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Astronomy 350
Cosmology
Professor Lynn Cominsky
Department of Physics and Astronomy
Offices: Darwin 329A and NASA EPO
(707) 664-2655
Best way to reach me:
[email protected]
March 11, 2003
Lynn Cominsky - Cosmology A350
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Group 6
 Justin Beck
 Tiffany Henning
 Pamela Riek
 Ryan Silva
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Lynn Cominsky - Cosmology A350
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Stellar evolution made simple
Puff!
Bang!
BANG!
Stars like the Sun go gentle into that good night
More massive stars rage, rage against the dying of the light
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Exploding Stars
Supernova 1987A in
Large Magellanic Cloud
HST/WFPC2
 At the end of a star’s life, if it is large enough,
it will end with a bang (and not a whimper!)
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Supernova Remnants
Vela Region
CGRO/Comptel
 Radioactive decay of chemical elements
created by the supernova explosion
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Supernovae
 Supergiant stars
become (Type II)
supernovae at the
end of nuclear shell
burning
 Iron core often
remains as outer
layers are expelled
 Neutrinos and heavy
elements released
 Core continues to
collapse
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Chandra X-ray
image of Eta
Carinae, a potential
supernova
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Making a Neutron Star
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Three views of a Supernova
Lightcurve
Image
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Spectrum
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Crab nebula
movie
 Observed by
Chinese astronomers
in 1054 AD
 Age determined by
tracing back
exploding filaments
 Crab pulsar emits 30
pulses per second at
all wavelengths from
radio to TeV
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Crab nebula
Radio/VLA
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Infrared/Keck
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Crab nebula
Optical/Palomar
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Optical/HST WFPC2
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Crab nebula and pulsar
X-ray/Chandra
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Cas A
 ~320 years old
 10 light years across
 50 million degree shell
neutron star
Radio/VLA
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X-ray/Chandra
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Neutron Stars
 Neutron stars are formed from collapsed iron cores
 All neutron stars that have been measured have
around 1.4 Mo (Chandrasekhar mass)
 Neutron stars are supported by pressure from
degenerate neutrons, formed from collapsed
electrons and protons
 A teaspoonful of neutron star would weigh 1 billion
tons
 Neutron stars with very strong magnetic fields around 1012-13 Gauss - are usually pulsars due to
offset magnetic poles
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Neutron Stars: Dense cinders
Mass: ~1.4 solar masses
Radius: ~10 kilometers
Density: 1014-15 g/cm3
Magnetic field: 108-14
gauss
Spin rate: from 1000Hz
to 0.08 Hz
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Distances to Supernovae
Supernova 1987A in LMC
 Brightest SN in modern
times, occurred at t0
 Measure angular
diameter of ring, q
 Measure times when top
and bottom of ring light
up, t2 and t1
 Ring radius is given by
R = c(t1-t0 + t2-t0)/2
 Distance = R / q
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Lynn Cominsky - Cosmology A350
D = 47 kpc
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Distances to Supernovae
 Type Ia supernovae are “standard candles”
 Occur in a binary system in which a white dwarf star




accretes beyond the 1.4 Mo Chandrasekhar limit and
collapses and explodes
Decay time of light curve is correlated to absolute
luminosity
Luminosity comes from the radioactive decay of Cobalt and
Nickel into Iron
Some Type Ia supernovae are in galaxies with Cepheid
variables
Good to 20% as a distance measure
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Standard Candles
 If you have two light
sources that you know are
the same brightness
 The apparent brightness
of the distant source will
allow you to calculate its
distance, compared to the
nearby source
 This is because the
brightness decreases like
1/(distance)2
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movie
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Cosmological parameters
W = density of the universe / critical density
W< 1 hyperbolic
geometry
W = 1 flat or
Euclidean
W > 1 spherical
geometry
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Cosmological parameters
 In order to find the density of the Universe, you
must measure its total amount of matter and
energy, including:



All the matter we see
All the dark matter that we don’t see but we feel
All the energy from starlight, background radiation, etc.
 The part of the total density/critical density that
could be due to matter and/or energy = WM
 Current measurements : WM < 0.3
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Supernovae & Cosmology
WM = matter
WL = cosmological
constant
Redshift
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0
0.2
0.4
0.6
Lynn Cominsky - Cosmology A350
0.8
1
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Einstein meets Hubble
WM = 8 p G r
3 Ho 2
WL = L
3 Ho2
W(total) = WM + WL
Perlmutter et al.
40 supernovae
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Accelerating Universe
 Results from Perlmutter et al. (and also by another
group from Harvard, Kirshner et al.) strongly
suggest that if WM = 0.3 :
 WL
= 0.7
There is some type of dark energy which is
causing the expansion of the Universe to
accelerate
 Other results indicate that Wtotal = 1
 This will be discussed later at much greater length

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Distributions
 If sources are located
randomly in space, the
distribution is called
isotropic
 If the sources are
concentrated in a
certain region or along
the galactic plane, the
distribution is
anisotropic
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Classifying Bursts
 In this activity, you will be given twenty cards
showing different types of bursts
 Pay attention to the lightcurves, optical
counterparts and other properties of the
bursts given on the reverse of the cards
 How many different types of bursts are there?
Sort the bursts into different classes
 Fill out the accompanying worksheet to
explain the reasoning behind your
classification scheme
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What makes Gamma-ray
Bursts?
 X-ray Bursts
 Properties
 Thermonuclear Flash Model
 Soft Gamma Repeaters
 Properties
 Magnetar model
 Gamma-ray Bursts
 Properties
 Models
 Afterglows
 Future Mission Studies
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X-ray Bursts
 Thermonuclear flashes on Neutron Star
surface – hydrogen or helium fusion
 Accreting material burns in shells, unstable
burning leads to thermonuclear runaway
 Bursts repeat every few hours to days
 Bursts are never seen from black hole
binaries (no surface for unstable nuclear
burning) or from (almost all) pulsars
(magnetic field quenches thermonuclear
runaway)
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X-ray Burst Sources
 Locations in Galactic Coordinates
bursters
non-bursters
Globular Clusters
• Most bursters are
located in globular
clusters or near the
Galactic center
• They are therefore
relatively older
systems
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X-ray Burst Source Properties
 Neutron Stars in binary systems
Weaker magnetic dipole: B~108 G
 NS spin period seen in bursts ~0.003
sec.
 Orbital periods : 0.19 - 398 h from X-ray
dips & eclipses and/or optical
modulation
 > 15 well known bursting systems
 Low mass companions
 Lx = 1036 - 1038 erg/s

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X-ray Emission
X-ray emission from
accretion can be
modulated by
magnetic fields,
unstable burning and
spin
 Modulation due to
spin of neutron star
can sometimes be
seen within the burst

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X-ray Burst Sources
 Burst spectra are thermal black-body
L(t) =
4 p R2 s T(t)4
Temperature
Radius Expansion
c2
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Cominsky PhD 1981
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Soft Gamma Repeaters
 There are four of these objects known to date
 One is in the LMC, the other 3 are in the Milky
Way
SGR 1627-41
LMC
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Soft Gamma Repeater Properties
 Young Neutron Stars near SNRs







Superstrong magnetic dipole: B~1014-15 G
NS spin period seen in bursts ~5-10 sec,
shows evidence of rapid spin down
No orbital periods – not in binaries!
4 well studied systems + several other
candidate systems
Several SGRs are located in or near SNRs
Soft gamma ray bursts are from magnetic
reconnection/flaring like giant solar flares
Lx = 1042 - 1043 erg/s at peak of bursts
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SGR 1900+14
 Strong burst
showing ~5
sec pulses
 Change in 5 s
spin rate leads
to measure of
magnetic field
 Source is a
magnetar!
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SGR burst affects Earth
 On the night of August 27, 1998 Earth's upper
atmosphere was bathed briefly by an invisible burst
of gamma- and X-ray radiation. This pulse - the most
powerful to strike Earth from beyond the solar system
ever detected - had a significant effect on Earth's
upper atmosphere, report Stanford researchers. It is
the first time that a significant change in Earth's
environment has been traced to energy from a distant
star. (from the NASA press release)
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Gamma Ray Burst Properties
 A cataclysmic event of unknown origin







Unknown magnetic field
No repeatable periods seen in bursts
No orbital periods seen – not in binaries
Thousands of bursts seen to date – no
repetitions from same location
Isotropic distribution
Afterglows have detectable redshifts which
indicate GRBs are at cosmological distances
(i.e., far outside our galaxy)
Lg = 1052 - 1053 erg/s at peak of bursts
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The first Gamma-ray Burst
Vela satellite
 Discovered in 1967 while looking for nuclear test
explosions - a 30+ year old mystery!
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Compton Gamma Ray Observatory
BATSE
• Eight
instruments
on corners of
spacecraft
• NaI
scintillators
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CGRO/BATSE Gamma-ray Burst
Sky
 Once a day, somewhere in the Universe
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The GRB Gallery
When you’ve
seen one
gamma-ray
burst, you’ve
seen….
one
gamma-ray
burst!!
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Near or Far?
Isotropic distribution implications:
Very close: within a few parsecs of the Sun
Why no faint bursts?
Very far: huge, cosmological distances
What could produce such a vast amount of energy?
Sort of close: out in the halo of the Milky Way
A comet hitting a neutron star fits the bill
Silly or not, the only way to be sure was to find
the afterglow.
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Breakthrough!
In 1997, BeppoSAX detects X-rays from a GRB
afterglow for the first time, 8 hours after burst
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The View From Hubble/STIS
7 months
later
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On a clear night, you really can see
forever!
990123 reached 9th magnitude for a few moments!
First optical GRB afterglow detected simultaneously
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The Supernova Connection
GRB011121
Afterglow faded like supernova
Data showed presence of gas like a stellar wind
Indicates some sort of supernova and not a NS/NS merger
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Hypernova
movie
 A billion trillion times the power from the Sun
 The end of the life of a star that had 100 times the
mass of our Sun
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Iron lines in GRB 991216
 Chandra observations show link to hypernova
model when hot iron-filled gas is detected
from GRB 991216
Iron is a signature of a
supernova, as it is
made in the cores of
stars, and released in
supernova explosions
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Catastrophic Mergers
 Death spiral of 2 neutron stars or black holes
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Which model is right?
The data seem to indicate two kinds of GRBs
• Those with burst durations less than 2 seconds
• Those with burst durations more than 2 seconds
Short bursts have no detectable afterglows so far
as predicted by the NS/NS merger model
Long bursts are sometimes associated with
supernovae, and all the afterglows seen so far
as predicted by the hypernova merger model
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Gamma-ray Bursts
 Either way you
look at it –
hypernova or
merger model
 GRBs signal the
birth of a black
hole!
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Gamma-ray Bursts
 Or maybe
the death of
life on
Earth?
No, gammaray bursts did
not kill the
dinosaurs!
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How to study Gamma rays?
 Absorbed by the Earth’s
atmosphere
 Use rockets, balloons or
satellites
 Can’t image or focus gamma
rays
 Special detectors: crystals,
silicon-strips
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Lynn Cominsky - Cosmology A350
GLAST
balloon test
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HETE-2
 Launched on 10/9/2000
 Operational and finding about 2 bursts
per month
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Swift Mission
To be launched in 2003
 Burst Alert
Telescope (BAT)
 Ultraviolet/Optical
Telescope (UVOT)
 X-ray Telescope
(XRT)
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Swift Mission
 Will study GRBs with “swift” response
 Survey of “hard” X-ray sky
 To be launched in 2003
 Nominal 3-year lifetime
 Will see ~150 GRBs per year
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Gamma-ray Large Area
Space Telescope
 GLAST Burst
Monitor (GBM)
 Large Area
Telescope (LAT)
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GLAST Mission
 First space-based collaboration between
astrophysics and particle physics communities
 Launch expected in 2006
 Expected duration 5-10 years
 Over 3000 gamma-ray sources will be seen
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GRBs and Cosmology
 GRBs can be used as standard candles, similar to Type
1a supernovae
 However, the supernovae are only seen out to z=0.7
(and one at z=1.7), whereas GRBs are seen to z=4.5,
and may someday be seen to z=10
 Schaefer (2002) has constructed
a Hubble diagram for GRBs,
using the cosmological
parameters from supernova data.
When more burst redshifts
become available (e.g., from
Swift), the parameters can be
determined independently
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The Great Interplanetary
GRB Hunt
 Using data from several satellites in the
solar system, you will use a “light ruler”
to figure out the direction to a gammaray burst
 This is similar to the way that the
Interplanetary Network (IPN) really
works
 See http://ssl.berkeley.edu/ipn3/
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Web Resources :
 GLAST E/PO web site http://glast.sonoma.edu
 Swift E/PO web site http://swift.sonoma.edu
 Imagine the Universe!
http://imagine.gsfc.nasa.gov
 Science at NASA’s Marshall Space Flight
Center http://science.nasa.gov
 Supernova Cosmology Project
http://panisse.lbl.gov/
 Ned Wright’s ABCs of Distance
http://www.astro.ucla.edu/~wright/distance.htm
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Web Resources
 Robert Duncan’s magnetar page
http://solomon.as.utexas.edu/~duncan/magnetar.html
 Chandra observatory http://chandra.harvard.edu
 Jochen Greiner’s Gamma-ray bursts and SGR
Summaries http://www.mpe.mpg.de/~jcg
HETE-2 mission http://space.mit.edu/HETE/
 Compton Gamma Ray Observatory
http://cossc.gsfc.nasa.gov/
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