
GRB090510 – our bright short GRB
De Pasquale et al. 2010
Ackermann et al. 2010 ApJ 716
LAT E>1GeV
LAT E>100MeV
BGO 260keV-5MeV
NaI 8keV-260keV

18. GRB090510 – Prompt Emission Models
● Non-thermal leptonic emission – Synchrotron and Self-Compton Synchrotron (SSC)
✗ If magnetic field strong → SSC component too weak to explain LAT observations
– If magnetic field weak SSC component strong→
✗ Strong internal absorption secondary MeV emission ~simultaneous with keV→ →
disagrees with observed 0.2s delay.
✗ Model also faces synchrotron “line-of-death problem” (Band =-0.5+-0.07 > -2/3)α
● Forward-shock from the early afterglow – Synchrotron emission
– Peak of the LAT emission == deceleration time of the relativistic blast wave.
✗ Model requires too high (~1) radiative efficiency for synchrotron emission
– Models fail to explain the lower-energy (E<20keV) extension of the PL
● Hadronic models
– Photohadronic and proton/ion synchrotron processes induce EM cascades secondary e→ -
e+
pairs emit through synchrotron and Inverse Compton
✗ Large makes photopion efficiency lowΓ → requires too large energy release
✗ Stronger magnetic field higher photopion efficiency→ → predicts softer LAT spectrum
✗ Proton synchrotron in strong magnetic field
• Delayed onset == time to accelerate, accumulate, and cool the ultrarelativistic p.
✗ Requires a very collimated beam (=~1o
)→ such strong beaming not found in S.GRBs
 See our GRB090510 paper (Ackermann et al. ApJ 2010 716) for more discussion and references.

19. GRB 090902B – Highest Energy ever detected
● Highest-energy photon detected ever from a
GRB:
– 33.4 GeV at 82s from z=1.822
– 94 GeV in the GRB frame!
● Also this is the second GRB after 090510 (a
short GRB) in which the PL extended at lower
energies.
Abdo et al. 2009, ApJ 706L, 38A

20. GRB 090926A – The first observed cut-off PL
• Extra power-law:
• Starts delayed and persists at longer times (5ks).
• First time ever a cutoff on the extra PL observed.
• Significant at bin c – sharp spike
• Marginally significant at bin d
• Permits direct measurement of Γ=~200–700
• Sharp spike at bin c
• It peaks at all energy ranges synchronized (<50ms)
and with similar widths → Implies PL and Band
related; (co-located or otherwise causally
correlated) ?
LAT E>1GeV
LAT E>100MeV
LAT All events
BGO 260keV-5MeV
NaI 14.3keV-260keV
NaI 8keV-14.3keV
Ackermann et al. 2009
?

21. •Forward Shock (FS) as the jet propagates in the external medium
• Onset time == time required for forward-shock to sweep up enough material and brighten
• Hard to explain rapid HE variability observed in some bursts (e.g. GRB090926A)
• Requires large Γ (larger than that of GRB090926A) or a dense circumburst medium
✗ Synchrotron? → cannot explain correlated LCurves (e.g. GRB090926A)
• IC of Band photons by HE electrons at the FS? → possible & can explain correlated LCurves
• Hadronic models (pair cascades, proton synchrotron)
• 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 low energy extension of the PL
✗ Scenarios require substantially more energy (1-3 orders of magnitude) than observed
✗ Hard to produce correlated variability at low- and high-energies (e.g. spikes of GRB 090926A)
•Leptonic models (inverse-Compton or SSC)
✗ Hard to produce a delayed onset longer than spike widths
✗ Hard to produce a low-energy (<50 keV) power-law excess (as in GRB 090510, 090902B)
✗ Hard to account for the different Band α and of the HE component spectral index.
• But photospheric emission models could explain these properties
●
SSC during late internal shocks
●
Thermal photosphere made by the powerful relativistic wind
●
Magnetic reconnection in Poynting-flux dominated outflows
●
These are just some of the models.. for more see discussions in our papers
More models for the extra component

22. The First LAT Catalog of
Gamma­Ray Bursts

23. The Fermi-LAT GRB Catalog
● First systematic study of GRB properties at high (E>20MeV) energies.
● Covers a 3 year period starting from August 2008 (32 detections)
➢ Will include tabulated data describing important GRB parameters
– Usual GRB properties:
• Duration, average flux, peak flux, time of the peak flux, fluence
– High-energy extended-emission parameters:
• Temporal decay slope, spectral evolution, start/end time
– Prompt emission parameters:
• Delayed onset of the LAT emission, spectral evolution & components
➢ Includes discussions on the unique properties of individual bursts (extra spectral
components, HE spectral cut-offs, analysis caveats).
➢ Includes details on the tools and methods involved in the analysis.
➢ To be submitted soon

24. Extended Emission
PRELIMINARY
GRB090510
[s]
GRB090902B
[s]
GRB080916c
[s]
● Flux decays as a power law in time.
– Power-law temporal decay: f(t)µ ta
with a ~ -1 – -2
– Radiative or adiabatic fireball (Ghisellini et al. 2009)
– No obvious breaks or other features.
GBM Duration

25. Extended Emission - Spectra
● No obvious spectral-evolution
pattern.
● Spectral index typically
averages around -2
GRB090902B
[s]
GRB080916c
[s]
GRB090510
[s]

26. Extended vs Prompt Emission Spectral Index
●ΓEXT
→ Average Spectral index of
extended emission
●β → Spectral index of Band function
in the prompt phase
●Prompt and extended phase spectra
not correlated
βα

27. Energetics
● Long GRBs (filled dots): LAT fluence typically between 10% and 1% of GBM fluence
● Short GRBs (open dots): LAT>GBM fluence
● We detected 4 exceptionally bright bursts: GRB 080916c, 090510, 090902B, 090926A
● They do not appear bright because they are systematically closer to us.
LAT=GBM
LAT=0.01xGBM

28. Highest-Energy Photon Detected
●LAT-detected emission frequently
reaches several-tens-of-GeV
energies (in the GRB frame).
●Good signs for the detection
prospects of VHE observatories.

29. Systematic Study of non
LAT­detected GRBs

30. Explaining the absence of LAT detections
● ~Half of GBM GRBs happen in the LAT FOV, however only ~10% are detected at E>100MeV.
– We investigated why we didn't detect the rest 90%.
● 1st
step: Estimated fraction of GRBs that should had been detected.
– Calculated LAT (0.1-10GeV) ULs (over GBM duration) and compare with predictions from
extrapolation of the GBM-fit (9keV–40MeV).
– Sample: GBM bursts that occurred in the LAT FOV with no LAT detection (161) & Band function
is the preferred model (-20%): Total 126 bursts.
– Result: Number of GRBs with predicted flux>LAT UL (i.e. should have been detected) is ~50%
PRELIMINARY

31. 2nd
step – Examine “bright BGO sample”
● Sample: 30 GBM bursts with Δβ<0.5, rate>75cts/s in
the BGO, and no LAT detection
● Similar to the larger sample: rate of GRBs with higher
predicted LAT flux ~50%.
– Bright sample representative of the parent sample
for purposes of this work
● Performed joint GBM+LAT spectral fits
– HE Spectral index β becomes considerably softer
in the joint fits → fraction of detectable but not
detected bursts down to ~23%. Not detectable by LAT
Detectable by LAT

32. 3rd
step: Repeat joint fits with modified spectral model
● Added spectral softening in the model between BGO
& LAT (exponential cutoff or a step function at 50
MeV) and repeated joint fits.
● The extra softening significantly improved the fit in
6/30 (20%) of these bursts → they require some form
of spectral softening at tens of MeV energies.
● Rest 80% of the bursts consistent with just a softer β.
● Note: Softening vs cutoff ↔ constant vs variable Γ?
GBM only Joint GBM-LAT Joint+step function at 50MeV

33. LAT non-detections – ULs on Γ
● Assuming that the spectral cutoffs in these 6 GRBs are because of internal opacity
effects, we can set ULs on the bulk Lorentz Factors of their jets.
● We only know the redshift for 091127 so we set Γmax
(z) for the rest.
● Results to be published soon.

34. Standard LAT data
LLELLE
BGO
NaI
Spectral cutoffs & the LLE Event Class
● Standard LAT event selections (“Transient”
class) run out of effective area at E<100MeV.
● “LAT Low Energy” (LLE) event selection → Very
relaxed set of cuts → plenty of statistics in the
tens-of-MeV-energy gap to probe GRB spectral
cutoffs.
• See plots for application on GRB110328
LLE
P6_V3_Transient
100MeV
Effective Area for GRB110328

35. 3a. Constraining Lorentz­
Invariance Violation

36. •There is a fundamental scale (the Planck scale λPl
≈10-35
m) at which quantum gravity
(QG) effects are expected to strongly affect the nature of space-time.
• Lorentz symmetry implies a scale-free space-time (all scales are equivalent) → QG
effects may cause violations of Lorentz Invariance (LIV) → speed of light in vacuum may
acquire a dependence on its energy → υγ(Eγ
)≠c.
•The Lorentz-Invariance violating terms are typically expanded using a series of powers
of the photon energy Eγ
over the Quantum Gravity mass MQG
:
where sn
={-1,0,+1} is a model-dependent factor.
•The Quantum-Gravity Mass MQG
• Sets the energy (mass) scale at which QG effects become important.
• Is expected to be of the order of the Planck Mass and most likely smaller than it
Lorentz-Invariance Violation

37. • Since , the sum is dominated by the lowest-order term (n) with sn
0≠ ,
usually n=1 or 2 (“linear” and “quadratic” LIV respectively):
,where sn
=+1 or -1 for subluminal and superluminal speeds respectively.
• There are many models that allow such LIV violations, and some others that actually require
them (e.g. stringy-foam model J. Ellis et al. 2008).
• If the speed of light depends on its energy, then two photons with energies Eh
>El
emitted
together will arrive at different times. For sn
=+1 (speed retardation):
• We want to constraint LIV Set lower limits on M→ QG,n
➢We accomplish that by setting upper limits on the time delay t between photons of differentΔ
energies.
Lorentz-Invariance Violation

38. LAT All events
LAT >100MeV
LAT >1GeV
GBM NaI
8-260keV
GBM BGO
0.26-5MeV
LAT – E vs T
 We set upper limits on the delay Δt by
associating the 31GeV photon with a lower-
energy emission interval.
 The starting time of that interval sets an
upper limit on the time delay Δt
 Most conservative case: 31GeV photon was
not emitted before the start of the GRB:
Δt≤860ms ↔ MQG,1
≥1.19MPl
 Photon was emitted some time after the start
of the main <MeV emission:
Δt≤300ms ↔ MQG,1
≥3.42MPl
 Photon was emitted some time after the start
of the >MeV emission:
Δt≤178ms ↔ MQG,1
≥5.72MPl
 Photon was emitted some time after the start
of the >1GeV emission:
Δt≤99ms ↔ MQG,1
≥10.0MPl
GRB 090510

39. Method #2 – Dispersion Cancellation
 Any energy-dependent time delays in our data would deform the high-energy peaks in
the LAT light curve.
 We can search for the spectral-lag value that cancels any such dispersions and
maximizes the sharpness of the lightcurve.
 A non-zero spectral-lag value would be a result of LIV and/or intrinsic to the GRB.
A simulated GRB light curve with a
20ms/GeV spectral lag.
The same light curve after applying an
opposite lag (peaks now maximally sharp)
*
Scargle J. D. et al. astro-ph/0610571v2

40.  Searched for spectral lags using all the LAT detected events (35MeV-31GeV).
 The curve shows a measure of the sharpness of the light curve (Shannon information)
versus the trial spectral lag.
 The solid vertical line denotes the minimum of the curve, which is our effective spectral-lag
measurement.
 The containment interval denoted by the vertical dashed lines is an approximate error
region, but does not reflect statistical uncertainties.
Finding the spectral lag
 Our effective spectral-lag
measurement:
➔ The lightcurve was already maximally
sharp.
✔ Similar results were obtained after
small changes to the upper energy
limit and the time interval of the used
dataset.

41. Estimating the Statistical Error
 We applied the same method on randomized
datasets (shuffled the times between events)
to measure the uncertainty of the measured
spectral-lag value.
– 99% of the times the randomized data
sets corresponded to a spectral lag
smaller than ±30ms/GeV (90% of the
times in ±10ms/GeV).
 Combined result: symmetric upper limit on the spectral lag coefficient:
|Δt/ΔΕ|<30ms/GeV ↔ MQG,1
>1.22MPl
(99% C.L.) on possible linear (n=1) dispersion of either sign (sn
=±1).
 Limit almost the same as the most conservative limit of the previous method.
Distribution of the
best trial-spectral
lag values in 100
randomized
datasets.

42. Upper Limits Table
●
We constrained small changes in the speed of light caused by linear and quadratic
perturbations in (Eγ
/MQG
).
●
Using two independent techniques, we have placed strong limits on linear perturbations for
both super- and sub-luminal speeds that were all higher than the Planck Mass.
●
Our results support Lorentz invariance and disfavor models in which a quantum nature of
space-time alters the speed of light, giving it a linear dependence on photon energy.
●
More in our paper Abdo et al. Nature 2009, 462, 331A

43. 3b. Constraining the Opacity
of the Universe to High­
Energy Gamma Rays

44. The Extra-Galactic Background Light
● Accumulation of all energy releases in the
form of electromagnetic radiation.
● Includes everything but CMB and the local
foreground emissions (Milky Way, Solar
System, etc.).
● Opacity effect: E>GeV Gamma-rays from
extragalactic sources interact with it through
γγ → e-
e+
● Why is it important?
● Contains information about the evolution
of matter in the universe: SFR, dust
extinction, light absorption and re-
emission by dust, etc.
● Its knowledge is necessary to infer the
actual spectra of extragalactic gamma-
ray sources.
● Observations of spectra that show no signs
of absorption and that extend to >10 GeV
energies from extra-galactic sources can set
upper limits on the opacity of the universe or
equivalently on the density of the EBL.

45. GRB Observations and the EBL
1. Assume intrinsic spectrum extends “as is” (with no extra curvature, breaks, etc.) from
unabsorbed-by-the-EBL energies (say under ~10 GeV) to higher energies.
2. Calculate probability of this assumed intrinsic spectral model giving a detected photon of
energy E≥Emax
(for our actual observation of the source).
● Stecker et al. ('06) Baseline and Fast Evolution
models predict too much opacity → probability for
E≥Emax
applied on our GRB090902B and 080916C
observations too low.
● These results are part of a more comprehensive
paper (Abdo et al. 2010ApJ...723.1082A) that uses
multiple methods on multiple source types (blazars
and GRBs).
● Overall results significantly (>11σ) reject these two
EBL models.
Application to the
Stecker et al.
Baseline model.
The Fast Evolution
model predicts an
even higher
opacity
Area E>Emax
=2x10-4

46. Conclusion
● The Fermi LAT and GBM allowed us to
– detect the keV-MeV-GeV emission from a large sample of bursts and
systematically characterize it,
– explore the relation between the high and low energy emissions,
– constrain current theoretical models on GRBs and guide future research, and
– use GRBs as probes to explore other non-GRB sciences such as particle
physics and cosmology.
• The LAT observations during these first three years have spurred the
development of numerous theories and models for GRB high-energy emissions.
• Now in our next three years, we have to find which ones are correct!
Thank you

47. Backup

48. Beaming corrections to emitted energy
● There are many reasons to believe that GRB
emission is beamed (relativistic beaming,
GRB emission mechanism)
● Beaming angle can be measured by breaks
in the afterglow lightcurves
● After correcting for the case of a beamed
geometry, isotropic energy released
~5*1050
erg
● GRB emission now comparable with the
emission from supernovae
D. A. Frail. Astro-ph/0311301

50. e+ e–
γ
The Large Area Telescope
• Precision Si-strip Tracker
– 18 XY tracking planes
– Single-sided silicon strip detectors (228 µm
pitch), 880,000 channels
– Tungsten foil converters (1.5 X0)
– Measures the photon direction; gamma ID
• Hodoscopic CsI Calorimeter
– Array of 1536 CsI(Tl) crystals in 8 layers
– 3072 spectroscopy chans (8.5 X0)
– Hodoscopic array supports bkg rejection and
shower leakage correction
– Measures the photon energy; images the
shower
• Segmented Anticoincidence Detector
– 89 plastic scintillator tiles
– Rejects background of charged cosmic rays;
segmentation minimizes self-veto effects at
high energy
• Electronics System
– Includes flexible, robust hardware trigger and
software filters
Sub-systems work together to identify and measure the flux of cosmic gammaSub-systems work together to identify and measure the flux of cosmic gamma
rays with energy between 20 MeV and 300 GeVrays with energy between 20 MeV and 300 GeV
Calorimeter
Tracker
ACD
[surrounds 4x4
array of TKR
towers]

51. High Energy Emission from GRBs
● SMM: detected GRBs in the 0.3-9MeV range
– 60% had significant emission above 1MeV
● EGRET: 0.03-30GeV range
– Detected photons above 100MeV from 4 GRBs
– GRB940217: 2 photons at ~3GeV, 1 photon at
18GeV 90 mins after the prompt emission
● Combined BATSE and EGRET data from
GRB941017
● A distinct high energy component extending to
at least 200MeV with no sign of a cutoff.
Gonzalez, et al.,
Nature 424, 847 (2003).

52. Duration Estimation
● GRB T90s are calculated based on the time development of the cumulative
background-subtracted lightcurve.
● In low statistics lightcurves (as in the LAT) → individual fluctuations can
introduce uncertainties in the choice of the plateau and can also “drive” the final
T05/T95.
● To characterize these fluctuations we
perform duration estimations on
simulated lightcurves that are
statistically compatible with the actual
detected lightcurve.
● The final result comes from the
median and +-1σ quantiles of the
simulated T05/T95/T90 distributions.
● Method under development and
verification.
● Improvements include removing the
effects of variable exposure
observations.
GRB090328
Preliminary

53. Method #1
 Associations with individual spikes constrain both
positive and negative time delays (sn
=±1)
 Such associations are not as secure → used as
intuition builders (what we could do)
 31GeV Photon lies at the center of a 20ms-wide
pulse. We constrain both a positive and a negative
time delay:
|Δt|<10ms↔ MQG,1
>102MPl
 750MeV photon & precursor. We place one more
limit on a negative time delay:
|Δt|<19ms↔ MQG,1
>1.33MPl

54. Absorption by the EBL