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Gamma-Ray Bursts as seen by FGST

Gamma-Ray Bursts as seen by FGST. Nicola Omodei, INFN Pisa for the Fermi Collaboration. 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. GRB history, in bullets.

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Gamma-Ray Bursts as seen by FGST

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  1. Gamma-Ray Burstsas seen by FGST Nicola Omodei, INFN Pisa for the Fermi Collaboration

  2. 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

  3. 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: Let’s launch a big mass of scintillator material, with full sky coverage and Localization capabilities => 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)

  4. Gamma-Ray Burst prompt spectrum • Well studied by BATSE (< 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 <150 keV: Epeak not constrained in most of the cases

  5. 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 ??

  6. 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 T45into Epeakand 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. • =>GRB as cosmological probe

  7. 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?

  8. Many questions need an answer Large Area Telescope-LAT US (NASA+DOE), France, Italy, Japan Sweden Gamma-ray Burst Monitor – GBM Marshall SFC, UAH, MPE

  9. The Gamma-Ray Burst Monitor Multitude of trigger algorithms ⇒ 58 GRB in ~2+ months ~300 GRB per year including XRF • GBM Localization: < 15o initially, update <5o

  10. Tracker ACD [surrounds 4x4 array of TKR towers] e– e+ Calorimeter The Large Area Telescope (LAT) • Fermi LAT is opening a new discovery window: • Extended energy range: up to hundreds GeV! • Exploring dt’s down to µsec! • Early result: 1 GRB/Month seen by LAT !

  11. 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 Mission Operations Center (MOC) Science Support Center HEASARC Schedules GRB Coordination Network (GCN) GBM Instrument Operations Center Alerts Data, Command Loads

  12. 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 (???) NaI BGO LAT

  13. *-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 -> 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 <0.1 degrees, suitable for TeV observation • TeV Absorbed => 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

  14. 1st LAT GCN: GRB080825C

  15. The bright 080916C

  16. GRB080916C Black region: Earth limb (removed)

  17. 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 β. • 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 (GBM+LAT) and observers

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