Nicola Omodei University of Siena and INFN, Pisa

1
GLAST and GRB probing the photon propagation
over cosmological distances

• Nicola Omodei (University of Siena and INFN,
  Pisa)
• Johann Cohen-Tanugi (INFN, Pisa)
• Francesco Longo (University and INFN, Trieste)
• On behalf of the GLAST GRB and Solar Flare
  Science Team

2
Quantum Gravity effect

• Within various theoretical frameworks
• String Theory/ Loop Quantum Gravity
• Discretized or non-commutative spacetimes
• A Lorentz invariance breaking dispersion law
  rises

Mp is a mass scale Planck Mass 1019 GeV
"Photons propagating in vacuum may exhibit non
trivial refractive index"!
3
Puzzling experimental facts

• G.Amelino Camelia Phys.Lett. B528 (2002) 181-187
• Space-time quantum solves three experimental
  paradoxes
• 3 experimental puzzles recently found in
  different areas
• 1) UHECR paradox observations of cosmic rays
  with energy incompatible with the GZK cutoff
• 2) Mrk501 paradox 24 TeV photon which should
  require
• a very un-physical spectrum of the source
  given the
• gamma-gamma absorbtion with the 3K background
• 3) Pion stability paradox longitudinal
  development of
• hadronic showers UHE pions more stable than
  lower energy.
• Unique solution proposed
• deformation of standard dispersion relation

4
Phenomenology for photons
2 photons, E1 and E2, emitted with delay ?t will
arrive with a delay
measures sensitivity
Test of Quantum Gravity a simple view
5
Sensitivity

• ?E the level arm
• for the instrument (Instrumental limit)
• for the observed energies (Observing a source)
• ?t the time resolution
• the time resolution of the instrument
  (Instrumental limit)
• the binning time to have enough statistic
  (Observing a source)
• L the typical distance of the sources
• If the instrument doesnt see any delay
• Eqg gt (L?E)/(c ? t)
• If I can see a delay ?t
• Eqg (L?E)/(c ? t)

6
The case for GLAST
From GLAST characteristics ?E (GBM LAT) 10
keV 300 GeV dt 100 µs Assuming z 1 for
the source (L1028cm)
Estimated Upper limit for GLAST
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The case for GLAST
EGRET highest limit for Eqg (104 lower than GLAST)
But, we have also to consider the low rates at
high energies. To have enough statisticsThe CRAB
(flaring) dt10-3
The GRBs are dim sources, far and with a short
time scale
GRB are good candidates to probe the Quantum
Gravity up to the Plank scale !!
8
On going activities Simulation

• At high energy (gt100 MeV) the number of
  expected photons from a GRB is low
• (lt 103-4) it is reasonable simulate the
  signal from GRB photon by photon.
• Simulation of the incoming flux
• Background Albedo, Cosmics, Diffuse gamma
• GRB simulation QG dispersion law
• The detector is illuminated in the correct way,
• taking into account the orbit and the
• tilting of the satellite
• Simulation of the Detector
• Geant4, Digitization, Reconstruction
• Triggers and GRB Alerts
• The onboard trigger are taken into account
• All the algorithms can be tested on the simulated
  
• data

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On going activities GRB modelling
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On going activities GRB modeling
The intrinsic delay
Gamma- Ray Burst may exhibit an intrinsic delay
related to the emission processes (i.e.
synchrotron cooling time) or geometry effect
(the angular spreading).This delay, could even
erase the effect of QG !

• Shock hydrodynamic Crossing Time
• The number of accelerated electrons depends on
  the time (depends on the geometry of the shell,
  but not on the energy of the observed photons).

11
On going activities GRB modeling
The intrinsic delay
Gamma- Ray Burst may exhibit an intrinsic delay
related to the emission processes (i.e.
synchrotron cooling time) or geometry effect
(the angular spreading).This delay, could even
erase the effect of QG !

• Radiative cooling time
• Depends on the energy of the photons

Electron all accelerated Time ta
Electron that are cooling Time ta tsyn(hv)
gc(t)
gm
gM
gm
gM
Electron all accelerated Time ta
Electron that are cooling Time ta tsyn(hv)
gc(t)
gm
gM
gm
gM
12
QG observation?

• Gamma-Ray Burst at z 1.
• Observing the burst at GBM energies
• For Eqg 1019 GeV at 100 KeV dt 10-5 s
• All the delay will be intrinsic (different
  cooling time, geometry effects,)!
• Observing the burst at LAT energies
• For Eqg 1019GeV at 10 GeV dt 1 s, at 1 GeV
  dt .1 s !
• Minimizing the uncertainties with
• Physical GRB model (computes the intrinsic delay
  at high energy)
• GRB statistics (different redshift different
  QG effect, but same intrinsic delay)

The events at LAT energies reconstructed by
GLAST For Energies up to 1GeV the peak is time
resolved!! At 10 GeV only few photons
13
QG observation?

• Broken or Deformed Lorentz Symmetry?
• Study of Noncommutative SpaceTimes
• Broken Lorentz symmetry (Matusis et al. 2000)
• Deformed Lorentz symmetry (AmelinoCamelia, 2002b)
• Testing with GLAST
• p2 ? E2 m2 w?plnE2n
• w gt 0 for Broken symmetry
• w lt 0 for Deformed symmetry