Signal from GammaRay Bursts

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Signal from Gamma-Ray Bursts

• Nicola Omodei
• INFN Pisa
• Siena Univ.

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Outline

• Tentative GRB definition
• GRB Observations
• Distribution in the sky
• The afterglow
• Energy and spectrum of GRB
• Temporal properties
• Analysis of the Light curves
• Fitting the spikes
• The Auto Correlation Function
• Wavelet analysis
• Power Density Spectra
• Interpreting the data
• Physical model assumption
• The central engine
• What can we learn ?
• A case of interest The QG effect with GRB
• Possibility of further research

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Tentative GRB definition

• GRB Gamma-Ray Bursts )
• Flashes of some seconds of high energy photons in
  the sky
• Isotropic distribution in the sky
• Never seen 2 GRBs coming from the same direction
  (destructive phenomena)
• Cosmological origin accepted -gt Fastest GRB
  observed z 5 (Billion of light years !)
• Phenomena extremely high energetic, and extremely
  short the biggest amount of energy (released in
  a short time) after the Big Bang!
• Related to some kind of explosion Supernovae,
  hypernovae, collapse of massive binaries systems
  (NS,BH)
• Sometime a residual source of X-ray and then
  Optical radiation is seen days or months after
  the burst (afterglow)
• We have data, we have theories, but we dont know
  the nature of this phenomena.

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GRB 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)
• BeppoSax -gt X afterglow (!!)
• HST,Keck -gt Optical Afterglow Founded

GRBs Rate 103 Bursts/yr (BATSE 1
GRB/day) (EGRET 1 GRB/yr)
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Distribution of GRB in the Sky
There is not any correlation with the Galactic
Plane Cosmological Origin!
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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 !!

?
Data from Band et al. 1993, Cohen et al. 1997
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High Energy Emission
2 photons _at_ 3 GeV During the BATSE burst
940217
1 photon _at_ 18 GeV 95 minutes later
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The Afterglow era
BeppoSax Costa et al. 1997
HST Co. look for Other
Afterglow
J.S. Bloom et al. 1997
Magnitudes of the host Galaxy
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Temporal Behavior Time Scales

• Prompt Emission
• Burst Duration -gt 1s 100 s,
• Bimodal
• Variability -gt ms (observed)

• Afterglow
• g AG gt 1 h (105 sec)
• X AG gt 1 d (106 sec)
• Opt AG gt 1 mo (108 sec)

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Light Curves

• The signals from GRB can vary on time scales on
  the order of ms
• The shapes of Light curves are very different
• One smooth spike
• One short spike
• Several peaks
• Intermittent activity
• No periodicity observed

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4 Channel BATSE Data
20-50 KeV
50-100 KeV
100-300 KeV
gt 300 KeV

• Collecting the data in 4 different energy
  channels
• Harder spikes are shorter

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Analysis of the Light Curves

• Empirical shape of the spike (no physics behind)
• Pulse Paradigm Harder spikes are shorter and
  come before (blue lines)

(Norris et al., 1996)
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Fitting the Light Curves
From Andrew Lee PhD thesis (slac-r-553)
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The Auto Correlation Function

• The ACF has a general shape in GRB !

Fenimore et al.1995
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Wavelet analysis

• Flickering many small amplitude flares,
  isolated or crowded
• Wavelets can show the statistical evidence of
  fluctuation above the Poisson noise!
• For a given bin size the wavelet activity is
  defined as the average of the squares of the
  product between the wavelet and the light curve,
  for all relative offset

Haar Wavelet

• For 5 bursts on each time scales the Wavelet
  Activity is divided by the expected activity from
  normal Poisson fluctuations.
• If the activity 1 -gt only Poisson
• The Activity gt 1 for t gt tmin
• tmin represents the rise time of the LC
• 256 µs lt Tmin lt 33 ms

Walker Schaefer, 1998
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GRB PDS
Beloborodov, Stern, Svensson astro-ph/0001401

• The Light curves are different, but their PDF is
  very similar
• The power law they observe in the average PDS of
  GRBs can be interpreted as that GRBs are random
  short realization of a standard process which is
  characterized by the PDS slope a -5/3

Sample 527 Light curves (T90 gt 100 s) 64 ms
resolution
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PDS in different energy channels

• S/N low in channel I and channel IV
• The red power (Pf at low f) decreases at high
  energy photons
• Break at 1 Hz observed
• The affect of the cosmological redshift affect
  the PDS (the net normalization increases, the
  slope in different channels)
• If GRB are SC -gt Dim burst (far) should have a
  flattening of the PDS (in the III ch.)!
• This effect is not seen !!
• -gt GRB can not be associated with Standard
  Candles!
• (the cosmological redshift is not the cause of
  dim burst).
• Dim burst are intrinsically weak!
• (same conclusion from Norris et al 94)

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Interpreting the Signal

• Physical model assumption
• The emitting region moves with relativistic
  motion (otherwise gg absorption)
• Physics of the emission processes (energy
  conversion into radiation)
• Non thermal spectrum (Synchrotron and Inverse
  Compton scattering)
• Nature of the central engine

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Fireball model

• Central engine emits shells of matter (e-, e)
• The shells move with Lorentz Factors 103
• The faster shells collide against the slower
  ones, producing an inelastic shock
• The shock wave accelerates e- and e, which emit
  by synchrotron radiation
• Inverse Compton scattering (self synchrotron
  Compton) can produce high energy photons
• The afterglow is due to the interaction with the
  ISM (External Shock)

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Hypothesis on the 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.
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What can we learn from the signal ?

• Spectrum
• Power law -gt Physics of the synchrotron radiation
  (Magnetic field, particle acceleration)
• Absorption features (observed Fe) suggestion on
  the environment, supernova connection.
• Light Curve
• Temporal scale Rise time and decay time can be
  related to the
• Characteristic time to accelerate particles
• Cooling time of the radiative processes
  (Synchrotron1/vE)
• Other
• Scale length observing a variability dt one can
  always relate dt with the distance dr cdt (the
  causality region) I.e. dt 10-3 s -gt dr 107
  cm (this should be the typical scale of the
  emitting shells)

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The central engine

• The central engine is hidden (the shells, in the
  first stage of their evolution, are optically
  thick and they shield the central engine)
• The temporal variability of the signal seems to
  be connected with the activity of the central
  engine (emitting shells)
• Orbital motion of the two merging objects ?
• Ejection of matter along the jet ?
• Multi-step explosion ?
• In a different scenarios (external shock) the
  variability is related with clouds or blobs
  in the ISM (Dermer et al.) and the variability
  time scales is related to the dimensions of the
  clouds ( 10 AU)

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Simulation of GRB

• We have a simulation tools based on a physical
  model
• Evolution of shells, shocks, Synchrotron
  Inverse Compton
• Temporal variability spectral evolution !

Integrated Spectrum
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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"!
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Phenomenology for Photons
Easiest case NO intrinsic delay (
)
E1
E1
E2
E2
tti
ttf

• D 2 1028 cm, EQG 1019 GeV, c3 1010 cm Þ
  Dt(ms) 60 DE(GeV)

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2 steps in analysis of a time lag

• Quantify intrinsic delay, the pulse paradigm
  can hide the QG effect !!
• This point remains unsolved "no conspiration"
  hypothesis (if there is no observed lag between
  peaks -gt they exclude the possibility to have an
  intrinsic delay that hides the QG effect-gt An
  upper limit is fixed !)
• Find correlations for peaks in different bands.
• Ellis et al.(APJ. 535 (2000) 139) solve 2) by
  looking at simple lonely peak
• Ellis et al.(CERN-TH/2002-258) add a mathematical
  requirement, which works for more complex
  multi-peak structure (Ep gt 6.9x1015 GeV with
  wavelet analysis on BATSEOSSE data)

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Possibility of future research

• Analysis tools development
• CGRO data available on-line
• GRB Simulation available (from physical model)
• Full simulation chain for GLAST satellite (MeV -
  TeV)
• Research for new features in GRB signal
• Could new analysis tools say something more about
  GRBs?
• Diffusion entropy, past history memory
• High energy region almost unobserved GLAST is
  coming, but we have to face new statistical
  problems linked to the low photon rates!!
• Quantum Gravity Effect
• The signal analysis could disentangle the
  intrinsic delay and the QG effect

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Geometry and beaming

• Geometry
• Scale length observing a variability dt one can
  always relate dt with the distance dr cdt (the
  causality region) I.e. dt 10-3 s -gt dr 107
  cm (this should be the typical scale of the
  emitting shells)
• Jets Different aperture angle of a jet and the
  different inclination of the jet with respect to
  the line of view can affect
• The energetic balance Ejet Eiso(W/4p)
  (standard candle ?)
• The knee observed in the afterglow light curve

G high
Log (Flux)
G Low
Log (Time)