GammaRay Bursts The Biggest Explosions Since the Big Bang

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Gamma-Ray Bursts The Biggest Explosions Since
the Big Bang
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Outline

• History and basic observational facts
• BATSE observations, the Great Debate, and the
  determination of the distance scale
• Basic Physics Compactness baryon loading
  problems
• The fireball model, afterglows, and jets
• Energetics and relation to supernovae
• Future directions GRBs as a cosmological tool

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The Cold War Gamma-Ray Bursts
The Vela (Watchmen) satellites were launched by
the DoD and the AEC in 1963 to detect Soviet
nuclear tests in space and on the Moon (Nuclear
Test Ban Treaty) While searching for ??ray
emission from supernova shock breakout (Colgate
1968) Klebesadel et al. found strong bursts of
?-rays coming from random locations on the sky.
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Gamma-Ray Bursts Observed Properties

• Observed rate 3 GRBs/day
• 0.5 Gpc-3 yr-1
• (SNeIc 4.8x104 Gpc-3 yr-1 )

GRBs are rare, diverse, and potentially arise
from more than one population of sources
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Where Do They Come From? 1970-1991

• Interstellar warfare
• Evaporating black holes
• Flares on nearby stars
• Star quakes
• Stars falling into black holes
• Comets falling on neutron stars
• Comet anti-comet annihilation
• (nuclear goblins)

Before 1991 135 theories, less than 100 ?-ray
bursts!
Problem Uncertainty in position and hence
distance Solutions triangulation, coded-aperture
masks, counterparts
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Statistical Methods
Location on the sky indicated that GRBs do not
originate in the plane of the Galaxy, the Virgo
cluster, local galaxies, or Abell clusters
logN/logS, ?V/Vmax? vary based on uniformity in
space. S-3/2 if uniform and shallower if
bounded ?V/Vmax?0.5 if uniform and lower if
bounded
logN/logS is the number of objects in a flux
bin Vmax is the max volume to which an object can
be detected in an experiment with clim. V is the
volume corresponding to the detection count-rate
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BATSE the Great Debate
logN/logS ? S-0.8
The Compton ?-Ray Observatory (launched 1991)
Paczynski claimed that this points to a
cosmological origin Lamb claimed that a Galactic
population of high-velocity neutron stars is also
consistent with the data
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Compton Takes the Dive
NASAs Compton Gamma Ray Observatory re-entered
the Earth's atmosphere at approximately 210 a.m.
EDT on June 4, 2000. It landed in the Pacific
Ocean approximately 2,400 miles southeast of
Hawaii
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Afterglows Determination of Cosmological Origin
Optical observations revealed a fading source
associated with a faint, extended source, similar
to a galaxy at z 1. The next burst provided a
direct confirmation via an absorption spectrum.
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Physics IThe Compactness and Baryon Loading
Problems
A cosmological origin indicates gt1051 erg of MeV
??rays released in a few seconds (i.e. small
region, c?t) ? Fireball - a region optically
thick due to electron-positron pair production
(e1e2 gt mec2 )
ee- pairs provide opacity ? expansion,
acceleration, and thermalization ? a thermal
gamma-ray burst
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Physics IThe Compactness and Baryon Loading
Problems
Another problem is baryon loading - astrophysical
fireballs entrain baryons which precipitate the
conversion of radiation to kinetic energy
(???const). Even if M 10-5 M? all the
radiation is converted to kinetic energy so no
GRB is produced. However,
The kinetic energy of the baryons is re-converted
to radiation via shocks (internal or external)
variability points to internal shocks
The unavoidable baryon contamination provides a
mechanism for delaying optical thinness and
ensuring a high Lorentz factor required for GRB
production. The new baryon loading problem is
how to get only 10-5 M?
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The Fireball Model
?ngine ? energy transport ? conversion to
??rays ? afterglow
Collapsar Coalescence
Baryonic Magnetic
Internal Shocks Magnetic instability
External Shock
Determine the engine properties using afterglow
observations
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Progenitors
Coalescence versus Collapse
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Neutron Star Binaries
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Collapsars
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Collapsars as the Progenitors of Long GRBs
1. Association with supernovae
2. Statistical distribution of GRB-host offsets
3. Association with star-forming galaxies
Missing evidence for Wind-modified ambient
medium Missing progenitors of short-duration GRBs
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GRBs Internal Shocks

• External shocks cannot explain the gamma-ray
  variability
• In external shocks the timescale is determined by
  angular spreading gt burst duration ? variability
  is smoothed out.
• The only way to overcome this is
• a jet lt 1/????0.01 rad / a very patchy ISM.

Internal shocks take place on a timescale ??c?t.
This preserves the emission structure, but
multiple shells are needed. Efficient internal
shocks require equal mass shells with a large
velocity spread The emission mechanism in
internal shocks is not known yet The efficiency
of internal shocks is low (a few )
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Physics IIAfterglows External Shocks
Afterglow starts when swept-up mass is about
1/??of ejecta mass 100 sec
Shock jump conditions
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Physics IIAfterglow Evolution Blandford-McKee
The evolution of the spectrum is tied to the
dynamics BM solution
Sari et al. 1998
By studying the afterglow emission we can
determine the energy, density, and structure
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Afterglows Reverse Shock
The afterglow is accompanied at early time by
emission from the reverse shock which plows
back into the ejecta
How many GRBs are equally bright? Swift
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Collimation (Jets)

• Egde effect
• Sideways expansion

Zhang et al. 2003