Rittenhouse Astronomical Society January 14, 2009
Exploring the Extreme
Universe with Fermi
Gamma-ray Space
Telescope
Dave Thompson
NASA GSFC
Deputy Project Scientist
S. Ritz
NASA GSFC and U. Maryland
Why?
How?
What?
Modified by J. Bazo
What
supposedly
first turned
David Banner
into the
Hulk?
Gamma Rays!
Because
gamma rays
are powerful
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But what if you had gamma-ray vision?
Features of the EGRET gamma-ray sky
EGRET all-sky (galactic coordinates) E>100 MeV
diffuse extra-galactic background (flux ~ 1.5x10-5 cm-2s-1sr-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-IDd)
An essential characteristic: VARIABILITY in time!
Field of view important for study of transients.
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-energy
particles in cosmic sources. Since
gamma-rays are produced by
interactions of such energetic
particles, his work is the foundation
for many of the studies being done
with the Fermi Gamma-ray Space
Telescope, formerly GLAST.
U. S. Postal Service
Fermi
FERMI
The Observatory
Large AreaTelescope (LAT)
20 MeV - >300 GeV
Gamma-ray Burst Monitor (GBM)
NaI and BGO Detectors
8 keV - 30 MeV
KEY FEATURES
Huge field of view
LAT: 20% of the sky at any
instant; 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, including
largely unexplored band 10 GeV 100 GeV. Total of >7 energy
decades!
Successors to EGRET and BATSE
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Large AreaTelescope (LAT)
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 Anticoincidence
Detector (ACD) Reject
background of charged cosmic
rays; segmentation removes
self-veto effects at high energy.
Electronics System Includes
flexible, robust hardware trigger
and software filters.
ACD
[surrounds
4x4 array of
TKR towers]
e+
Tracker
e
Calorimeter
Atwood et al, ApJ submitted
Systems work together to identify and measure the flux of cosmic gamma
rays with energy 20 MeV - >300 GeV.
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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 of
energy lots of energy.
Who do we call for help?
Prof. Einstein, what do we do with
something that is just a large amount
of energy?
Energy? Thats E, and E = mc2
Convert the energy to mass.
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Fermi Large Area Telescope (LAT)
Pair-Conversion Telescope
Anticoincidence
Detector (background rejection)
Conversion Foil
Particle Tracking
Detectors
e+
Calorimeter
(energy measurement)
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 positions of the charged particles, the blue lines show the reconstructed track trajectories, and the yellow line shows
the candidate gamma-ray estimated direction. The red crosses show the detected energy depositions in the calorimeter.
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Fermi Science
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 relics
from the Big Bang? Other fundamental physics checks.
Huge increment in capabilities.
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 and
ground 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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MISSION ELEMENTS
GPS
msec
Large Area Telescope
& GBM
DELTA
7920H
Telemetry 1 kbps
Fermi Spacecraft
TDRSS SN
S & Ku
GN
Schedules
Mission Operations
Center (MOC)
GRB
Coordination
Network (GCN)
LAT Instrument
Science
Operations Center
Science
Support Center
Schedules
Alerts
White Sands
HEASARC
GBM Instrument
Operations Center
Data, Command Loads
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What is Fermi seeing?
A key point - because gamma rays are detected one at a
time 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 Sky
Survey
Full sky every 2 orbits (3 hours)
Uniform exposure, with each
region viewed for ~30 minutes
every 2 orbits
Best serves majority of science,
facilitates multiwavelength
observation planning
Exposure intervals
commensurate with typical
instrument integration times for
sources
EGRET sensitivity reached in
days
Pointed observations when appropriate (selected by peer review in later
years) with automatic earth avoidance selectable. Target of Opportunity
pointing.
Autonomous repoints for onboard GRB detections in any mode.
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LAT Working Very Well On Orbit!
Energy resolution
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:
Effective Area
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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 is
constantly 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
( E / Ec )b
N ( E) N0 E e
Consistent with b=1
(simple exponential)
1.51
0.05
0.04
Ec 2.9 0.1 GeV
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
Vela: P=89.3 ms
Crab: P =33 ms
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LAT discovers a radio-quiet pulsar!
pulsars found in blind searches
of LAT data.
P ~ 317 ms
Characteristic 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
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Over half the bright sources seen with LAT appear to
be associated with Active Galactic Nuclei (AGN)
Power comes from
material falling
toward a
supermassive
black hole
Some of this
energy fuels a jet
of high-energy
particles that
travel at nearly the
speed of light
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Gamma rays from blazars
PKS 1502+106 - a blazar 10
billion light years away,
never detected by EGRET,
flared up overnight to
become one of the
brightest things in the
gamma-ray sky.
3C454.3 - LAT saw
it flare
Blazars
up 5 times
brighter than
supermassive
ever
blackEGRET
holes with
hugemeasured.
jets of
particles and
radiation pointed
right at Earth.
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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 gammaray blazars PKS 1502+106,
PKS 1454-354
Flares from known gammaray blazars: 3C454.3, PKS
1510-089,3C273, AO
0235+164, PSK 0208-512,
3C66A, PKS 0537-441,
3C279
Galactic plane transients:
J0910-5041, 3EG J09033531
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Two ATels - Astronomers Telegrams
www.astronomerstelegram.org
These announcements encourage cooperation from other telescopes, like
Swift, to help understand how these powerful jet sources work.
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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.
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Multiple detector light curve
The bulk of the emission
of the 2nd peak is moving
toward later times as the
energy increases
Clear signature of
spectral evolution
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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 the
extreme Universe.
Beyond pulsars, blazars, and gamma-ray bursts,
other sources remain mysteries. Nearly 20% of the
brightest sources do not seem to have obvious
counterparts at other wavelengths.
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