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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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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Physics IThe Compactness Problems
Fireball - a region optically thick due to
electron-positron pair production (e1e2 gt mec2 )
Low baryon load
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The Fireball Model
?ngine ? energy transport ? conversion to
??rays ? afterglow
Collapsar Coalescence
Baryonic Magnetic
Internal Shocks Magnetic instability
External Shock
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Progenitors
Coalescence versus Collapse
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Collimation (Jets)

• Egde effect
• Sideways expansion

Zhang et al. 2003
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The Energetics of Gamma-Ray Bursts
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The Energetics of Gamma-Ray Bursts
??rays measure the energy in ??gt 100
material X-rays measure the energy in ?? 10
material The energy of GRBs is nearly constant
the opening angles vary widely
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A Common Origin / Energy Scale
GRBs have a standard total energy yield of a few
1051 erg (?gt 2), but the fraction coupled to
ejecta with ??gt?100 varies widely What
properties of the progenitor or engine determine
the partition of energy?
Berger et al. 2003a
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SN 1998bw - A Transition Object?

• The radio emission from SN 1998bw points to an
  engine
• ? 2 with Erel 1050 erg
• Double peak indicates a variable injection of
  energy

SN 1998bw is a transition object between typical
SNe and GRBs
1.
SN 1998bw is a typical GRB viewed well away from
the jet axis
2.
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The Origin of SN 1998bw-like Events
1. The rate of SN 1998bw-like events is
completely independent of the GRB rate. 25 of
the Ib/c SNe within 100 Mpc.

• 2. ?fb-1?500 RGRB/RIbc10-5
• 0.5 of the Ib/c SNe within 100 Mpc.

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Engine- vs. Collapse-Driven Explosions
lt 2 of all local SNe are related to GRBs The
fastest ejecta observed are 100,000 km/sec
(compared to 10,000 km/sec in the
optical) Summary GRBs and SNe produce nearly
the same amount of kinetic energy 1051 erg.
However, in GRBs the baryon load is 10-3 - 10-5
solar masses and in SNe it is a few solar masses.
Some objects may bridge the gap
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The Cosmic Star Formation History
Challenges Few observational windows
UV/optical, submillimeter/FIR, radio Selection
effects dust in the optical/UV, sensitivity in
submm/FIR/radio Individual problems extinction
corrections, redshift identification, AGN Unknown
overlap between the different samples who has it
right?
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The Host Galaxies of Gamma-Ray Bursts
GRBs offer an alternative galaxy-selection
technique which is not susceptible to current
selection effects

• Redshift easily obtained from afterglow
  absorption spectra (z2)

• Selection independent of dust thanks to
  dust-penetrating ??rays
• Selection independent of emission properties in
  any band
• GRBs can be detected at very high redshifts (z gt
  10)

The GRB host sample is relatively small (50
galaxies) biases?
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GRB Host Galaxies
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The Host Galaxies of Gamma-Ray Bursts
Redshift peaks at z1 (bias?), but 25 at z gt
2 15 are fainter than 25.5 mag and have a
redshift - a unique sample
Photo-z
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Blue Galaxies
GRB host galaxies are very blue with a median R-K
2.5 mag. This includes the host galaxies that
are detected in the sub-mm. Is this due to a
selection bias against dusty environments?
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Blue Galaxies
GRB hosts are blue primarily because they are
faint in the K-band. A lower total mass?
Relatively young stellar population? What can we
learn about the objects with lt 0.01 L ?
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GRB Host Galaxies Young Starbursts?

• Blue colors
• Low LNIR (low total mass)
• Low Lopt (low inst. SFR)
• ? Starburst galaxies in the early stages of the
  starburst process

Berger et al. 2003c
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Absorption Spectroscopy
Cosmology with gamma-ray bursts

• Address the interaction and interplay between
  SF, ISM and IGM.
• Advantages over quasars no proximity effect,
  bright(er), high-z
• Advantages over traditional galaxy studies
  arbitrarily faint

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Absorption Spectroscopy
Time-dependent photoionization?
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Probing the High-Redshift Universe
It is currently believed that cosmic
re-ionization was primarily driven by the first
generation of massive stars (z 20) If these
stars died as GRBs then we can probe the IGM to
significantly higher redshifts than what is
possible with quasars