GammaRay Bursts as seen by FGST

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Gamma-Ray Burstsas seen by FGST

• Nicola Omodei, INFN Pisa
• for the Fermi Collaboration

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Outline

• What is known and what is left open
• Gamma-Ray Burst and the Fermi Gamma-Ray Space
  Telescope (FGST)
• Early results
• Perspectives and conclusions

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GRB history, in bullets

• DISCOVERY SPECULATION 1967 1991 (Vela,
  Ginga, SMM)
• What are those things? Disruption of NS? Are
  their distribution isotropic? Are there lines in
  their spectra? Do they repeat?
• Answer Lets launch a big mass of scintillator
  material, with full sky coverage and Localization
  capabilities gt CGRO/BATSE
• POPULATION STUDIES 1991 1997 (CGRO/BATSE)
• Isotropy, Uniformity, and Bimodality
• Some hint at high energy (EGRET)
• COUNTERPARTS! or THE REAL ASTRONOMERS GET
  INVOLVED... 1997 2006 (Beppo Sax )
• X-rays afterglow discovered by Beppo-Sax.
• Begins the Afterglow era (many observatories
  involved)
• Swift connection of the prompt phase (BAT) to
  X-ray (XRT) and UV (UVOT) afterglow.
• Crucial role of GCN.
• Still many issues open
• GAMMA RAYS REDUX 2006 -- ??
• IACT decrease their threshold to sub-100 GeV
  range
• Agile launched in 2007 (x-rays tens of GeV)
• Fermi (previously known as GLAST) Launched in
  2008 (keV to hundreads of GeV)

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Gamma-Ray Burst prompt spectrum

• Well studied by BATSE (lt few MeV)
• Mostly Smoothly joint broken power law (Band at
  al. 93)
• Some Comptonized model exponential cut-off at
  high-energy
• Some Pure power-law (Epeak not constrained)
• Swift spectra lt150 keV Epeak not constrained in
  most of the cases

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Afterglow

• Discovered by BeppoSax (97)
• Swift
• Connection to the Prompt emission
• X-Ray Flashes in the afterglow
• Steep-Shallow-Steep decay
• Also short bursts have an afterglow!
• Fading to lower frequencies
• (New questions/new answer) possibly increased ??

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Connecting Prompt and Afterglow The
Relationships

• Amati ( et al. 2002) Epeak-Eiso (Epeak,
  redshift)
• Ghirlanda (et al. 2004) Epeak-Egamma (Epeak,
  redshift, Tbreak)
• Firmani (et al. 2006) bring T45 into Epeak and
  Eiso.
• Fenimore Ramirez-Ruiz (2000) Eiso -
  variability in Gamma-rays
• Norris (et al. 2000) lag-luminosity relation.
  Short lag luminous.
• (Lags measured in observer frame i.e. not
  z-corrected)
• Use these relationships to infer a
  pseudo-redshift from a measurement of spectral
  parameters or lags and luminosities in observer
  frame.... many more bursts without measured z
  than with.
• gtGRB as cosmological probe

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The puzzle of the High energy some open issues
This bursts remains an enigma, and it is a
perfect key study for GLAST
Agile burst (Giuliani et al 08) Extended
Emission?
GRB941017 BATSE EGRET joint analysis 2
separate components Independent time evolution
(extended HE emission) Spectral index of the HE
component -1 Cut-off at higher energies
where? How common in GRB?
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Many questions need an answer
Large Area Telescope-LAT US (NASADOE), France,
Italy, Japan Sweden
Gamma-ray Burst Monitor GBM Marshall SFC, UAH,
MPE
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The Gamma-Ray Burst Monitor
Multitude of trigger algorithms ? 58 GRB in 2
months 300 GRB per year including XRF
GBM Localization lt 15o initially, update lt5o
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The Large Area Telescope (LAT)

• Fermi LAT is opening a new discovery window
• Extended energy range up to hundreds GeV!
• Exploring dts down to µsec!
• Early result 1 GRB/Month seen by LAT !

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MISSION ELEMENTS
Large Area Telescope GBM
m

sec
GPS

-

Telemetry 1 kbps
Fermi Spacecraft

TDRSS SN S Ku
DELTA 7920H


S
-
-

GN

LAT Instrument Science Operations Center
White Sands
Schedules
HEASARC
Mission Operations Center (MOC)
Science Support Center
Schedules
GBM Instrument Operations Center
GRB Coordination Network (GCN)
Alerts
Data, Command Loads
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Study GRB with the Fermi observatory

• Spectrum studied over 7 decades!
• Bright burst study of the cut-off, if any.
• Detailed temporal/spectral evolution
• Is there any extra component?
• How common is the extended/delayed GeV emission?
• Pseudo-redshift estimators
• Epeack, Egamma,Duration, Variability, lag
  provided by Fermi
• Need redshift, Tbreak
• improve statistics
• new relations?
• Time Lag in pulses as a function of energy
• Intrinsic lag vs cosmological effect (QG)
• Observations are needed to understand how
  particles are accelerated in GRB, up to what
  energies, and how they emit gamma-rays are
  emitted. Constrain the LF of the expanding
  shells.
• DISCOVERIES (???)

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-GeV connection

• This scenario gets even more exiting if other
  observatories are involved
• X-ray- GeV connection
• Simultaneous detection of X-Ray afterglow with
  swift -gt Better location with XRT!
• X-Ray flashes GeV emission at late time (Galli
  et al. 2007)
• Opt, NIR Telescopes - Afterglow
• TeV connection
• LAT can send trigger to GCN in few sec., with
  localization.
• Few GRB/yr will have LAT location lt0.1 degrees,
  suitable for TeV observation
• TeV Absorbed gt Low threshold CTA!
• Are there Very-High-Energy delayed emission?

EGRET - 5 GRBs in spark chamber. Difficult to
(i) make predictions based on these low numbers
(ii) extrapolate from BATSE based on possibility
of distinct temporal and spectral components at
high energies
Normal Incidence 60 off axis
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1st LAT GCN GRB080825C
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The bright 080916C
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GRB080916C
Black region Earth limb (removed)
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LAT Status, perspectives and conclusions

• LAT looks on ground for emission from all known
  GRB triggers.
• GBM and LAT see 2 bursts in common implying 1
  burst in common with LE instruments per month.
  Magnitude of flux seen in LAT seems related to
  prompt emission GBM spectral index ß.
• LAT ground processing also performs blind search
  for GRB.
• Currently no independent onboard LAT GRB
  trigger... now under test
• LAT trigger will operate both based on GBM
  trigger and independently based on clustering in
  time and space.
• Independent trigger would have found GRB 080916c,
  but not GRB 080825c.
• Also coming soon Automatic Repoint Recommendation
  to put bright/hard GBM bursts in LAT FoV for
  late prompt and afterglow emission 1 per week
  TBD for 5 hours TBD.
• Much analysis of common bursts in progress, both
  for prompt and afterglow emission
• Multi-frequency approach crucial to continued
  progress of GRB science
• Good coordination and quick response from the
  Fermi GRB team (GBMLAT) and observers