Gamma-ray Large Area Space Telescope
GLAST is a satellite designed to
measure the direction, energy
& arrival time of celestial
gamma rays
Launch is in 2007
Purpose
Study AGN’s
Study GRB’s
Search for Dark Matter
Dark Matter?? What the heck is
that?! Why are gamma rays
helpful? What are gamma rays
anyway?
Richard E. Hughes
Dark Matter & GLAST; p.1
Dark Matters…..
This is luminous matter
This is dark matter
It is tempting to look at the universe, seeing stars and galaxies, clusters of
galaxies and come to the conclusion that what you SEE is the matter, and what
you don’t see is empty space. But, you would be wrong! There is general
agreement that, in fact, MOST of the matter in the universe is in a form that we
can’t SEE. This matter is imaginatively referred to as “Dark Matter”.
Richard E. Hughes
Dark Matter & GLAST; p.2
Rotation Velocities in Our Solar System
The curve below is exactly what one
expects if most of the mass is
inside the orbits of the planets. In
our case, most of the mass in the
solar system is due to the sun.
Another way of viewing things is to
say that if you know the orbital
speed of ANY planet, you can
determine how much massive
“stuff” must be INSIDE that orbit.
GM r
v r  
r
2
Richard E. Hughes
Dark Matter & GLAST; p.3
The Milky Way Galaxy
Our sun is in the Milky way galaxy, about 28,000 light-years from its center.
The speed of the solar system relative to the galactic center is approximately
220 km/s. At this speed, it takes about 200 million years to make one
complete revolution.
A galaxy is composed of stars and other material which are held together by
gravity.
The name ‘Milky Way’ comes from the band of light that can be seen during
dark nights in the summer. This band is actually an edge-on view of the galaxy,
and it is believed that when viewed “head on” it is a spiral glaxy.
The Celestial River
Richard E. Hughes
Dark Matter & GLAST; p.4
The Milky Way Galaxy
 The COBE satellite was designed to investigate a phenomenon called the
Cosmic Microwave Background.
 COBE is sensitive to infra-red (IR) wavelengths of light.
 The Milky Way viewed in the visible, is obscured by dust.
 However, viewed in the IR, the Milky Way shows a clear central bulge
overlaying a thin disk, as expected of an edge-on view of a spiral galaxy:
Richard E. Hughes
Dark Matter & GLAST; p.5
The Milky Way Galaxy
The image shown is a
rendition of what we
believe the Milky Way
galaxy looks like if it were
viewed head on:
 The radius is about
50,000 light-years
 The sun is about 28,000
light-years from the
center
 Near the Orion arm
 Between the arms
Perseus and Sagittarius
Richard E. Hughes
Dark Matter & GLAST; p.6
The Rotation Curve For the Milky Way
The same sort of rotation curve can be made for the Milky Way galaxy.
Given that the Sun is on the outer edge of the galaxy (about 2/3 out), we
expect that most of the mass is inside the galactic radius of the Sun. So
we should see a decreasing rotation curve, like we do for the solar system.
But instead, it is FLAT (if not increasing).
Richard E. Hughes
Dark Matter & GLAST; p.7
How about other galaxies?
NGC 6503: Galaxy in Constellation Draco
Richard E. Hughes
Dark Matter & GLAST; p.8
Yet another galaxy…
Richard E. Hughes
Dark Matter & GLAST; p.9
Why are the rotation curves flat?
 Stars and gas in the galactic disks follow circular orbits
whose velocity depends on the inner mass only:
 A flat rotation curve means that the total M(<r) increases
linearly with r, while the total luminosity approaches a
finite asymptotic limit as r increases. Clearly a large amount
of invisible gravitating mass (more than 90% of the total
mass in the case of the Milky Way and other examples) is
needed to explain these flat rotation curves.
 This invisible mass is referred to as DARK MATTER
 Is there any other supporting evidence?
Richard E. Hughes
Dark Matter & GLAST; p.10
Gravitational Lensing
Richard E. Hughes
Dark Matter & GLAST; p.11
Example of Gravitational Lensing
Foreground cluster of
galaxies CL0024+1654
(constellation Pisces)
Blue galaxy
behind the
cluster
“lensed” copy of
blue galaxy
Richard E. Hughes
Dark Matter & GLAST; p.12
What is causing the Lensing?
The majority of the dark matter is distributed broadly and smoothly in the cluster,
covering a region on the sky more than 1.6 million light-years across. The mass of
the individual cluster galaxies appears as pinnacles on this mountain of dark matter
mass. Overall, the dark matter in the cluster outweighs all the stars in the cluster's
galaxies by 250 times!
From http://www.bell-labs.com/org/physicalsciences/projects/darkmatter/darkmatter1.html
Richard E. Hughes
Dark Matter & GLAST; p.13
What and where is the dark matter?
The dark matter can’t be in the central disk of galaxies. Why?
Interstellar clouds would be much thinner (due to gravitational forces
of the dark matter.
So the dark matter must be in “halos” of the galaxies.
What the dark matter is NOT:
1) Stars: even faint one would radiate some light.
2) Dust: we would not be able to see our own galaxy or others, since
dust absorbs and scatters light
What the dark matter MIGHT be:
1) Black holes
2) Dim, old white dwarfs which are no longer bright
3) Proto-stars in which fusion did not start
4) Some new form of elementary matter
Richard E. Hughes
Dark Matter & GLAST; p.14
Searching for Dark Matter
 If we believe that Dark Matter really does exist, how do we
look for it?
 Well, we need a model. And one which is pretty handy is the
Standard Model!
 Well, actually not the Standard Model, but a close relative,
which involves something called “SuperSymmetry”
 A particle predicted by the SuperSymmetry theory is called
the Neutralino
 This particle is predicted for reasons having NOTHING to
do with dark matter, but – in a happy coincidence – it
COULD BE that the neutralino is the mysterious source of
Dark Matter.
 Once the neutralino is made, it can’t decay into something else
 UNLESS: it meets its antiparticle.
Richard E. Hughes
Dark Matter & GLAST; p.15
The Neutralino
 Postulates:
 The dark matter particle is the neutralino
 There are enough dark matter particles in the halo of galaxies that
the dark matter particles will collide from time to time
 Since the dark matter particle is its OWN anti-particle, when the
particles collide, they will ANNIHILATE
Richard E. Hughes
Dark Matter & GLAST; p.16
Seeing the Annihilation of the Neutralino
 Annihilation of the dark matter
particles
 This will result in other particles
 Sometimes only 2 photons
(gamma rays) of very distinct
energy (equal to the mass x c2)
of the dark matter particle
 Sometimes a number of photons
of lower energy
g lines
50 GeV
300 GeV
 We can see these photons
 Using telescopes on earth (if the
energies are large)
 Using satellites designed to see
lower energy photons
GLAST is such a satellite.
Richard E. Hughes
Dark Matter & GLAST; p.17
GLAST Mission
GLAST measures the direction,
energy & arrival time of
celestial gamma rays
GLAST is two instruments:
- Large Area Telescope(LAT)
measures gamma-rays in the
energy range ~20 MeV - >300
GeV
- Gamma-ray Burst Monitor(GBM)
provides correlative observations
of transient events in the energy
range ~20 keV – 20 MeV
Launch: March 2007
Orbit:
550 km,
28.5o inclination
Lifetime: 5 years
(minimum)
Richard E. Hughes
Dark Matter & GLAST; p.18
Why study g-rays ?
Gamma-rays carry a wealth of information
 g-rays offer a direct view into Nature’s largest
accelerators
 the Universe is mainly transparent to g-rays: can probe
cosmological volumes.
 g-rays readily interact in detectors, with a clear signature.
 g-rays are neutral: no complications due to magnetic fields;
point directly back to sources, etc.
Richard E. Hughes
Dark Matter & GLAST; p.19
GLAST is an International Mission
NASA - DoE Partnership on LAT
• LAT is being built by an international team
• Si Tracker: Stanford, UCSC, Japan, Italy
• CsI Calorimeter: NRL, France, Sweden
• Anticoincidence: GSFC
• Data Acquisition System: Stanford, NRL, Ohio State
GBM is being built by US and Germany
• Detectors: MPE
Sweden
Italy
Germany
USA
Richard E. Hughes
France
Japan
Dark Matter & GLAST; p.20
GLAST LAT Collaboration
United States










California State University at Sonoma
University of California at Santa Cruz - Santa Cruz Institute of Particle Physics
Goddard Space Flight Center – Laboratory for High Energy Astrophysics
Naval Research Laboratory
The Ohio State University
Stanford University – Hanson Experimental Physics Laboratory
Stanford University - Stanford Linear Accelerator Center
Texas A&M University – Kingsville
University of Washington
Washington University, St. Louis

Centre National de la Recherche Scientifique / Institut National de Physique
Nucléaire et de Physique des Particules
Commissariat à l'Energie Atomique / Direction des Sciences de la Matière/
Département d'Astrophysique, de physique des Particules, de physique Nucléaire et
de l'Instrumentation Associée
France

Italy


Istituto Nazionale di Fisica Nucleare
Istituto di Fisica Cosmica, CNR (Milan)
Japanese GLAST Collaboration
 Hiroshima University
 Institute for Space and Astronautical Science
Richard
RIKEN
E. Hughes
124 Members (including 60
Affiliated Scientists)
16 Postdoctoral Students
26 Graduate Students
Dark Matter & GLAST; p.21
Launch of Satellite
Satellite Launch: Delta II Rocket
• 891 to 2,142 kg (1,965 to 4,723 lb) to
geosynchronous transfer orbit (GTO) and
2.7 to 6.0 metric tons (5,934 to 13,281 lb)
to low-Earth orbit (LEO).
• used for deep-space explorations such as
NASA's missions to Mars, a comet or
near-Earth asteroids.
Richard E. Hughes
Dark Matter & GLAST; p.22
Gamma-ray Large Area Space Telescope
GLAST Mission
GLAST Observatory
• spacecraft
• LAT
• GBM
 high-energy gamma-ray
observatory; 2 instruments
- Large Area Telescope
(LAT)
- Gamma-ray Burst
Monitor (GBM)
 launch (March 2006):
Delta 2 class
 orbit: 550 km, 28.5o
inclination
 mission operations
 science
- LAT Collaboration
- GBM Collaboration
- Guest Observers
 lifetime:
5 years (minimum)
Richard E. Hughes
LAT Inst. Ops. Center
LAT data handling
Instrument performance
Level 1 data processing; selected
higher level processing
Support LAT Collaboration Science
Investigation
Routine Data
Alerts
Large loads
TOO commands
LAT Data
Spacecraft,
GBM data
Status
Command Loads
Mission Ops Center
Observatory safety
Spacecraft health
Commanding
Mission scheduling
Level 0 processing
GBM data handling
Burst and transient Alerts
GRB
Coordinates
Network
Science Support Center
Science scheduling
Archiving
Guest Observer Support
Standard product processing
Schedules
Spacecraft data for
archiving
GBM Data
GBM Inst. Ops. Center
Status
Command Loads
Instrument performance
Standard product processing
Dark Matter & GLAST; p.23
GLAST LAT Overview: Design
g
Si Tracker
ACD
pitch = 228 µm
8.8 105 channels
12 layers × 3% X0
+ 4 layers × 18% X0
+ 2 layers
Segmented
scintillator tiles
0.9997 efficiency
 minimize self-veto
Grid (& Thermal
Radiators)
CsI Calorimeter
e+
e–
Hodoscopic array
8.4 X0 8 × 12 bars
2.0 × 2.7 × 33.6 cm
 cosmic-ray rejection
 shower leakage
correction
Richard E. Hughes
3000 kg, 650 W (allocation)
1.8 m  1.8 m  1.0 m
20 MeV – 300 GeV
Flight Hardware & Spares
Data
acquisition
16 Tracker Flight Modules + 2 spares
16 Calorimeter Modules + 2 spares
1 Flight Anticoincidence Detector
Data Acquisition Electronics + Flight Software
Dark Matter & GLAST; p.24
GLAST LAT Overview: Design
g
Si Tracker
ACD
pitch = 228 µm
8.8 105 channels
12 layers × 3% X0
+ 4 layers × 18% X0
+ 2 layers
Segmented
scintillator tiles
0.9997 efficiency
 minimize self-veto
Grid (& Thermal
Radiators)
CsI Calorimeter
e+
e–
Hodoscopic array
8.4 X0 8 × 12 bars
2.0 × 2.7 × 33.6 cm
 cosmic-ray rejection
 shower leakage
correction
Richard E. Hughes
3000 kg, 650 W (allocation)
1.8 m  1.8 m  1.0 m
20 MeV – 300 GeV
Flight Hardware & Spares
Data
acquisition
16 Tracker Flight Modules + 2 spares
16 Calorimeter Modules + 2 spares
1 Flight Anticoincidence Detector
Data Acquisition Electronics + Flight Software
Dark Matter & GLAST; p.25
GLAST Burst Monitor (GBM)

The secondary instrument onboard is the GLAST Burst Monitor, or GBM. The GBM is
designed to observe gamma ray bursts, which are sudden, brief flashes of gamma rays
that occur about once a day at random positions in the sky. These bursts are still a
mystery to astronomers; no one knows what causes them, or what physical forces are at
work. All that is known is that they are among the most powerful explosions in the
Universe. The GBM has such a large field-of-view that it will be able to see bursts from
over 2/3 of the sky at one time, providing locations for follow-up observations of these
enigmatic explosions. The GBM is composed of two sets of detectors Ð 12 sodium iodide
(NaI) scintillators and two cylindrical bismuth germanate (BGO) detectors. When gamma
rays interact with these crystalline detectors, they produce flashes of visible light, which
the detector can use to locate the gamma-ray burst on the sky
Richard E. Hughes
Dark Matter & GLAST; p.26
The Large Area Telescope (LAT)

The LAT will detect gamma rays by using a technique known as pair-conversion. When a
gamma ray slams into a layer of tungsten in the detector, it creates a pair of subatomic
particles (an electron and its anti-matter counterpart, a positron). These particles in turn
hit another, deeper layer of tungsten, each creating further particles and so on. The
direction of the incoming gamma ray is determined by tracking the direction of these
cascading particles back to their source using high-precision silicon detectors.
Furthermore, a separate detector counts up the total energy of all the particles created.
Since the total energy of the particles created depends on the energy of the original
gamma ray, counting up the total energy determines the energy of that gamma ray. In this
way, GLAST will be able to make gamma-ray
images of astronomical objects, while also
g
determining the energy for each detected gamma ray.
e+
Richard E. Hughes
e–
Dark Matter & GLAST; p.27
Gamma Ray Bursts



Gamma Ray Bursts are intense
flashes of gamma rays lasting from
fractions of a second to hours, some
with fading afterglows visible for
months. What are they?
BATSE map of its 2704 detected GRBs
 collisions between highly dense
neutron stars or black holes?
 signatures of the birth of a black
hole?
Example: GRB 990123
Distance: 10 billion light-years
Size: emitting region is lightseconds across
Power: at maximum up to
1,000,000,000,000,000,000
(quintillion) times the Sun's power or
roughly equal to the energy released
by 100 billion Suns in a year's time
GLAST should observe more than a
200 bursts per year, measuring
energy spectra of bursts from a few
keV to hundreds of GeV in the short
time after onset when the majority
of the high-energy is released
Richard E. Hughes
Artists conception of a GRB
Dark Matter & GLAST; p.28
Active Galactic Nuclei (AGN)



AGN are a special class of glaxies
that are the source of tremendous
energy, shining with power equivalent
to trillions of suns. It is believed
that at the center of these objects
there lies a supermassive black hole,
which ejects jets of matter in
opposite directions at nearly the
speed of light.
If one of the jets is directed toward
us the AGN is referred to as a
Blazar
GLAST will detect thousands of
blazars and will try to answer
questions like:
Hubble Heritage image of M87
Schematic diagram of an AGN
 How are the jets formed?
 How is the matter in the jets
accelerated to such fantastic
speeds?
 Is a billion-solar-mass black hole
really the central power source?
Richard E. Hughes
Dark Matter & GLAST; p.29