Gamma Ray Bursts

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Gamma Ray Bursts

• Shamelessly stolen from Chris Fryers summer
  school lectures

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GRBs The Historical Perspective

• 1967 Discovery Vela Satellites
• 1972-1991 Golden Age for Theorists - no
  constraints and a world of proposals
• 1991 Constraining the theories CGRO (BATSE)
  finds isotropic distribution
• 1996 Localization BeppoSAX localizes bursts
  to get redshifts and host-galaxy information

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Gamma-Ray Bursts and the Cold War
In the 1950s, the US and USSR decided to ban The
testing of nuclear weapons.
How do we check?

• Seismic Waves
• Low Frequency Sound Waves
• Gamma-Rays

Crashed Balloon Became Roswell Alien!
Mogul Project
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Gamma Rays in the Cold WarVela Satellites
Stats e.g. Vela 5A

• Scintillation X-ray Detectors
  3-12keV,6-12keV Area 26cm2
• CsI Gamma-Ray Detectors -
  150-750keV Volume 60 cm3

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First Detected Gamma-Ray Burst
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GRBs The Golden Age for Theory
What Theorists Know Constraints on Theory
What we know/dont know from observations

• Not Russian tests!
• Lots of gamma-ray emission
• No distances Total energy and location unknown!
• Too few objects to get spatial distribution!

• Cant be thermal emission alone!
• Options I)
  Relativistic Boosting from jet or compact object!
  II) Nuclear Lines
  (e.g. Nickel Decay)
• III) Magnetic Fields.

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Creativity of Theorists
With so few constraints, theorists came up with
all Sorts of models relying on a range of physics.
Three Classes based on location
Galactic
SS

• Solar System
• Galactic
• Cosmological (outside of the Milky Way)

Energy Observed Flux d2
Energy Requirements Vary over 20 orders Of
magnitude!
Cosmological
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The first gamma- Ray burst model Appeared
before The Vela results Were published! By 1992,
over 100 models Existed! Despite this Number,
the Currently favored Model is not on This list!
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Gamma-Ray Bursts in the Solar System

• Lightning in the Earths atmosphere (High
  Altitude)
• Relativistic Iron Dust Grains
• Magnetic Reconnection in the Heliopause

Red Sprite Lightning
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Gamma-Ray Bursts in the Milky Way

• Accretion Onto White Dwarfs
• Accretion onto neutron stars
  I) From binary companion
  II) Comets
• Neutron Star Quakes
• Magnetic Reconnection

X-ray Novae
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Extragalactic Models

• Large distances means large energy requirement
  (1051erg)
• Event rate rare (10-6-10-5 per year in an L
  galaxy) Object can be exotic

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Cosmological Models

• Collapsing WDs
• Stars Accreting on AGN
• White Holes
• Cosmic Strings
• Black Hole Accretion Disks
  I) Binary Mergers II)
  Collapsing Stars

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Black-Hole Accretion Disk (BHAD) Models Binary
merger or Collapse of rotating Star
produces Rapidly accreting Disk (gt0.1 solar Mass
per second!) Around black hole.
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BATSE - Burst And Transient Spectrometer
Experiment on Compton Gamma-Ray Observatory
BATSE Module
BATSE Consists of two NaI(TI) Scintillation
Detectors Large Area Detector (LAD) For
sensitivity and the Spectroscopy Detector (SD)
for energy coverage
8 Detectors Almost Full Sky Coverage Few
Degree Resolution 20-600keV
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BATSE Results - Isotropy
Galactic models
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Gamma-Ray Burst Lightcurves
GRB990316
GRB Lightcurves have A broad range of
Characteristics
Fast Rise Exponential Decay FREDs
GRB970508
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Gamma-Ray Burst Lightcurves
GRB990123
Double bursts and Extended Structures
No standard shape Exists!
GRB980703
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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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Gamma-Ray Burst Duration vs. Energy Spectrum
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BeppoSAX Instruments
HPGSPC(Phoswitch)
LECS/MECS

• Xenon Gas Scintillator
• Energy Range .1-1keV (1-10keV)
• 1 arc minute resolution
• Goal Localize Object

• HPGSPC - High Pressure Xenon/He Gas
• Phoswitch - NaI(Tl), CsI(Na) Scintillators
• 4-120keV (15-300keV)
• Goal Broad Energy resolution in X-ray narrow
  field

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GRB970228 first good localization
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GRB070228 Optical Counterpart Discovered (with
corresponding optical localization!)
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GRB 970508 Optical Counterpart
BeppoSAX X-ray Localization Allowed a The
Optical Transient to Be detected While still on
The rise. OT allowed Spectral Measurement!
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GRB970508 Absorption Lines z0.835
flux
Metzger et al. 1997
Wavelength
Optical Emission
Absorption
Mg II
Mg II I
Fe II
flux
Fe II
Wavelength
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Radio Scintillation can also be used to estimate
the GRB distance consistent with z0.835
Just as the Earths Atmosphere Causes light To
scatter Causing point Sources to twinkle, the
Interstellar Medium causes Radio emission To
twinkle. When The burst gets Large enough, Like
planets, the Twinkling stops.
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A crash Course in Scintillations
Scintillations determine the size of the source
in a model independent way. The size (1017cm)
is in a perfect agreement with the prediction of
the Fireball model.
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GRBs in the Swift Era

• Thanks to Neil Gehrels

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Eiso
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Location, Location, Location(In addition to
detecting hosts, we can determine where a burst
occurs with respect to the host.
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If we take These Positions At face Value, We
can Determine The Distribution Of bursts With
respect To the half- Light radius Of
host Galaxies! This Will Constrain The models!
Distribution Follows Stellar Distribution
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GRB locations within galaxies
GRBs show higher gas densities and
metallicities, And have significantly lower
(Si,Fe,Cr)/Zn ratios, Implying a higher dust
content Star Formation Region
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GRB Environments II Studying the environment
using radio and optical observation of GRBs

• Density profiles are different for different
  environments massive stars will be enveloped by
  a wind profile.
• These different density profiles produce
  different radio, optical emission.

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For gt1/2 of Gamma-Ray Bursts, afterglows
consistent with constant density or inconsistent
with wind bubbles. (radio And R-band Data best
Diagnostics!
GRB021004
Roger Chevalier
Li Chevalier 2003
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Jet Signatures
GRB 010222
Piran, Science, 08 Feb 2002
Stanek et al. (2001)
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Energy and Beaming Corrections

• The dispersion in isotropic GRG energies results
  from a variation in the opening (or viewing)
  angle
• The mean opening angle is about 4 degrees (i.e.
  ?fb-1? 500 )
• Geometry-corrected energies are narrowly
  clustered (1?2x)

15 events with z and t_jet
Frail et al. (2001)
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Summary of GRB Energetics

• Gamma-ray bursts and their afterglows have
  (roughly) standard energies
• Robust result using several complementary methods

E? ? gamma-rays Ek ? X-rays Ek ? BB modeling Ek
? Calorimetry
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SN/GRB connection!

• GRBs have SN-like outbursts.
• But these bursts are beamed, and we wont see all
  explosions as a GRB.
• What do we make of the SN/GRB connection
• All GRBs produce SNe?
• All SNe are GRBs (only those observed along the
  jet axis are GRBs)?
• Are either of these true?

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How Common are Engine-Powered SNe?
VLA/ATCA survey of 34 Type Ib/c SNe to detect
off-axis GRBs via radio emission
Berger PhD

• Most nearby SNe Ib/c do not have relativistic
  ejecta
• Two distinct populations
• Ek(GRB)ltlt1 foe (hydo collapse)
• lt10 are 1998bw-like

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Fireball Model Prediction vs. Postdiction

• Prediction (from Latin prae- before dicere
  to say) A foretelling on the basis of
  observation, experience or scientific reasoning.
• Postdiction (from Latin post- after dicere
  to say) To explain an observation after the
  fact.
• If your model predicts all possible outcomes,
  it is not a prediction. This merely states that
  you can not constrain the answer with your
  current model.

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Internal Shocks Shocks between different shells
of the ejected relativistic matter

• dTR/cg2 d/c D/cT
• The observed light curve reflects the activity of
  the inner engine. Need TWO time scales.
• To produce internal shocks the source must be
  active and highly variable over a long period.

dT
T
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Internal Shocks ?Afterglow

• Internal shocks can convert only a fraction of
  the kinetic energy to radiation
• (Sari and Piran 1997 Mochkovich et. al.,
  1997 Kobayashi, Piran Sari 1997).
• It should be followed by additional emission.
• It ain't over till it's over (Yogi Berra)

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Gamma-Ray Burst 4 Stages
1) Compact Source, Egt1051erg 2) Relativistic
Kinetic Energy 3) Radiation due to Internal
shocks GRBs 4) Afterglow by external shocks
The Central Compact Source is Hidden
Plus burst of optical emission!
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The Internal-External Fireball Model
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The Resolution of the Energy Crisis

• Etot - The total energy
• Eg iso - Observed (iostropic) g-ray energy

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JETS and BEAMING
Particles remain within initial cone
Radiation is beamed into a narrow cone
g q -1
Jets with an opening angle q expand forwards
until g q-1 and then expand sideways rapidly
lowering quickly the observed flux (Piran, 1995
Rhoads, 1997 Wijers et al, 1997 Panaitescu
Meszaros 1998).
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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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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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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
ltbperp2gt2b2/3
Isotropic velocities
vpar
vperp
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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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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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Gratuitous Mushroom Cloud Picture
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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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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 lt10-100 Millisecond
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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Two Jet Drivers Neutrinos
e,e- pair plasma
Neutrino Annihilation
Scattering
Absorption
Densities above 1010-1011 g cm-3 Temperatures
above a few MeV
Disk Cools via Neutrino Emission
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Two Engine Drives Neutrinos Magnetic Fields

• Source of Magnetic Field Dynamo in accretion
  disk.
• Source of Jet Energy -
  I) Accretion Disk
  II) Black Hole Spin

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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)Though frequency not
• Energetics and rate roughly correct. (rate very
  uncertain)

• Disadvantages
• Size of disk 10-30km Duration lt1s not a
  working model for long-duration bursts
• Many predictions dont match afterglow era
  science (long GRBs).

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Black Hole Accretion Disk Models
Collapsar (aka hypernova
Supernova explosion of a very massive star
Iron core collapse forming a black hole
Material from the outer shells accreting onto the
black hole
Accretion disk gt Jets gt 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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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 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 gt 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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Binary Evolution is Important for nearly All
GRB progenitors! For merging binaries, It is
essential that The binaries be Close.
Definition of Terms

• Massive Star Star that, if not affected by
  binary mass transfer would undergo core-collapse
  (MSN 8-10 solar masses)

Fryer, Woosley Hartmann 1999
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Definition of Terms

• Black Hole Mass (MBH) transition mass for black
  hole formation.
• He core helium core of massive star helium
  core masses will also have transitions for
  neutron star and black hole formation.
• Mp,Ms masses of primary (most massive) and
  secondary (least massive) stars in a binary.

Fryer, Woosley Hartmann 1999
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• NS-NS binaries
• (also known as Double
• Neutron Star Binaries)
• 3 primary mechanisms
• exist
• I) Primary collapses to a NS.
• Common envelope evolution
• tightens binary so that a close
• NS-NS binary is formed after
• the collapse of the secondary.

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• NS-NS binaries
• (also known as Double
• Neutron Star Binaries)
• 3 primary mechanisms exist
• I) Primary collapses to a NS.
• Common envelope evolution
• tightens binary so that a close
• NS-NS binary is formed after
• the collapse of the secondary.
• II) Both stars evolve off the
• main sequence at roughly
• the same time. Hydrogen
• and Helium CE phases
• tighten binary.

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• NS-NS binaries
• (also known as Double
• Neutron Star Binaries)
• 3 primary mechanisms exist
• I) Primary collapses to a NS.
• Common envelope evolution
• tightens binary so that a close
• NS-NS binary is formed after
• the collapse of the secondary.
• II) Both stars evolve off the
• main sequence at roughly
• the same time. Hydrogen
• and Helium CE phases
• tighten binary.
• III) No common envelope phase.
• Well placed NS kick creates
• a tight binary.

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For most equations of state, NS-NS mergers
produce A black hole surrounded by An accretion
disk.
Equatorial view of disk Conditions density,
Temperature, electron Fraction and entropy
Ruffert Janka 1999
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Disk Structure for NS-NS Mergers
Densities exceed 1011 g cm-3, Temperatures exceed
a few MeV, Disk masses range from 0.03-0.25 solar
masses
Ruffert Janka 1999
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NS-NS Mergers Neutrino Emission
These dense, hot disks emit copious
neutrinos 1053 erg/s
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NS-NS Mergers
Disk profiles leave a vacuum along the orbital
axis.
This opening funnels the explosion. Although it
will not produce few degree jets without the aid
of magnetic fields, it does produced beamed
explosions.
Ruffert et al. 1997
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Merger Rates Dependence On Kick Velocities
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Comparison To localized long-duration GRBs
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With delays and formation rates, We can predict
GRB rates as A function of redshift. But lots
of uncertainties Still abound!
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A first look at Collapsars

• Collapsar Progenitors
• Single Stars
• Binary Systems
• NS/BH merger with He-star
• Collapsar Types
• Collapse to black hole after supernova engine
  fails
• Fallback black hole after weak supernova
  explosion
• Direct Collapse to a Black Hole
• Jets and the collapsar model

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Constraints For Forming Collapsar GRBs

• Star must collapse to form black hole.
• Star must lose its hydrogen envelope so that it
  remains compact. Jet must travel through star
  roughly on the GRB duration timescale.
• Star must be rapidly rotating so disk forms
  around black hole.

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I Massive Single Star with High metallicity
loses its hydrogen envelope via winds. If it
retains enough mass and rotation to form a
BHAD, a GRB is produced.
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I Massive Single Star with High metallicity
loses its hydrogen envelope via winds. If it
retains enough mass and rotation to form a
BHAD, a GRB is produced. II Common envelope
Evolution removes Hydrogen envelope. Binary
required, but mass and rotation constraints
easier.
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I Massive Single Star with High metallicity
loses its hydrogen envelope via winds. If it
retains enough mass and rotation to form a
BHAD, a GRB is produced. II Common envelope
Evolution removes Hydrogen envelope. Binary
required, but mass and rotation constraints
easier. III Neutron star or Black Hole
formed first. Common Envelope evolution place
Compact remnant at center Of companion. Good
for Angular momentum constraints!
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Collapsar Types

• Type I Initial model. Star collapses but does
  not make a supernova explosion and ultimately
  forms a BHAD.
• Type II Star collapses to form neutron star
  with weak supernova explosion. Fallback causes
  collapse to black hole and formation of BHAD.
• Type III Collapse directly to a black hole from
  very massive stars.

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Jets in the Collapsar Model

• Energy deposition is far from beamed.
• But the funnel created by the thick disk will
  beam the explosion.
• Prediction of the collapsar model is that GRBs
  must be beamed. But how beamed requires
  relativistic calculations.

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Three main Progenitors Collapse of Single Star
Difficult to Get rotation rate. Collapse of
Merged Binary system Requires specific Binary
parameters Merger of NS/BH With helium core
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• Three Collapsar
• Types
• Failed supernova Probably the most likely under
  the neutrino-driven mechanism (but hard to make)
• Weak Supernovae May not be able to make jets
  strong enough to explain GRBs but easy to make!
• Only Population III stars do not work with
  neutrinos!

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All Collapsar models will produce JETS! GRBs
must Be beamed!