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Exploring the Extreme
Universe with FermiGamma-ray Space
TelescopeDave ThompsonNASA GSFC
Deputy Project Scientist
S. Ritz
NASA GSFC and U. Maryland
• Why?• How?
• What?
Rittenhouse Astronomical Society January 14, 2009
Modified by J. Bazo
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Whatsupposedlyfirst turnedDavid Bannerinto the
Hulk?
Gamma Rays!
Becausegamma raysare powerful
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But what if you had gamma-ray vision?
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EGRET all-sky (galactic coordinates) E>100 MeV
diffuse extra-galactic background (flux ~ 1.5x10-5
cm-2
s-1
sr -1
)galactic diffuse (flux ~30 times larger)
high latitude (extra-galactic) point sources (typical flux from EGRET sources O(10-7 - 10-6) cm-2s-1)
galactic sources (pulsars, un-ID’d)
An essential characteristic: V ARI ABILITY in time!
Field of view important for study of transients.
Features of the EGRET gamma-ray sky
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About that Name
Enrico Fermi was an Italian
physicist who immigrated to the
United States before World War II.
He was the first to suggest a viable
way to produce high-energyparticles in cosmic sources. Since
gamma-rays are produced by
interactions of such energetic
particles, his work is the foundationfor many of the studies being done
with the Fermi Gamma-ray Space
Telescope, formerly GLAST.
U. S. Postal Service
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Fermi
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FERMI
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The Observatory
Gamma-ray Burst Monitor (GBM)NaI and BGO Detectors
8 keV - 30 MeV
Large AreaTelescope (LAT)20 MeV - >300 GeV
KEY FEATURES
• Huge field of view –LAT: 20% of the sky at anyinstant; in sky survey mode,expose all parts of sky for~30 minutes every 3 hours.GBM: whole unocculted sky
at any time.• Huge energy range, includinglargely unexplored band 10 GeV -100 GeV. Total of >7 energydecades!
Successors to EGRET and BATSE
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Large AreaTelescope (LAT)
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e+ e –
Overview of LAT: How it works
• Precision Si-strip Tracker (TKR)
Measure the photon direction;gamma ID.
• Hodoscopic CsI Calorimeter
(CAL) Measure the photon
energy; image the shower.
• Segmented AnticoincidenceDetector (ACD) Reject
background of charged cosmic
rays; segmentation removes
self-veto effects at high energy.
• Electronics System Includesflexible, robust hardware trigger
and software filters.
Systems work together to identify and measure the flux of cosmic gamma
rays with energy 20 MeV - >300 GeV.
Calorimeter
Tracker
ACD[surrounds4x4 array ofTKR towers]
Atwood et al, ApJ submitted
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FERMI
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How does a high-energy gamma-ray telescope work?
• The key is “high-energy”
• A gamma ray is a packet ofenergy – lots of energy.
• Who do we call for help?
Prof. Einstein, what do we do withsomething that is just a large amount
of energy?
Energy? That’s E, and E = mc2
Convert the energy to mass.
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e+ e – Calorimeter(energy measurement)
Particle TrackingDetectors
Conversion Foil
AnticoincidenceDetector (background rejection)
Pair-Conversion Telescope
Fermi Large Area Telescope (LAT)
• Gamma rays interact by pair production, the conversion of the
gamma-ray energy into two particles – an electron and a positron
(really an antiparticle); LAT is a particle detector.
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LAT Gamma Candidate Events
The green crosses show the detected posit ions of the charged p art ic les, the blue l ines show the reconstru cted track trajector ies, and the yel low line shows
the candidate gamma-ray est imated direct ion. The red crosses show the detected energy deposit ions in the calor imeter.
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A very broad menu that includes:
• Systems with supermassive black holes (Active Galactic Nuclei)• Gamma-ray bursts (GRBs)
• Pulsars
• Supernova remnants (SNRs), PWNe, Origin of Cosmic Rays
• Diffuse emissions
• Solar physics• Probing the era of galaxy formation, optical-UV background light
• Solving the mystery of the high-energy unidentified sources
• Discovery! New source classes. Particle Dark Matter? Other relicsfrom the Big Bang? Other fundamental physics checks.
Huge increment in capabilities.
Fermi Science
Draws the interest of both the High Energy Particle Physics and
High Energy Astrophysics communities.
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The Accelerator
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Launch!
• Launch from Cape Canaveral
Air Station 11 June 2008 at
12:05PM EDT
• Circular orbit, 565 km altitude
(96 min period), 25.6 deg
inclination.
• Communications:
– Science data link via
TDRSS Ku-band, average
data rate 1.2 Mbps.
– S-band via TDRSS andground stations
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A moment later…
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… and then …
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… on its way!
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GN
HEASARC
-
-
DELTA
7920H•
White Sands
TDRSS SN
S & Ku
LAT InstrumentScience
Operations Center
GBM Instrument
Operations Center
GRB
Coordination
Network (GCN)
• Telemetry 1 kbps •
- •
S
Alerts
Data, Command Loads
Schedules
Schedules
Mission Operations
Center (MOC)
Science
Support Center
• m sec •
•
•
Fermi Spacecraft
Large Area Telescope
& GBMGPS
MISSION ELEMENTS
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What is Fermi seeing?
• A key point - because gamma rays are detected one at atime like particles, the Fermi telescopes do not have
high angular resolution like radio, optical or X-ray
telescopes. No pretty pictures of individual objects.
• Instead, Fermi trades resolution for field of view. The
LAT field of view is 2.4 steradians (about 20% of the
sky), and the GBM field of view is over 8 steradians.
• The Fermi satellite is operated in a scanning mode,
always looking away from the Earth.
• The combination of huge field of view and scanning
means that the LAT and GBM view the entire sky every
three hours!
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Operating modes
• Primary observing mode is SkySurvey
– Full sky every 2 orbits (3 hours)
– Uniform exposure, with eachregion viewed for ~30 minutesevery 2 orbits
– Best serves majority of science,facilitates multiwavelengthobservation planning
– Exposure intervalscommensurate with typicalinstrument integration times forsources
– EGRET sensitivity reached indays
• Pointed observations when appropriate (selected by peer review in lateryears) with automatic earth avoidance selectable. Target of Opportunitypointing.
• Autonomous repoints for onboard GRB detections in any mode.
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• PSF on-orbit as expected (note
intrinsic energy dependence =>
localization is source-dependent)
– verify using on-pulse photons
from Vela, compare with detailed
MC simulation:
LAT Working Very Well On Orbit!
Effective Area
Energy resolution
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Large Area Telescope First Light!
The full gamma-ray sky projected onto a surface - Galactic coordinates
The Fermi Large Area Telescope sees the whole gamma-ray sky every three
hours. This is an important feature, because the high-energy sky isconstantly changing. This image represents just four days of observations.
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Three months of LAT scanning data
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Pulsars - rapidly rotating neutron stars
Vela pulsar -
brightest persistent
source in the
gamma-ray sky.
The actual rotation of the star takes less than 1/10 second.
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Vela Pulsar – Phase-averaged SED
b
c E E e E N E N )/(
0)(
0.051.51
0.04
2.9 0.1 GeVc
E
Consistent with b=1(simple exponential)
b=2 (super-exponential)
rejected at 16.5s
No evidence for magnetic
pair attenuation:
Near-surface emission
ruled out
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The Pulsing Sky
Pulses at
tenth true
rate
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Pulsars
Geminga: P=237 ms
Crab: P =33 ms
Vela: P=89.3 ms
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LAT discovers a radio-quiet pulsar!
P ~ 317 msCharacteristic age ~ 10,000 yrs
Location of EGRET source 3EG J0010+7309,the Fermi-LAT source, and the central X-ray
source RX J0007.0+7303
pulsars found in blind searchesof LAT data.
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Over half the bright sources seen with LAT appear to
be associated with Active Galactic Nuclei (AGN)
• Power comes frommaterial falling
toward a
supermassive
black hole
• Some of thisenergy fuels a jet
of high-energy
particles that
travel at nearly the
speed of light
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Blazars –
supermassiveblack holes with
huge jets of
particles and
radiation pointed
right at Earth.
Gamma rays from blazars
3C454.3 - LAT saw
it flare up 5 times
brighter thanEGRET ever
measured.
PKS 1502+106 - a blazar 10billion light years away,
never detected by EGRET,
flared up overnight to
become one of the
brightest things in the
gamma-ray sky.
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Flaring sources
• Automated search for
flaring sources on 6 hour, 1
day and 1 week timescales.
• Astronomers telegrams
– Discovery of new gamma-
ray blazars PKS 1502+106,
PKS 1454-354
– Flares from known gamma-
ray blazars: 3C454.3, PKS
1510-089,3C273, AO
0235+164, PSK 0208-512,
3C66A, PKS 0537-441,3C279
– Galactic plane transients:
J0910-5041, 3EG J0903-
3531
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Two ATels - Astronomer’s Telegrams www.astronomerstelegram.org
These announcements encourage cooperation from other telescopes, like
Swift, to help understand how these powerful jet sources work.
Gamma-Ray Bursts (GRBs): the most
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Gamma Ray Bursts (GRBs): the most
powerful explosions since the Big Bang
• Originally discovered by
military satellites, GRBs are
flashes of gamma rays
lasting a fraction of a second
to a few minutes.
• Optical afterglows reveal that
many of these are at
cosmological distances
• The GBM and LAT extend the
energy range for studies of
gamma-ray bursts to higher
energies, complementing
Swift and other telescopes.
Multiple detector light curve
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• The bulk of the emissionof the 2nd peak is movingtoward later times as theenergy increases
• Clear signature ofspectral evolution
Multiple detector light curve
Wh N f F i?
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What Next for Fermi?
• We have only scratched the surface of what the Fermi
Gamma-ray Space Telescope can do. – The gamma-ray sky is changing every day, so
there is always something new to learn about theextreme Universe.
• Beyond pulsars, blazars, and gamma-ray bursts,other sources remain mysteries. Nearly 20% of thebrightest sources do not seem to have obviouscounterparts at other wavelengths.