GammaRay Bursts

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Lecture 18 Gamma-Ray Bursts
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Nuclear Test Ban Treaty, 1963 First Vela
satellite pair launched 1963
Velar to watch
The Vela 5 satellites were placed in orbit by
the Advanced Research Projects of the DoD and
the AEC. Launched on May 23, 1969 into high
earth orbit (118,000 km), this pair of
satellites and their predecessors, Vela 4,
discovered the first gamma-ray bursts. The
discovery was announced by Klebesadel, Strong,
and Olson (ApJ, 182, 85) in 1973.
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First Gamma-Ray Burst
The Vela 5 satellites functioned from July, 1969
to April, 1979 and detected a total of 73
gamma-ray bursts in the energy range 150 750
keV (n.b,. Greater than 30 keV is gamma-rays)
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Typical durations are 20 seconds but there
is wide variation both in time- structure and
duration. Some last only hundredths of a
second. Others last thousands of
seconds. Typical power spectra peak at 200 keV
and higher.
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Paciesas et al (2002) Briggs et al (2002)
Koveliotou (2002)
Shortest 6 ms GRB 910711
Longest 2000 s GRB 971208
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In total about 5000 gamma-ray bursts have been
detected
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Observed
Expected if havent reached any edge yet
log number of sources
log sensitivity
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This posed a problem for the models that had GRBs
in our own Galaxy ..


















X









We are not at the center of our galaxy so we
should see more bursts towards the center than
in the opposite direction,









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• Isotropy could mean three things (in 1990)
• Very nearby bursts centered on the Earth e.g.,
• the Oort cloud of comets
• A very extended spherical halo around the Galaxy
  much bigger than the distance from here to
  the center of the Galaxy
• Bursts very, very far away billions of light
  years

What we really needed was source identifications.
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BeppoSax (1996-2002)
(Italian-Dutch X-ray astronomy mission)
MEC S and LECS (medium and lowenergy x-ray
sensors, 1 arc min positions)
(2-30 keV 20x20 degree FOV angular resolution
5 arc min)
The scintillator anti-coincidence shields of the
Phoswich detector are able to detect gamma-rays
60-600 keV and get crude angular information
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BeppoSax GRB 970228 (discovered with WFC)
Feb 28, 1997 (8 hr after GRB using MECS)
March 3, 1997 (fainter by 20)
Each square is about 6 arc min or 1/5 the moons
diameter
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GRB 970228
William Hershel Telescope
Isaac Newton Telescope
Groot, Galama, von Paradijs, et al IAUC 6584,
March 12, 1997
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GRB 970228
HST images March 26 and April 7, 1997
showed afterglow was in a faint galaxy
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another GRB much later GRB 990123
Two HST images of GRB 990123. The image on the
left was taken February 8, 1999, the one on the
right March 23, 1999. Each picture is 3.2 arc
seconds on a side. Three orbits of HST time were
used for the first picture two for the second
hence the somewhat reduced exposure.
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GRB 970228
Spectrum of the host galaxy of GRB 970228
obtained at the Keck Telescope. Prominent
emission lines of oxygen and neon are indicated
and show that the galaxy is located at a
redshift of z 0.695. (Bloom, Djorgovski, and
Kulkarni (2001), ApJ, 554, 678. See also GCN
289, May 3, 1999.
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GRB 970228
From the red shift a distance can be inferred
billions of light years. Far, far outside our
galaxy. From the distance and brightness an
energy can be inferred.
1.6 x 1052 erg in gamma rays alone
This is 13 times as much energy as the sun will
radiate in its ten billion year lifetime, but
emitted in gamma-rays in less than a minute. It
is 2000 times as much as a really bright
supernova radiates in several months.
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As of July 2002, 4677 GRBs had been
detected http//grbcat.gsfc.nasa.gov/grbcat/grbca
t.html As of May 2005, 53 redshifts had been
determined, the largest is still 4.5.
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The typical energy is 1053 erg or about 5 of the
mass of the sun turned to pure energy according
to E mc2
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GRB 990123
2 x 1054 erg
Had this burst occurred on the far side of our
Galaxy, at a distance of 60,000 light years, it
would have been as bright in gamma-rays as
the sun. This is ten billion times brighter than
a supernova and equivalent to seeing a fifty
million trillion trillion megaton explosion.
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The majority consensus
32 bursts are plotted here. Now (5/05) about
53 redshifts are known. Overall the distribution
remains similar
Zeh et al (2004)
http//www.mpe.mpg.de/jcg/grbrsh.html
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Djorgovski et al (2002)
almost always galaxies experiencing an unusual
rate of star formation
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Are the energies required really that great?
Earth
If the energy were beamed to 0.1 of the sky,
then the total energy could be 1000 times less
Earth
Nothing seen down here
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• GRBs are produced by highly relativistic flows
  that have been collimated into narrowly
  focused jets

Quasar 3C273 as seen by the Chandra x-ray
Observatory
Quasar 3C 175 as seen in the radio
Artists conception of SS433 based on
observations
Microquasar GPS 1915 in our own Galaxy time
sequence
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Minimum Lorentz factors for the burst to be
optically thin to pair production and to avoid
scattering by pairs. Lithwick Sari, ApJ, 555,
540, (2001)
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It is a property of matter moving close to the
speed of light that it emits its radiation in a
small angle along its direction of motion. The
angle is inversely proportional to the Lorentz
factor
This offers a way of measuring the beaming angle.
As the beam runs into interstellar matter it
slows down.
Measurements give an opening angle of about 5
degrees.
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• GRBs have total energies not too unlike
  supernovae

Frail et al. ApJL, (2001), astro/ph 0102282
Despite their large inferred brightness, it is
increasingly believed that GRBs are
not inherently much more powerful than
supernovae. From afterglow analysis, there is
increasing evidence for a small "beaming angle"
and a common total jet energy near 3 x 1051 erg
(for a conversionefficiency of 20).
See also Freedman Waxman,
ApJ, 547, 922 (2001) Bloom,
Frail, Sari AJ, 121, 2879
(2001) Piran et al. astro/ph
0108033 Panaitescu Kumar,
ApJL, 560, L49 (2000)
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Supernovae
Looking for bumps
must correct for redshift
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Results
Zeh, Klose, Hartmann (2004)

• among 36 GRBs with optical afterglows (end of
  2002), 21 have a sufficient data quality and a
  known redshift
• nine late time bumps are found in afterglow light
  curves

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SN 1998bw/GRB 980425
NTT image (May 1, 1998) of SN 1998bw in the
barred spiral galaxy ESO 184-G82Galama et al,
AA S, 138, 465, (1999)
WFC error box (8') for GRB 980425 and two NFI
x-ray sources. The IPN error arc is also shown.
Type Ic supernova, d 40 Mpc Modeled as the 3 x
1052 erg explosion of a massive CO
star (Iwamoto et al 1998 Woosley, Eastman,
Schmidt 1999) GRB 8 x 1047 erg 23 s
a very unusual supernova!
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The smoking gun
GRB030329/ SN2003dh
z 0.1685 One of the brightest GRBs ever HETE2
Stanek et al., Chornock et al. Eracleous
et al., Hjorth et al., Kawabata et al.
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Lpeak implies (again) 0.5 solar masses of 56Ni
Exceptionally --- bright fast high
velocity radio bright
Supernova simultaneous with the GRB (- 2 days).
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Summary Requirements (long-soft bursts)

• Provide adequate energy at high Lorentz factor
  (G gt 200 KE 3 x 1051 erg)
• Collimate the emergent beam to approximately 0.1
  radians
• Make bursts in star forming regions
• In the internal shock model, provide a beam
  with rapidly variable Lorentz factor
• Allow for the observed diversity seen in GRB
  light curves
• Last approximately 20 s, but much longer in some
  cases
• Explain diverse events like GRB 980425
• Produce a (Type Ib/c) supernova in some cases

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SWIFT so far May 28, 2005 39
bursts detected starting December 17, 2004
9 red shifts determined One short, hard
burst localized to the outskirts of an
elliptical galaxy at redshift 0.255. Press
conference this week.
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Models
It is the consensus that the root cause of
these energetic phenomena is star death that
involves an unusually large amount of angular
momentum (j 1016 1017 cm2 s-1) and quite
possibly, one way or another, ultra-strong
magnetic fields (1015 gauss). These are
exceptional circumstances. A neutron star or a
black hole is also implicated.
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Dana Berry
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Merging neutron star - black hole pairs
Strengths a) Known event b)
Plenty of angular momentum c)
Rapid time scale d) High
energy e) Well developed
numerical models
Weaknesses a) Outside star forming regions
b) Beaming and energyprobably
inadequate for long
bursts c) Do not make
supernovae
But this model may still be good for a class of
bursts called the short hard bursts for which
we have no counterpart information yet.
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Models based on massive stars Millisecond
Magnetars Supranovae
Collapsars
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e.g. Wheeler, Yi, Hoeflich, Wang (2001)
Usov (1992, 1994, 1999)
The ms Magnetar Model
But now there exist magnetars and AXPs
But this much angular momentum is needed in all
modern GRB models
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The ms Magnetar Model (continued)
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Critical Comments on ms magnetar model

• Isotropic explosion would be not lead to
  adequate material with high Lorentz
  factor (even with 1053 erg Tan, Matzner,
  McKee 2001)
• Jetted explosion would require too much
  momentum
• (and too much baryons) to achieve high
  Lorentz factor.
• Need to wait for polar regions to
  clear, but during that
• time the neutron star would probably
  become a black hole.
• Jets, by themselves are inefficient at
  producing 56Ni.
• So need a combination energetic
  isotropic explosion
• to start with, then a powerful focused
  jet.
• Or maybe the ms magnetar has an accretion disk
  and wind?

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The Collapsar Model
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Usually massive stars make supernovae. Their iron
core collapses to a neutron star and the energy
released explodes the rest of the star. But what
if the explosion fizzled? What if the iron
core collapsed to an object too massive to be a
neutron star a black hole. A star without
rotation would then simply disappear.... But
what if the star had too much rotation to all
go down the (tiny) black hole? If supernovae are
the observational signal that a neutron star has
been born, what is the event that signals the
birth of a black hole?
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Collapsar Progenitors
Two requirements

• Core collapse produces a black hole - either
  promptly or very shortly thereafter.
• Sufficient angular momentum exists to form a
  disk outside the black hole (this virtually
  guarantees that the hole is a Kerr hole)

Fryer, ApJ, 522, 413, (1999)
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The more difficult problem is the angular
momentum. This is a problem shared by all current
GRB models that invoke massive stars...
In the absence of mass loss and magnetic fields,
there would be abundant progenitors. Unfortunatel
y nature has both.
15 solar mass helium core born rotating rigidly
at f times break up
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Black hole formation may be unavoidable for low
metallicity
Solar metallicity
Low metallicity
With decreasing metallicity, the binding energy
of the core and the size of the silicon core
both increase, making black hole formation more
likely at low metallicity. Woosley, Heger,
Weaver, RMP, (2002)
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The Star Collapses (log j gt 16.5)
alpha 0.1
alpha 0.001
Nucleons
7.6 s
7.5 s
Neutrino flux low, 56Ni low
Neutrino flux high, 56Ni high
MacFadyen Woosley ApJ, 524, 262, (1999)
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7.6 s after core collapse high viscosity case.
In the vicinity of the rotational axis of the
black hole, by a variety of possible processes,
energy is deposited.
It is good to have an energy deposition mechanism
that proceeds independently of the density and
gives the jet some initial momentum along the
axis
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Blandford Znajek (1977) Koide et al. (2001) van
Putten (2001) Lee et al (2001) etc.
MHD Energy Extraction
The efficiencies for converting accreted matter
to energy need not be large. B 1014 1015
gauss for a 3 solar mass black hole. Well below
equipartition in the disk.
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The Production of 56Ni

• Needed to power the light curve of the supernova
  if one is to be visible. Need 0.1 to 0.5
  solar masses of it.
• A bigger problem than most realize The
  jet doesnt do it too little mass
  Forming the black hole depletes the innermost
  core of heavy elements Pulsars may
  have a hard time too if their time scale is gt 1 ms

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The disk wind MacFadyen Woosley (2001)
Neglecting electron capture in the disk
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The Jet-Star Interaction
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Break out at 9 s Density
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Break out at 9 s Lorentz factor
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GRB
G 200
5o , internal shocks
GRB 980425
G 10 - 100
Hard x-ray bursts
20o , external shocks?
G 1
Unusual supernova (polarization, radio source)
A Unified Model for Cosmological Transients
(analogous to AGNs)
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A 1053 erg event situated 30,000 light years
away (distance from here to the Galactic center)
would give as much energy to the earth in 10
seconds as the sun equivalent to a 200 megaton
explosion. Does it matter having an extra sun in
the sky for 10 seconds? Probably not. This is
spread all over the surface of the earth and the
heat capacity of the Earths atmosphere is very
high. Gamma-rays would deposit their energy
about 30 km up. Some bad nitrogen chemistry
would happen. Noticeable yes, deadly to all
living things No.
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Biological Hazards of Gamma-Ray Bursts
Distance Events
Megatons Results (kpc)
/10 by

• 100 1000 200
  Some ozone damage, EMP
  acid
  rain
• 1 1 10
  20,000 Ozone gone, acid rain,
  blindness
  2nd and 3rd degree
  burns
• 0.1 0.01 0.1 two
  million Shock waves, flash incineration,
  
  tidal waves, radioactivity (14C)
• 
  End of life as we know it.

Depends on uncertain efficiency for conversion
of energetic electrons to optical light