GRB Engines: Past and Current Models

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GRB Engines Past and Current Models

• Lecture 4 Using fireball constraints to
  determine true engine C. Fryer (UA/LANL)

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Sedov Solution Useful Relativistic version
needs some tuning

• rE/r1/5t2/5E/r01/5rw/5t2/5
• r (1-w/5) (E0/r0)1/5t2/5
• r (E0/r0)1/(5-w)t2/(5-w)
• v dr/dt (E0/r0)1/(5-w) 2/(5-w) t(w-3)/(5-w)
• v0(t/t0)(w-3)/(w-5)

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Fireball Model - Summary

• Thusfar, the fireball model has made very few
  true predictions
• Current Favorite form of the Fireball model uses
  internal and external shocks.

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Fireball Model - Summary

• Basic Fireball model simple Relativistic shocks
  with synchrotron inverse Compton emission
• Question on gm gm determined simply by assuming
  a constant fraction of the shock energy goes into
  electrons.

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Particles in a B-field radiate

• Relativistic Particles
• Psynchrotron 2q2/3c3 g4 q2B2/(g2m2c2)vperp2
• 2/3 r02cbperp2g2B2
• where r0e2/mc2
• Psynch4/3sTcb2g2B2/(8p)
• where sT8pr02/3 is the Thompson Cross-Section

B
2b2/3
Isotropic velocities
vpar
vperp
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Fireball Model - Summary

• Basic Fireball model simple Relativistic shocks
  with synchrotron inverse Compton emission
• Internal Shocks produce optical burst and
  gamma-rays, External Shocks produce afterglow
• Jets alter the spectra in an observable way.

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Jets Make a Sharp Break in the light curve
synch emission does not include the effects of
cooling.
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Observations place several constraints on the
Engine!

• Few times 1051 erg explosions (few foe)
• Most of energy in gamma-rays (fireball model
  works if explosion relativistic)
• Rapid time variability
• Duration ranging from 0.01-100s
• Accompanied by SN-like bursts
• Occur in Star Forming Regions
• Explosion Beamed (1-10 degrees)

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With the fireball model, these constraints are
strengthened!

• Relativistic factors above 100!
• Some explosions must occur in windswept media
• Some (all?) explosions are jets

10
GRB Engines

• Energy sources and conversion on earth and in
  astrophysics
• Variability constraints Compact object models
• With observational constraints, models now fall
  into two categories
• I) Black hole accretion disk models (compact
  binary merger, collapsar)
• II) Neutron Star Models (magnetar, supranova)

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Energy Sources What Powers These Explosions?
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Occams Razor
Pluralitas non est ponenda sine neccesitatem
Plurality should not be posited without
necessity - William of Ockham
We are to admit no more causes of natural
things than such as are both true and sufficient
to explain their appearances."
- Isaac Newton
All Saints Church, Ockham
Corollary Dont invent new physics unless you
need it!
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Energy Source Gravitational Potential Energy
Hydroelectric Power
Energy from Falling Water Drives Turbines!
1-2000 MegaWatts
Hoover Dam - Arizona/Nevada
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Chemical Energy
Reforming Bonds in Molecules Tighter
Configurations give off Energy
Ames, Iowa 500MW Refuse Derived Fuel and
Coal Fired Power Plant
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Nuclear Energy
Diablo Canyon, CA
Breaking Bonds Within the Atom 1) Fission
1,000 MWatts
Large Atom (e.g. Uranium) Captures an Electron
Causing It to Become Unstable and Break
Apart. Some Mass is Converted to Energy
EMc2
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Nuclear Energy
II) Fusion Combining Atoms To Form A Larger
Atom Can Also Release Energy!
Atomic Bomb Can Be Fusion or Fission
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Energy Conversion
Energy released through gravitational potential,
chemical, or nuclear sources must be converted
into useful energy. Useful Electricity on
Earth (to Most of Us) Explosion
Energy (to the Astronomy Observer) This usually
requires a mediator something to transport
the energy Magnetic Fields e.g. a
Dynamo Radiation e.g. Photons (light), Neutrinos
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MagneticDynamo
Water (Accelerated by Gravity Or Thermal
Pressure) Spins Large Magnets. The Motion Of
the Magnets Drives an Electric Current --
Electricity!
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Radiation
Photons (Light) or Neutrinos Can Transport Energy
Nuclear Energy Released in the Sun Must Make Its
Way Out of the Sun Via The Transport of Light or
Neutrinos.
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Radiation
Light from the sun travels to the Earth where we
convert it To Electricity.
Photo-Cells
Solar Power Plant Mojave Desert
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Energy Sources

• Chemical rarely important in astrophysics
• Nuclear Physics Stars, Type Ia Supernovae,
  X-ray Bursts
• Gravitational Potential Energy All other
  supernovae, X-ray Binaries, Pulsars (neutron star
  spin arises from potential energy) and, for most
  theories, GRBs!

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Energy Sources

• Chemical rarely important in astrophysics
• Nuclear Physics Stars, Type Ia Supernovae,
  X-ray Bursts
• Gravitational Potential Energy All other
  supernovae, X-ray Binaries, Pulsars (neutron star
  spin arises from potential energy) and, for most
  theories, GRBs!

Garcia-Senz et al. 1999
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Energy Sources
Helium Detonation on NS

• Chemical rarely important in astrophysics
• Nuclear Physics Stars, Type Ia Supernovae,
  X-ray Bursts
• Gravitational Potential Energy All other
  supernovae, X-ray Binaries, Pulsars (neutron star
  spin arises from potential energy) and, for most
  theories, GRBs!

Zingale et al. 2001
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Energy Sources

• Chemical rarely important in astrophysics
• Nuclear Physics Stars, Type Ia Supernovae,
  X-ray Bursts
• Gravitational Potential Energy All other
  supernovae, X-ray Binaries, Pulsars (neutron star
  spin arises from potential energy) and, for most
  theories, GRBs!

Fryer Heger 1999
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Energy Sources

• Chemical rarely important in astrophysics
• Nuclear Physics Stars, Type Ia Supernovae,
  X-ray Bursts
• Gravitational Potential Energy All other
  supernovae, X-ray Binaries, Pulsars (neutron star
  spin arises from potential energy) and, for most
  theories, GRBs!

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Energy Sources

• Chemical rarely important in astrophysics
• Nuclear Physics Stars, Type Ia Supernovae,
  X-ray Bursts
• Gravitational Potential Energy All other
  supernovae, X-ray Binaries, Pulsars (neutron star
  spin arises from potential energy) and, for most
  theories, GRBs!

27
Energy Sources

• Chemical rarely important in astrophysics
• Nuclear Physics Stars, Type Ia Supernovae,
  X-ray Bursts
• Gravitational Potential Energy All other
  supernovae, X-ray Binaries, Pulsars (neutron star
  spin arises from potential energy) and, for most
  theories, GRBs!

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GRB Energy Sources
Energy Needed 1052 erg Of useful energy (not
leaked Out in neutrinos or Gravitational waves
or Lost into a black hole)! Most GRB Models
invoke Gravitational potential energy As the
energy source. Collapse to a NS or
stellar Massed BH most likely source
E G M2/r 1-10 solar masses 3-10 km
E1053-1054 erg Allowing a 1-10 Efficiency!
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Gamma-Ray Burst Durations
Two Populations Short 0.03-3s Long
3-1000s Possible third Population 1-10s
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Burst Variability
Not only must any model Or set of models
predict A range of durations, But the bursts
must also Be rapidly variable!
Burst Variability on the level
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Durations and Variabilities
Variability size scale/speed of
light Again, Neutron Stars and Black Holes
likely Candidates (either in an Accretion disk
or on the NS surface). 2 p 10km/cs .6 ms cs
1010cm/s
NS, BH
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Durations and Variabilities
Duration Rotation Period / Disk Viscosity (a
0.1-10-3) Period 2 pr3/2/G1/2MBH1/2
.3 ms near BH surface Duration for
small disks 3-300ms
NS, BH
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Black Hole Accretion Disk Models Material
accreting Onto black hole Through disk
Releases potential Energy. If this Energy can
be Harnessed to Drive a relativistic Jet, a GRB
is formed.
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Harnessing the Accretion Energy
Mechanism I Neutrinos from hot disk annihilate
Above the disk Producing a Baryon-poor, High-
energy jet
Mechanism II Magnetic fields are Produced by
Differential rotation In the disk. This
Magnetic field produces a jet.
Accretion Disk
Details Lecture 5
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Black-hole neutron-star merger (NS-NS Mergers)
Black hole and neutron star (or 2 neutron stars)
orbiting each other in a binary system
Neutron star will be destroyed by tidal effects
neutron star matter accretes onto black hole

• Accretion disk

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NS/BH (NS/NS) Mergers

• Advantages
• Progenitors known (e.g. Hulse-Taylor Pulsar
  system)
• Energetics and rate roughly correct

• Disadvantages
• Size of disk 10-30km Duration working model for long-duration bursts
• Many predictions dont match afterglow era
  science.

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Black Hole Accretion Disk Models
Collapsar (aka hypernova
Supernova explosion of a very massive ( 25 Msun)
star
Iron core collapse forming a black hole
Material from the outer shells accreting onto the
black hole
Accretion disk Jets GRB!
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Collapsars

• Observations Explained
• Energetics explained
• Duration and variability explained

• Observations Predicted
• SN-like explosions along with GRB outburst
• Bursts occurring in star forming regions
• GRB Beaming

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Massive Star Models

• With the observations pushing toward massive
  stars, a number of other massive star models
  appeared pushing for neutron star mechanisms!
• Explosions from Magnetars
• Supranova (neutron star which later collapses to
  a black hole)

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Magnetic NSs in Collapse

• With the SN/GRB association, Wheeler and
  collaborators sought a new GRB mechanism arguing
  that all supernovae produce GRBs
• During Collapse, magnetic fields grow in
  proto-neutron star.

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Magnetic NSs in Collapse Cont.

• Using a pulsar-like mechanism (magical/magnetic
  fields strike again), Wheeler argued that this
  fast-spinning, newly born, neutron star will
  produce jets in most stellar collapses.

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Advantages of Magnetic NSs in Collapse

• Supernovae do not appear symmetric!

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Advantages of Magnetic NS models Polarization

• Supernovae are polarized.
• Polarization Increases with time (implying that
  we are uncovering a central engine that is
  asymmetric).
• Wheeler and collaborators argue that all
  supernovae (or maybe just all Ib/Ic supernovae)
  have jets only a fraction are observed as GRBs!

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Problems with the Magnetic NS model
Nickel distribution from asymmetric explosion

• Magnetic Field Model requires very strong
  magnetic fields! Only shown to work with
  hand-wavy approximation.
• SN spectra are different than the SN-like spectra
  in GRBs and hypernovae.

Hungerford et al. 2003
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Problems with the Magnetic NS model SNe vs.
SN-like outbursts spectra different!
Ic no H, no strong He, no strong Si
SiII
Ia
Ca
O
He
Ib
Hypernovae broad features blended lines
Large mass at high velocities
Ic
94I
97ef
Hyper -novae
98bw
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Problems with the Magnetic NS model
Nickel distribution from asymmetric explosion

• Magnetic Field Model requires very strong
  magnetic fields! Only shown to work with
  hand-wavy approximation.
• SN spectra are different than the SN-like spectra
  in GRBs and hypernovae.
• Why do we get 3 branches of supernova energies?

Hungerford et al. 2003
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Supernovae/Hypernovae
Nomoto et al. (2003)
EK
Failed SN?
13M?15M?
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But Most Supernovae are not GRBs!!!!! Death Of
the Pulsar Model for GRBs
Radio shows A definite Break between GRBs and
Normal type Ib/Ic SNe! At Most, 5 Of
supernovae Are GRBs (Berger et al. 2003). Must
be right, Done by GRB observers!
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Supranova Model For GRBs
If a neutron star is rotating extremely rapidly,
it could escape collapse (for a few months) due
to centrifugal forces.
Neutron star will gradually slow down, then
collapse into a black hole collapse triggers
the GRB
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Advantages of the Supranova Model

• GRB occurs after supernova explosion
• Iron produced in supernova can then be lit up by
  gamma-ray burst producing iron lines!
• Iron lines observed!

X-ray spectrum of GRB010220 From XMM-Newton. The
solid line shows a power Law fit, the residuals
to this Fit indicate, to some, the Presence of
an emission line.
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Iron Lines hard to Explain with Collapsar Model

• Bottcher et al. (1999,2001) tried to explain
  these lines using the excretion disk of a binary
  merger in the collapsar model.

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Iron Lines hard to Explain with Collapsar Model

• Although they could produce iron lines, they
  could not produce iron lines that survived long
  enough to explain all observations Supranova
  model can easily explain all iron lines.

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But are these iron lines real? If not, the
supranova has no real advantage over the
collapsar model.
Analysis by Bob Rutledge (McGill) suggests
This line Can be Explained Away as Noise!
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Disadvantages of the Supranova Model
Mass thing Duration cant be Longer than
3-3000ms
NS, BH
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Disadvantages of the Supranova Model
Duration Rotation Period / Disk Viscosity (a
0.1-10-4) Duration cant be Longer than
3-3000ms Current bursts with Iron lines are all
Long-duration!
NS, BH
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The straw that broke the camels back
observations (not physics)!

• In the supranova model, the supernova explosion
  should occur months before the GRB
• Observations (again limited to long-duration
  bursts) find that the supernova-like explosion
  occurs alongside the GRB.

http//www-cfa.harvard.edu/jbloom/valencia
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Summary of Burst Models

• NS/NS, BH/NS Mergers Durations too short for
  long-duration GRBs
• Pulsar-like, Magnetar models although favored
  for Soft gamma-ray repeaters (SGRs), predicts
  that most SNe are GRBs a prediction proved
  false by observations
• Supranovae predict that the SN outburst occurs
  BEFORE GRB also disproved by observations (hard
  to explain long-duration bursts in any event).
• Collapsar Still the Favored Model

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Energy Sources

• Two Main Energy Sources
• In Astrophysics
• Gravitational Potential
• Energy
• II) Nuclear Energy

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Energy Transport

• Radiation (photons, neutrinos)

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Energy Transport

• Radiation (photons, neutrinos)
• Magnetic Fields

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Variabilities of GRBs limits models to compact
objects (NS, BH)
Variability size scale/speed of
light Again, Neutron Stars and Black Holes
likely Candidates (either in an Accretion disk
or on the NS surface). 2 p 10km/cs .6 ms cs
1010cm/s
NS, BH
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Black Hole Accretion Disk Models Compact
Mergers And collapse of Stars.
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Neutron Star Models

• Magnetar Models ruled out because most
  supernovae do NOT produce GRBs.
• Supranova models ruled out for long-duration
  bursts by duration times

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Tomorrow- Black Hole Accretion Disks

• Structure of a Relativistic Disk
• Neutrino Driven Explosions from a Black Hole
  Accretion Disk
• Magnetic Field Driven Explosions from a Black
  Hole Accretion Disk