Gamma-Ray Burst Working Group

1
Gamma-Ray Burst Working Group
Co-conveners Abe Falcone, Penn State,
afalcone_at_astro.psu.edu David A. Williams, UCSC,
daw_at_scipp.ucsc.edu
February 8, 2007
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VHE g-Rays from GRB?

• Amenomori et al. 1996 (Tibet AS-??
• 6? excess at 10 TeV in stacked analysis of 57
  BATSE GRB
• Padilla et al. 1998 (HEGRA AIROBICC)
• 2.7? excess above 20 TeV from GRB 920925c
• Atkins et al. 2000 Atkins et al. 2003
  (Milagrito)
• 3? excess (post-trials) from GRB 970417a gt650
  GeV no evidence from 53 other bursts
• Atkins et al. 2005 Saz Parkinson 2005 (Milagro)
• Limits gt100 GeV from 33 GRB during 20002003
• Connaughton et al. 1997 (Whipple)
• Limits gt250 GeV from follow-up observations of 9
  BATSE bursts
• Horan et al. 2006 (Whipple)
• Limits gt400 GeV from follow-up observations of 7
  GRB 20022004
• Jarvis et al. 2005 (STACEE)
• No detection in follow-up observations of 8 GRB
  20022005
• Albert et al. 2006 (MAGIC)
• Upper limits overlapping T90 for GRB 050713a and
  050904 afterglow limits for 7 other bursts

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Why so difficult?

• Many good talks yesterday describing high energy
  emission mechanisms for GRBs, often extending to
  TeV
• Where is it?
• Pair opacity in source can prevent escape
• EBL absorption significant gt100 GeV for most
  bursts
• Ground-based instruments with lowest energy
  thresholds have narrow field of view
• Atmospheric Cherenkov telescopes Whipple,
  STACEE, MAGIC, HESS, VERITAS
• Wide FoV instruments have thresholds which
  increase significantly with zenith angle
• Air shower detectors Milagro, ARGO-YBJ, Tibet
  AS-?
• No gold-plated burst yet to really reveal or
  constrain VHE emission

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Prompt Emission

• Inverse Compton scattering of keV/MeV emission
  should create GeV/TeV emission
• Fighting pair opacity in the burst itself
• Highest energy photons which escape related to
  bulk Lorentz factor
• Opacity less in external shock models
• May help discriminate
• Ground-based needed for GRBs with very high
  (gt500) Lorentz factor
• Probe the most extreme of the GRBs
• What sensitivity is needed and whats the best
  strategy for getting there?

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Early Afterglow (T901000 s)

• Transition period from prompt emission to
  afterglow
• Pair opacity declining as fireball expands
• Many possible processes could be in play
• Continued internal shocks
• External shock blast wave (i.e. nominal
  afterglow)
• Reverse shock
• Prolonged energy injection by central engine
• Delayed X-ray flares
• Hard component in GRB 941017 211 s duration
• Many possibilities for SSC and EC production of
  VHE ?s
• VHE light curve of IC from X-ray flare depends on
  which photon and electron populations are
  involved
• Requirements on FOV and slew speed are somewhat
  relaxed, but bigger/faster gives more detections

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Afterglow (gt1000 s)

• Time frame of EGRET high energy emission from GRB
  940217
• Understand the limits of mechanism
• External shock blast wave still present
• VHE afterglow related to shock parameters, e.g.
• Ambient density
• Magnetic field
• Fraction of energy in electrons
• Continued X-ray flares
• Pion decay could be significant
• Less flux, but higher likelihood that slewing
  instrument will react fast enough

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Links to Other Science

• Lorentz Invariance Violation
• Energy dependent delays of simultaneously emitted
  photons can limit (or measure) Lorentz invariance
• Energy dependent light curves?
• Limited redshift range because of EBL absorption
• Broad energy range, especially if delay between
  GLAST and TeV
• Probably more credible during prompt phase
• Does low energy really win by statistics?
• Cosmic Ray Acceleration
• What role do GRBs play in production of UHE
  cosmic rays?
• If significant UHE cosmic ray acceleration
• p synchrotron
• p? initiated cascades
• inelastic np initiated cascades (dominant?)
• Like blazars, it may be difficult to break
  degeneracy between IC and hadronic models
• Have the advantage of better constraints on
  Lorentz factor and smaller timescales/regions
• May still need neutrinos