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Fermi: Highlights of
GeV Gamma-ray
Astronomy
Dave Thompson
NASA GSFC
On behalf of the Fermi Gamma-ray Space
Telescope Large Area Telescope
Collaboration
Neutrino Oscillation Workshop
Otranto, Lecce, Italy
September, 2010
Outline
• Introduction - Gamma Rays and Neutrinos
• Distant GeV Gamma-ray Sources
– Gamma-ray Bursts (GRB)
– Active Galactic Nuclei (AGN)
– Local and Starburst galaxies
• Galactic GeV Gamma-ray Sources
• Pulsars and Pulsar Wind Nebulae
• Binary sources
• Supernova remnants
• Summary
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Gamma Rays and Neutrinos
– High-energy gamma rays are primarily produced by interactions
of energetic particles.
• Typical processes are inelastic nuclear collisions (pion
production) and inverse Compton scattering.
• Gamma rays tracing hadronic processes are of particular
interest for neutrino observations.
• Neutrino observations may be critical to learning the nature of
gamma-ray sources.
– The Universe is mainly transparent to gamma rays with energies
less than 20 GeV, so they can probe distant or obscured regions.
• Potential to identify locations and time variability for neutrinoproducing objects.
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The Fermi Gamma-ray Space Telescope
An International Mission
Large Area Telescope
(LAT)
KEY FEATURES
Spacecraft Partner:
General Dynamics
• 20 MeV -> >300 GeV
• 2.4 Steradian field of view
• Operated in scanning mode, so
views the entire sky every 3
hours.
• Peak effective area ~8000 cm2
• Single photon angular
resolution 0.8o at 1 GeV, better at
higher energies.
• Source location capability 1-10
arcmin.
• Energy resolution 10-20%
Gamma-ray Burst Monitor (GBM)
NaI and BGO Detectors
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8 keV - 40 MeV
The Gamma-ray Sky Seen with Fermi LAT
Galactic diffuse emission comes from cosmic-ray
interactions with the interstellar medium
Sources are seen against a strong diffuse background.
E > 1 GeV image.
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Gamma-ray Spectrum at Intermediate Galactic Latitudes
These pion-producing interactions imply
that there is a diffuse background of
neutrinos, too.
Observed

Total with Sources
Sources
Total Diffuse
Isotropic
Bremsstrahlung
Inverse Compton
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1451 Sources
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Extragalactic
Gamma-ray
Sources
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Gamma-ray Bursts (GRBs) - the Brightest and
Most Distant Sources Seen by Fermi
GRB 090510
Abdo et al. 2009, Nature 462, 331
GBM NaI
Collapsar:
Rapidly spinning stellar
GBM BGO
core collapse supernova, with
relativistic jets that can produce
long GRBs
LAT (>1MeV)
31 GeV
0.83 s
Constraints on the quantum gravity
mass scale (MQG) by direct measurement
of photon arrival times, testing Lorentz
Compact Merger: Two neutron
invariance violation.
stars, or a neutron star and a
black hole, merge, producing a
jet that gives
to a short
GRB
MQG,1rise
/MPlanck
> 1.19
• Both long (>2 sec) and short
(<2 sec) bursts have been
seen
• Some bursts show highenergy emission afterglow
• Constraint: lower limit of bulk
Lorentz factor: Γmin ~1000
• Some bursts have an extra
spectral component compared
to the standard Band model.
• These short, bright flashes
can be used as tools to probe
basic physics, as in the
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example here.
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
• Fermi LAT sees
primarily blazars,
for which the jet is
pointed toward
Earth.
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The Variable Gamma-ray Sky
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Blazar PKS 1502+106
Gamma rays
X-rays
Ultraviolet
Optical
Radio
The spectral Energy Distribution (SED) of this blazar is
complex, requiring multiple components that vary with time.
A key result for Fermi and multiwavelength studies: in
most cases, simple models for blazars are inadequate.
In some models of blazar jets, hadrons transport much
of the energy and have neutrino-producing interactions.
Seeing neutrinos from a blazar would be the key to
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verifying hadronic interactions.
Centaurus A - Radio Galaxy
Over ½ of the total >100 MeV observed LAT flux in the lobes
E > 200 MeV
22 GHz
LAT PSF
LAT counts map with background (isotropic
and diffuse) and field point sources subtracted
WMAP image provided by
Nils Odegard (GSFC)
Requires 0.1-1 TeV electrons in giant ‘relic’ lobes: accelerated in-situ
or efficient energy transport from the center of the galaxy.
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Galaxies Dominated by Cosmic-Ray
Interactions
o
NGC 253
Large Magellanic
Cloud
Starburst Galaxies
Small Magellanic Cloud Spectrum
Small Magellanic
Cloud
GeV gamma rays in these
galaxies come primarily from
the interactions of cosmic ray
hadrons and electrons with
interstellar matter and photon
fields.
M82
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Galactic
Gamma-ray
Sources
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The Pulsing g-ray Sky
Over 60 gamma-ray pulsars are now known.
Pulses at
1/10th true rate
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Pulsar Wind Nebulae - Powerful Particle Accelerators
Crab Nebula Spectral
Energy Distribution. Red
points are Fermi LAT
data, showing transition
from synchrotron to
Compton components.
Vela Pulsar
E>800 MeV Test Statistic
(significance) map. PWN Vela X
E>2 GeV counts map.
PWN HESS J1640-465
E>10 GeV counts map.
PWN MSH 15-52
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Cygnus X-3 - Binary System - 4.8 Hour Period
Gamma rays
X-rays
Neutron star or black hole binary system, accelerating particles
to high energies. The system remains largely a mystery.
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Cygnus X-3 - Binary System - 4.8 Hour Period
LAT
LAT
Fermi LAT only sees gamma-ray emission when Cyg X-3 has
strong low-energy X-ray flux and weaker high-energy X-ray flux.
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Supernova Remnants (SNR) - Spatially Resolved
Strong evidence for cosmic ray production in SNR.
Note: LAT does not resolve Cas A
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Fermi Observations of SNR
• Supernova remnant W44 - spatially resolved. 2-10 GeV
front-converting events, deconvolved image.
Green contours are from Spitzer IRAC, shocked H2
Black cross is PSR B1853+01, not seen as a pulsed
source.
Note: leptonic models are not excluded. Neutrinos could confirm the
acceleration of hadrons by SNR.
2121
Fermi Observations of SNR
• Supernova remnant W51C - spatially resolved. 2-10
GeV front-converting events, deconvolved image.
Magenta contours are region of shocked CO
Black diamond is possible PWN CXO J192318.5+143035
White ellipse is the outer boundary of W51.
LAT PSF
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A Surprise - A Gamma-ray Nova
• In early March, the LAT skywatchers found a new,
flaring gamma-ray source in the Cygnus region.
• To our surprise, we learned that an optical flare of the
symbiotic system V407 Cyg (red giant/white dwarf
binary) had occurred at about the same time.
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A Surprise - A Gamma-ray Nova
The energy spectrum
of the nova is
plausibly
produced by
accelerated
protons from the
expanding shell of
the nova colliding
with the wind from
the red giant star.
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What Next for Fermi?
• As we start the third year of operations, 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, X-ray binaries, SNR,
starburst galaxies and gamma-ray bursts, other
sources remain mysteries. Nearly 40% of the
sources in the First LAT Catalog do not seem to have
obvious counterparts at other wavelengths.
– Multiwavelength/multimessenger studies,
including neutrino observations, will be critical for
learning the nature of such sources.
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Dark-matter particles
annihilate with one
another, leading to gamma
rays
Light dark-matter particles
produce 511 keV (low-energy)
gamma rays
WIMP dark-matter particles
(neutralinos) produce 30 MeV to 10
GeV (medium-energy) gamma rays
Heavy dark-matter particles produce
300 to 600 GeV (high-energy) gamma
rays
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Illustrations by Gregg Dinderman/Sky & Telescope
Dark Matter Searches
Upper limits
on Dark
Matter
annihilation
cross
section.
Dwarf Spheroidal Galaxies are known to be dominated by Dark Matter.
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Summary
Gamma rays seen with the Fermi Gamma-ray Space
Telescope are revealing sites of particle acceleration and
interaction, ranging from distant Gamma-ray bursts and
Active Galactic Nuclei to sources in our Galaxy.
Gamma-ray bursts, flares from Active Galactic Nuclei,
Starburst Galaxies, Supernova Remnants, and Novae are
all good candidates for astrophysical neutrino sources.
All the Fermi gamma-ray data are public. The Fermi
Science Support Center, at http://fermi.gsfc.nasa.gov/ssc/
is the access center for these data.
Join the fun!
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