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Gamma Ray Bursts
Shamelessly stolen from Chris
Fryer’s summer school lectures
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
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
Gamma Rays in the Cold War
Vela 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
First Detected Gamma-Ray Burst
GRBs – The Golden Age for Theory
What we know/don’t
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!
What Theorists Know –
Constraints on Theory
• Can’t be thermal emission
alone!
• Options
I)
Relativistic Boosting from jet or
compact object!
II) Nuclear Lines (e.g. Nickel
Decay)
III) Magnetic Fields.
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:
• Solar System
• Galactic
• Cosmological
(outside of the
Milky Way)
Galactic
SS
Energy = Observed Flux d2
Energy
Requirements
Cosmological
Vary over 20 orders
Of magnitude!
The first gammaRay 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!
Gamma-Ray Bursts in the
Solar System
• Lightning in the
Earth’s
atmosphere
(High Altitude)
• Relativistic Iron
Dust Grains
• Magnetic
Reconnection
in the
Heliopause
Red Sprite Lightning
Gamma-Ray Bursts in the Milky
Way
• Accretion Onto White
Dwarfs
• Accretion onto neutron
stars
I) From binary
X-ray Novae
companion
II) Comets
• Neutron Star Quakes
• Magnetic Reconnection
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
Cosmological Models
• Collapsing WDs
• Stars Accreting on
AGN
• White Holes
• Cosmic Strings
• Black Hole Accretion
Disks
I) Binary Mergers
II) Collapsing Stars
Black-Hole Accretion
Disk (BHAD) Models
Binary merger or
Collapse of rotating
Star produces
Rapidly accreting
Disk (>0.1 solar
Mass per second!)
Around
black
hole.
BATSE - Burst And Transient Spectrometer Experiment
on Compton Gamma-Ray Observatory
BATSE Module
8 Detectors
Almost Full Sky Coverage
Few Degree Resolution
20-600keV
BATSE Consists of
two NaI(TI) Scintillation
Detectors: Large
Area Detector (LAD)
For sensitivity and the
Spectroscopy Detector
(SD) for energy coverage
BATSE Results - Isotropy
Galactic models
Gamma-Ray Burst Lightcurves
GRB990316
GRB Lightcurves have
A broad range of
Characteristics
Fast Rise Exponential Decay
“FREDs”
GRB970508
Gamma-Ray Burst Lightcurves
GRB990123
Double bursts and
Extended Structures
No standard shape
Exists!
GRB980703
Gamma-Ray Burst Durations
Two Populations:
Short – 0.03-3s
Long – 3-1000s
Possible third
Population
1-10s
Gamma-Ray Burst Duration
vs. Energy Spectrum
BeppoSAX Instruments
LECS/MECS
• Xenon Gas
Scintillator
• Energy Range: .11keV (1-10keV)
• ~1 arc minute
resolution
• Goal – Localize
Object
HPGSPC(Phoswitch)
• HPGSPC - High
Pressure Xenon/He
Gas
• Phoswitch - NaI(Tl),
CsI(Na) Scintillators
• 4-120keV (15300keV)
• Goal – Broad Energy
resolution in X-ray
narrow field
GRB970228 – first good localization
GRB070228 – Optical Counterpart
Discovered (with corresponding optical
localization!)
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!
Metzger et al. 1997
flux
GRB970508 – Absorption Lines: z=0.835
Wavelength
Optical Emission
Absorption
Fe II
Fe II
Mg II I
flux
Mg II
Wavelength
Radio Scintillation can also be used to estimate
the GRB distance: consistent with z=0.835
Just as the Earth’s
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.
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.
GRBs in the Swift Era
Thanks to Neil Gehrels
Eiso
Location, Location, Location
(In addition to detecting hosts, we can determine
where a burst occurs with respect to the host.
Distribution
Follows
Stellar
Distribution
If we take
These
Positions
At face
Value,
We can
Determine
The
Distribution
Of bursts
With respect
To the halfLight radius
Of host
Galaxies!
This Will
Constrain
The models!
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
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.
GRB021004
For >1/2 of
Gamma-Ray
Bursts,
afterglows
consistent with
constant density
or inconsistent
with wind
bubbles.
(radio
And R-band
Data best
Diagnostics!
Roger Chevalier
Li & Chevalier 2003
Jet Signatures
E   (1  cos b )E  ,iso
f b  (1  cos b )   b /2
2
 n o 
b  

E
  ,iso 
1/ 8
 t break 
 1 z 


3/8
Stanek et al. (2001)
GRB 010222
Energy and Beaming
Corrections
•
15 events with z and t_jet
•
•
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)
E  5 1050 erg
(for n o  0.1 cm -3 assumed)
Frail et al. (2001)
Summary of GRB Energetics
• Gamma-ray bursts and
their afterglows have
(roughly) standard
51
E

10
erg
energies 
E k  10 51 erg
E  gamma-rays
Ek  X-rays
Ek  BB modeling
Ek  Calorimetry
• Robust result using
several complementary
51
E

E

E

few

10
erg
shock

k
methods
  E / E shock  0.5
SN/GRB connection!
GRBs have SN-like outbursts.
But these bursts are beamed, and we won’t see
all explosions as a GRB.
What do we make of the SN/GRB connection:
I) All GRBs produce SNe?
II) All SNe are GRBs (only those observed
along the jet axis are GRBs)?
Are either of these true?
How Common are EnginePowered 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)<<1 foe (hydo
collapse)
• <10% are 1998bw-like
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.
Internal Shocks
Shocks between different
shells of the ejected
relativistic matter
D=cT
d=cdT
• dT=R/cg2= d/c

D/c=T
• 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.
d
T
T
Internal
Shocks
Afterglow
D=cT
d=cdT
• 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)
Gamma-Ray Burst: 4
Stages
1) Compact Source, E>1051erg
2) Relativistic Kinetic Energy
3) Radiation due to Internal shocks = GRBs
Plus burst of optical emission!
4) Afterglow by external shocks
The Central Compact Source is
Hidden
The Internal-External Fireball Model
-rays
Inner
Engine
Relativistic
Wind
Internal
Shocks
Afterglow
External
Shock
There are no direct observations of the inner engine.
The -rays light curve contains the best evidence on the
inner engine’s activity.
The Resolution of the Energy
Crisis
 Etot - The total energy
 Eiso - Observed (iostropic) ray energy
Etot   E iso
1
Beaming:
E- Actual
ray energy
Etot    E   
1
1

2
2
E iso
The two most powerful BeppoSAX bursts are
jets (Sari, Piran & Halpern; 1999).
JETS and BEAMING
Particles remain
within initial cone
Radiation is
“beamed” into
a narrow cone
Particles spreads
sideways
quickly
Radiation
is “beamed”
into a
large cone
1
Jets with an opening angle  expand forwards until 1 and then
expand sideways rapidly lowering quickly the observed flux (Piran,
1995; Rhoads, 1997; Wijers et al, 1997; Panaitescu & Meszaros
1998).
Fireball Model - Summary
• Basic Fireball model simple
– Relativistic shocks with
synchrotron + inverse
Compton emission
• Internal Shocks produce
optical burst and gammarays, External Shocks
produce afterglow
• Jets alter the spectra in an
observable way.
Sedov Solution Useful – Relativistic
version needs some tuning
[r]=[E/r]1/5[t]2/5=[E/r0]1/5rw/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)
Particles in a B-field radiate
Relativistic Particles
Psynchrotron = 2q2/3c3 4
q2B2/(2m2c2)vperp2
= 2/3 r02cbperp22B2
where r0=e2/mc2
Isotropic velocities
B
bperp2 =2b2/3
<
>
Psynch=4/3Tcb22B2/(8p)
where T=8pr02/3 is the
Thompson Cross-Section
vpar
vperp
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)
• …
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)
Gratuitous Mushroom Cloud
Picture
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
E=1053-1054 erg
Allowing a 1-10%
Efficiency!
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 <10-100
Millisecond level
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
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
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.
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
Two Jet Drivers: Neutrinos &
e+,e- pair plasma
Neutrino
Annihilation
Disk Cools via Neutrino
Emission
Densities above 1010-1011 g cm-3
Temperatures above a few MeV
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
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
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 <1s: not a
working model for
long-duration bursts
• Many predictions
don’t match afterglow
era science (long
GRBs).
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 =>
Jets => GRB!
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
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.
Magnetic NSs in Collapse – Cont.
• Using a pulsar-like
mechanism
(magical/magnetic fields
strike again), Wheeler
argued that this fastspinning, newly born,
neutron star will produce
jets in most stellar
collapses.
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!
Problems with the Magnetic NS model
• Magnetic Field Model
requires very strong
magnetic fields! Only
shown to work with handwavy approximation.
• SN spectra are different
than the SN-like spectra
in GRBs and hypernovae.
Nickel distribution from
asymmetric explosion
Hungerford et al. 2003
Problems with the Magnetic NS model: SNe vs.
SN-like outbursts – spectra different!
Ic: no H,
Ia
Ib
SiII
O Ca
He
Ic
Hyper
-novae
no strong He,
94I
97ef
98bw
no strong Si
Hypernovae：
broad features
blended lines
“Large mass at
high velocities”
Problems with the Magnetic NS model
• Magnetic Field Model requires
very strong magnetic fields!
Only shown to work with handwavy approximation.
• SN spectra are different than
the SN-like spectra in GRBs
and hypernovae.
• Why do we get 3 branches of
supernova energies?
Nickel distribution from
asymmetric explosion
Hungerford et al. 2003
Supernovae/Hypernovae
Nomoto et al. (2003)
EK
Failed SN?
13M~15M
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!
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
Advantages of the Supranova
Model
• GRB occurs after
supernova explosion
• Iron produced in
supernova can then
be lit up by gammaray 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.
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.
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.
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!
Disadvantages of the Supranova
Model
Mass thing…
Duration can’t be
Longer than 3-3000ms
NS,
BH
Disadvantages of the Supranova
Model
Duration = Rotation
Period / Disk Viscosity
(a = 0.1-10-4)
Duration can’t be
Longer than 3-3000ms
Current bursts with
Iron lines are all
Long-duration!
NS,
BH
The straw that broke the camel’s
back – observations (not physics)!
• In the supranova
model, the supernova
explosion should
occur months before
the GRB
• Observations (again
limited to longduration bursts) find
that the supernovalike explosion occurs
alongside the GRB.
http://www-cfa.harvard.edu/~jbloom/valencia
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
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 corecollapse (MSN~ 8-10
solar masses)
Fryer, Woosley & Hartmann 1999
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
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.
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.
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.
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
Disk Structure for NS-NS Mergers
Densities exceed 1011 g cm-3,
Ruffert & Janka 1999
Temperatures exceed a few MeV,
Disk masses range from 0.03-0.25 solar masses
NS-NS Mergers – Neutrino Emission
These dense,
hot disks
emit copious
neutrinos
~1053 erg/s
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
Merger
Rates
Dependence
On Kick
Velocities
Comparison
To localized
long-duration
GRBs
With delays and
formation rates,
We can predict
GRB rates as
A function of
redshift.
But lots of
uncertainties
Still abound!
A first look at Collapsars
•
I)
II)
III)
•
I)
II)
III)
•
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
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.
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.
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.
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!
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.
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.
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
Three Collapsar
Types:
I) Failed supernova:
Probably the most
likely under the
neutrino-driven
mechanism (but
hard to make)
II) Weak Supernovae:
May not be able to
make jets strong
enough to explain
GRBs but easy to
make!
III) Only Population III
stars – do not work
with neutrinos!
All Collapsar models will produce JETS! GRBs must
Be beamed!