GammaRay Bursts in the GLAST Era

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Gamma-Ray Bursts in the GLAST Era

• Nicola Omodei (Università di Siena, INFN Pisa)
• Francesco Longo (Università e INFN, Ferrara)

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Gamma Ray Bursts

• Osservazioni GRB
• Modello a Fireball
• Emissione ad Alte Energie
• Tests sulla Gravità Quantistica
• Requisiti strumentali e SW
• Contributi LAT e GBM (credits M.Kippen GBM)
• Osservazioni Multi-?

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Gamma Ray Bursts

• Some Observations
• - From their discovery to the CGRO
• Observational Properties
• - Energy
• - Spectral Properties
• - Temporal Properties
• - The afterglow era
• - High Energy Emission
• Theoretical Models
• - Standard fireball model
• - Internal Shocks model
• - External Shock model
• - Delayed MeV GeV photons
• Central Engine Models

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GRBs Observations
Observed for the first time in 1973 (Klebesadel,
Strong Olsen 1973) They were connected to NS in
the MWG Galactic Origin CGRO/EGRET, 1991 (20
MeV30 GeV) CGRO/BATSE, 1991 (25 KeV10
MeV) Isotropic Distribution in the
Sky Cosmological Origin (Fenimore,Meegan 1992,
Briggs 1995)
GRBs Rate 103 Bursts/yr (BATSE 1
GRB/day) (EGRET 1 GRB/yr)
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Energy of the GRBs

• Flux (g) (0.1-10) x 10-6 erg/cm2 (W/4p)

Galactic Origin E 1045 1046 erg
Cosmological Origin E 1051 1052 erg
M E/c2 1 Msol
Systematic Error !!
(Cohen, Narayan, Piran 1998,
Lingenfelter et al. 1997)
Data from Band et al. 1993, Cohen et al. 1997
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Spectral Properties
?
In the BATSE energy range (25 KeV 10 MeV)
Double Power Low
(D.L. Band et al. 1993)
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Temporal PropertiesDuration
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Light Curves
Complex in shape !
We can distinguish between two different time
scales The duration of the burst T 1-10
seconds The Variability of the burst (Duration
of the spikes) t 10-3 T (Millisecond
Variability)
(Walker, Schaefer Fenimore, 2000)
There is no evidence of the variability for E gt
10 MeV because the EGRETs deadtime was 100 ms

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Broadband Observations The Afterglow

• BeppoSAX 1997

Good Spatial resolution (lt arcmin) Observation of
the X-Afterglow (Costa et al. 1997)

• Optical Afterglow (HST, Keck)

Direct observation of the host galaxies
Distances determination (Groot 1997, van
Paradijs et al. 1997)
Redshift 1 5
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The Afterglow era
J.S. Bloom et al. 1997
Magnitudes of the host Galaxy
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High Energy Emission
2 photons _at_ 3 GeV During the BATSE burst
1 photon _at_ 18 GeV 95 minutes later
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High Energy Emission
There are photons _at_ 102 MeV During the BATSE burst
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Theoretical models of GRB
Standard Fireball Model - Relativistic motion of
the emitting region - Shock mechanism converts
the kinetic energy of the shells into radiation.
High Energy External Shock - Synchrotron SSC -
High conversion efficiency - Not easy to
justify the rapid variability
Soft Energies Internal Shocks - Synchrotron
Emission - Rapid time Variability - Low
conversion efficiency
Interaction Between the high energy particles
produced in the first stage with the external
e.m. field (Background Radiation)
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The compactness problem
Pair Production Compton Scattering Black
Body radiation !!
The Emitting region has to move Relativistic
Correction
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To beam or not to beam ?
Relativistic Beaming
The radiation emitted from a source that is
moving with a Lorenz factor G toward the
observer appears beamed in a cone of aperture 1/ G
1. If the emission is spherical it will appear
collimated
2. If the source has jets it will be visible
only if q G -1
f
q
G -1
Relativistic Doppler Effects
Relation between the time as measured by GLAST
and the time measured in the source frame
Observed Flux by GLAST
Relation between the energy of a photon as
observed by GLAST and the energy emitted of the
photon
(A.Dar 2001)
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The Fireballs model (an artistic view)

• The source has to be a compact object (from the
  observed time variability)
• The central engine is hidden
• The conversion of the kinetic energy into
  radiation is provided by shocks
• The observed variability seems to be directly
  connected with the variability of the central
  engine

Piran 1999
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The Shape of a spike
Rise Time Geometry of the Shell
dtrise 10-6 s
Decay Time Cooling Time
dtdecay 10-4 10-3 s
FRED Fast Rise Exponential Decay
If the first shell slows down the conversion
efficiency is higher !
(Fenimore Ramirez-Ruiz 1999)
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External Shock Model - (an artistic view)

• The Blast wave interacts with the surrounding
  medium (SN Remnant?)
• The initial Lorentz factor of the shell is
  closely related to the baryon loading
• As the blast wave sweeps up and captures material
  from the surrounding environment, it decelerates,
  becomes energized and emits radiation

Synchrotron - Synchrotron Self Compton -
Synchrotron Self Absorption gg ee-
(C.D. Dermer, J.Chang,E.Mitman, M. Böttcher,
1999)
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External Shock Model - HE Emission
The ESM predicts a Soft to Hard evolution of the
spectrum during the early afterglow phase
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External Shock Model The Afterglow era
In the late afterglow phase the evolution of the
spectrum is Hard to Soft
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Time evolution of the spectrum in the ESM
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Delayed MeV-GeV gamma ray photons
Cheng Cheng 1996
Without the presence of IMF !!
EGRET !
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Central Engine

• Merging of compact objects (NS-NS, NS-BH,
  BH-BH). These objects are observed in our Galaxy,
  the merging time is about 108 yr.
• Supranovae, Hypernavae very massive star that
  collapses in a rapidly spinning BH.
  Identification with SN explosion.

Cosmic Origin Very Massive BH (106 Msol)
one spike for each star it captures. (Carter
1992, Cheng Lu 2001) Galactic Origin NS
are expelled from the Disk of the Galaxy into the
Halo. (Li Dermer 1992,Podsiadlodwski, Rees
Ruderman 1995)
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Work in progress...
Starting from GRB fireball scenario Internal-
external shock model with particular attention to
the High Energy emission - Possibility of
recycling the electrons after the pair production
in the high energy production mechanisms
(Inverse Compton) - GRB event generator number
of photons emitted as function of time and
as function of the frequency (intrinsic
spectrum). - Absorption through the IGM and
propagation of the photons in the
expanding universe (luminosity distance,
redshift,) The main purpose is not to understand
what GRBs are, but is to obtain a simple tool for
testing the capabilities of the analysis
software. Developing a model that is consistent
with the observed properties and with the theory
of GRB that emulates the main features of the
signal that arrives at the detector.
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Test of Quantum Gravity
E Photon energy EQG Effective
quantum gravity energy scale Deformed dispersion
relation with function f model dependent
function of E/EQG c2 P2 E2 ( 1 a(E/EQG )
O(E/EQG ) 2) v dE/dP c ( 1 a(E/EQG
)) Vacuum as quantum-gravitational medium which
respond differently to the propagation of
particle of different energies. Medium
fluctuation at a scale of the order of Lp10-33
cm Dt a E/EQG D/c
c2 P2 E2 ( 1 f(E/EQG ))
D 2 1028 cm EQG 1019 GeV Dt(ms) 60
DE(GeV)
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Instrument Requirements

• GLAST - LAT will detect high-energy radiation
  from approximately 50 to 100 bursts per year (as
  compared to 1 per year for EGRET)
• GLAST will study the relationship between GeV
  emission and keV-MeV emission as a function of
  time during the burst. How does the high-energy
  spectral form and peak energy change with time?
• Measurements of intrinsic burst spectra at these
  energies can constrain bulk Lorentz factors of
  relativistic fireball models and provide
  measurements of cutoffs due to absorption on the
  extragalactic background light at energies as low
  as 100 GeV for large redshifts.

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Instrument Requirements

• Ability to quickly recognize and localize GRBs
• Field of view gt 2 sr to monitor a substantial
  fraction of the sky at any time.
• Spectral resolution better than 20, especially
  at energies above 1 GeV, for sensitive spectral
  studies and searches for breaks
• Sustain random photon rate of 10 kHz with less
  than 20 deadtime for determining correlations
  between low energy and high energy gamma-ray
  burst time structure
• A goal in the ability to repoint the spacecraft
  autonomously in lt 5 minutes
• Single photon angular resolution approaching 10
  arcminutes at high energies for good source
  localization.
• Rapid (few seconds) notification of the burst and
  its position to the ground.
• Burst notifications to be rapidly (few 10's of
  seconds) sent from the ground
• Capability to simultaneously measure the low and
  high-energy components.

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GLAST and Gamma-Ray Bursts
Composite spectrum of 5 EGRET Bursts

• Little is known about GRB emission in the gt50 MeV
  energy regime
• EGRET detected 5 high-energy bursts, but
  suffered from
• Small field of view (40), so few bursts were
  detected
• Small effective area (1000 cm2), so few detected
  photons per burst
• Large deadtime (100 ms/photon), so few prompt
  photons were detected
• Prompt GeV emission with no high-energy cutoff
  (combined with rapid variability) implies highly
  relativistic bulk motion at source G gt
  102103
• Extended or delayed GeV emission may require more
  than one emission mechanism

Dingus et al. 1997
No evidence of cutoff
Hurley et al. 1994
Extended/Delayed emission
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GLAST and Gamma-Ray Bursts

• The GLAST LAT will have
• Large 2 sr field of view, so more detected
  bursts (50100/yr)
• gt10? EGRET effective area, so more photons per
  burst
• 105? lower deadtime, so more detected prompt
  photons
• Improved sensitivity Egt10 GeV, for better
  locations and spectral range
• 5? better angular resolution, for arc-min GRB
  locations and better afterglow sensitivity
• On-board computing for providing rapid GRB
  locations to afterglow observers
• The GLAST LAT will not have
• Sensitivity lt10 MeV, where there is the most
  knowledge of GRBs
• Sensitivity outside its FoV
• Fast trigger for weak bursts

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Role of the GLAST Burst Monitor (GBM)

• LAT will provide ground-breaking new GRB
  observations, but it will be difficult to
  evaluate them in the context of current GRB
  knowledge

• GBM will enhance GLAST GRB science by providing
  low-energy context measurements with high time
  resolution
• Improved GBMLAT wide-band spectral sensitivity
• Compare low-energy vs. high-energy temporal
  variability
• Continuity with current GRB knowledge-base
  (GRO-BATSE)
• GBM will provide rapid GRB timing location
  triggers w/FoV gt LAT FoV
• Improve LAT sensitivity and response time for
  weak bursts
• Re-point GLAST/LAT at particularly interesting
  bursts for afterglow observations
• Provide rapid locations for ground/space
  follow-up observations IPN timing

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Instrument Requirements
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Instrument Design
12 Sodium Iodide (NaI) Scintillation Detectors
2 Bismuth Germanate (BGO) Scintillation Detectors
Data Processing Unit (DPU)

• Characteristics
• Energy range 5 keV to 1 MeV
• Major Purposes
• Provide low-energy spectral coverage in the
  typical GRB energy regime over a wide FoV
• Provide rough burst locations over a wide FoV

• Characteristics
• Energy range 150 keV to 30 MeV
• Major Purpose
• Provide high-energy spectral coverage to overlap
  LAT range over a wide FoV

• Major Purposes
• Flexible burst trigger algorithm(s)
• Automatic detector/PMT gain control
• Compute on-board burst locations
• Issue r/t burst alert messages

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Detector Placement Concept
Low-Energy NaI(Tl) Detectors (3 of 12)
LAT
High-Energy BGO Detector (1 of 2)
Top View
Side View
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GRB Spectral Performance

• Simulated GBM and LAT response to time-integrated
  flux from bright GRB 940217
• Spectral model parameters from CGRO wide-band fit
• 1 NaI (14 º) and 1 BGO (30 º)
• Baseline 8000 cm2 LAT _at_ 30º

• Good spectral response over 5.3 decades in
  energy!
• In addition to providing low-energy parameters,
  combined fit yields better constraints on
  high-energy power-law index than LAT-only fit

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Time-Resolved Spectroscopy Performance

• Simulation of bright GRB990123
• Model parameters taken from BATSE fits
• Same detector response as previous example
• GBM detectors can easily measure evolution of
  low-energy spectral parameters
• LAT alone cannot detect evolution of high-energy
  index b
• LATGBM can detect evolution of b

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Mission/Instrument Operations
LAT (IOC)
GLAST Science Operations Center (SOC)
GLAST Mission Operations Center (MOC)
Science User Community
GBM (IOC)

• Including Inst. Team members
• Scientific data analysis
• Including final GRB locations and joint GBM/LAT
  spectral timing analysis

• Archive processed science data
• Distribute data to user community

• GBM/LAT commands
• GBM/LAT monitoring
• Compute rapid GRB locations LAT/GBM
• Distribute GRB alerts via GCN

• Monitor instrument operation and performance
• Flight s/w updates
• Generate Inst. commands
• Routine science processing

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GRB Software
Module Input
Outputs Function
Utilities
Detection Time range,
Times, directions, Spatial Temporal
Photon retrieval
other SC info uncertainties,
Trigger real time
rates assignment photon
ACD, GBM LAT to burst
Pulse Profiles Photon, times,
Pulse Description Max Likelihood
Photon retrieval
energies
Bayesian decomposition
Exposure
Spectroscopy Photons, times, Spectral
Fit Spectral analysis
Photon retrieval
energies

Exposure
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GRB multi-? studies
GCN network
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GRB multi-? studies
SWIFT (2003-)
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Conclusions

• GRB unsolved mystery
• Towards Early Cosmology
• High Energy Emission
• New Physics(?)
• Unique Mission for GRB
• Great experience in Italy (BeppoSAX, AGILE, ..)