GRBs and EDGE

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GRBs and EDGE
Dick Willingale Paul OBrien Swift Science
Team
EDGE Meeting 25th April UCL
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Standard Fireball Model
External Shock
Internal Shock
The outflow slowed down by the surrounding
medium
Collisions between different parts of the flow

X
O
GRB
R
Afterglow
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Afterglow parameters

• The afterglow can potentially give us information
  about
• The explosion energy, E0 , and explosion
  mechanism
• The density and/or density profile of the CSM,
  n(R)
• The jet collimation angle, ?j
• Shock physics
• But we need to understand the afterglow
• The afterglow might contribute vital information
  required to produce a
• GRB Standard Candle

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Prompt Gamma-ray Emission by Internal Shocks
luminosity
External Shock emission Typical freq. X-ray-gt
optical -gt radio
GAP in observations
pre-Swift Era
time
10 sec
A few hrs
Days-months
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The Swift Era e.g. GRB 061121
Kim Page et al. ApJ, 2007
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Functional Fitting of GRB X-ray decay curves
observed by Swift
6 temporal 4 spectral
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80 have fast prompt decay 2nd component ?
afterglow
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20 slow prompt decay ? afterglow
The majority of the X-ray decay curves exhibit
flares, some at late times. These are most likely
an indication of continued (late) activity from
the central engine.
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• Currently we have 150 GRBs with BAT XRT
  detection. Essentially all GRBs can be detected
  by the Swift XRT if we slew quickly and if there
  are no observing constraints.
• Many are bright enough early on to allow EDGE to
  detect WHIM/ISM/IGM absorption features.

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I - Prompt emission
internal shocks? Poynting outflow?
relativistic beaming
relativistic expanding fireball
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II - Prompt decay
delayed off-axis/high latitude emission
Prompt source switches off suddenly
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III - Plateau late emission hump
synchrotron emission from external shock
fireball expansion slows down
interaction of fireball with CSM - external shock
develops energy injection from the fireball
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IV - Afterglow power law decay
synchrotron emission spectrum gradually red-shift
ed
beaming angle expands as external shock slows down
fireball dead
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V - Afterglow post jet-break
synchrotron spectrum still being red-shifted but
fades faster because edge of jet now visible
beaming angle now larger than jet
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Phases of decay - Times
Prompt emission stops Tp
External forward shock becomes visible TFS
Final decay Starts Ta Late break Tb
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• Two times Tp and TFS
• two radii and two gammas Rp, RFS,Gp,GFS
• Analysis of 10 GRB decay curves gives
• Gp/GFS2 and RFS/Rp4
• Rp1015-1016 cm
• If prompt emission is synchrotron require
  Rp3x1017 cm
• If prompt emission is synchrotron-self-Compton
  then size is OK but should see an optical flux
  many orders of magnitude brighter than observed

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What is the prompt emission?

• Baryonic outflow and internal/external shocks
  cant produce the prompt emission
• Require different shock physics or
• Magnetic field dominated outflow dissipation in
  outflow produces the gamma-ray prompt emission
• See Kumar et al. MNRAS, 2007

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Intrinsic (source frame) spectra of the prompt
emission
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The Amati relation between the peak photon energy
and isotropic energyUnderstanding this could be
a key to unlock the mystery of the prompt emission
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What is the final X-ray power law decay? As the
outflow slows we expect a simple coupling between
the temporal and spectral indices.
post jet break pre jet break
energy injection? 060218 oddball
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Transition from plateau to finalpower law decay
getting Softer getting harder
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Time of transition Ta ?
Post jet break Energy injection
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Extra break Tb gt Ta (blue)
Slow prompt ? final decay
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Potential jet breaks2 in accord
withpre-Swiftoptical jet breaks
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Decays where an expected jet break is absent
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Decays where as expected jet break is absent
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Collimated energy E?
Optical jet breaks - Frail/Bloom/Ghirlanda
energy offset factor 34 (tjet110Ta)
Lower limits using Tatjet
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Ghirlanda relation type II?
end of X-ray plateau tjTa
similar gradient
end of X-ray light curve tjTmax
Optical Jet Breaks tjopt
energy offset factor 26 (tjet90Ta)
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Summary of Swift results

• X-ray afterglows can stem from the plateau (80
  GRBs) and/or the initial decay of the prompt
  emission (20 GRBs)
• Only 50 of the final X-ray afterglows conform to
  the expectations of the standard model energy
  injection?
• Some afterglows get harder as they evolve not
  expected from the standard model
• 3 possible jet breaks identified in accord with
  pre-Swift optical jet breaks
• 8 X-ray decays - expected jet break is absent
• Study of the initial prompt decay indicates that
  the a baryonic shock may not be responsible for
  the emission
• The Amati relation still holds but what is the
  correlation between Epeak and Eiso really telling
  us?

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Observational requirements for the future

• Full time coverage from prompt to final afterglow
  decay a few seconds to weeks! We need fast
  slewing.
• Simultaneous multi-waveband observations to track
  the evolution of the SED. Again we need fast
  slewing.
• Measurement of the hard tail of the prompt
  emission spectrum ? Epeak value. We need a hard
  X-ray detector.
• Accurate positions for finding the optical
  afterglow, identifying the host galaxy, getting
  the redshift
• Large collecting area to provide excellent
  statistics for spectral/absorption studies early
  on.
• Measurement of both prompt and afterglow
  parameters to provide a route to the GRB
  Standard Candle.