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Exploring the Universe with High-Energy g-rays
Benoît Lott , SLAC/CENBG
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Specifity of high-energy gamma-ray astronomy

• Extented domain 30 keV-30 TeV 9 orders of
  magnitude
• high-energy g-
  rays Egt30 MeV
• EkT ? T1010 K for E1 MeV!
• Non-thermal emission emission by relativistic
  particles boosted to high energy by cosmic
  accelerators.
• electrons
• Bremsstrahlung by interaction with matter
  
• Synchrotron by interaction with magnetic fields
  (polarization, q1/G)
• Inverse Compton by interaction with target
  photons
• hadrons
• p0 ? 2g photoproduction or nuclear reactions
  (pp)
• p,- ? m,- nm
• ?
• e,-nenm
  electronsneutrinos

g-rays probe the Universes highest-energy
accelerators
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Instrumental specificities

• Few photons N(E) E-2 ? large collecting
  area!
• No focalization possible
• Photons dont reach ground (atmosphere is 27 X0
  thick) electromagnetic shower produced in the
  atmosphere can be detected.
• 2 types of detectors
• space-based telescopes low energy (Elt10 GeV
  ?), no telescopes in operation since 2001
  !
• ground-based Cherenkov telescopes high energy
  (Egt250 GeV ?).
• The 10 GeV-100 GeV domain remains little or
  not explored.
• No instrument with large field of view operating
  at Egt10 GeV.

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GLAST (Gamma-ray Large Area Space Telescope)
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LAT (Large Area Telescope)30 MeV-300 GeV
g
g

• Pair conversion telescope
• 16 towers
• Veto
• Tracker
• Calorimeter

Si-W tracker

-
LAT
EGRET
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The LAT performance
100 s
1 orbit
Flux limit( 10-6 g cm-2 s-1)
1 day
1 year
4 10-9 g cm-2 s-1 for 1 year
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Response of the LAT calorimeter to
electromagnetic showers (CERN-SPS)
relativistic heavy ions (FRS/GSI)
80 GeV e 1.5 X0
layer 1
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Counts
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blue data red Geant4
deposited energy (MeV)
Quenching factor k Lmeas/Ecalc
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HESS

• in operation in Namibia since 2004
• main partners are Germany and France
• energy threshold 100 GeV

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A little history
Space
Ground
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The high-energy gamma-ray sky
30 of photons from Extragalactic Diffuse
Emission (isotropic)
EGRET sky map for Egt100 MeV (Seth Digel)
60 of photons from Galactic Diffuse Emission
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Galactic Diffuse Emission
IC Inverse Compton bremss bremsstrahlung EBExta
galactic background
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3rd EGRET Catalog 271 sources
170 unindentified!
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Pulsars
Neutron stars Endpoint of evolution of massive
stars (1.4 M?ltMlt 3 M ?) Properties R10 km,
M1 M?, nuclear density, B1012G, superfluid
interior, deconfined-quark core? Cosmic
lighthouse , T10 ms 3 s gt1000 known in
radio, 8 in g 150 SNR, 12 known
associations The three brightest gray sources
are pulsars Geminga (400 ly),Crab (7000 ly),
Vela (800 ly) The electron wind  of young
pulsars can energize the ejecta nebula or
 plerion , like the Crab nebula.
Geminga
Crab
Galactic anticenter EGRET
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Pulsar phasograms
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Polar-cap and outer-gap emissions
WRc
RCrab1600 km
A. Harding
Radio coherent emission High-energy emission
two competing classes of models assuming
different
locations of the accelerating cavity within the
magnetosphere
- polar cap (small Wem)
- outer gap (large Wem)
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Plerion (Wind-Powered Nebula)
optical
Explosion on July 4, 1054 distance 6.3 103
light years Tpulsar 33 ms
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• First-order Fermi process

• Observed spectral energy distributions are power
  law
• E-a with a 2. If synchrotron or Inverse
  Compton, electrons have also a
• power law energy distribution E-n with n 2.
• Cosmic rays at earth exhibit a power law
  distribution n 2.71
• after correction for propagation effect
  within the galaxy, at the source n 2
• The acceleration process must be collisionless
  and produce a power law
• distribution with index 2.

• E1 E2 E1(averaged over angle)
• DE/E a v/c after n cycles, E can
• become very large.
• If one takes an escape probability
• into account, the resulting energy
• distribution is a power law.
• Rankine-Hugoniot equations for
• a strong shock n 2

E2
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Supernova Remnants (SNRs)
Some are  shell  SNRs (no active nebula) known
acceleration sites of electrons Old problem
Source of cosmic rays? Current paradigm CR up to
the knee are accelerated in shocks with ISM 1st
order Fermi mechanism (Elt1015eV) provides
naturally a power law
SNRs(?)
AGNs?
GRBs?
Paradigm plagued with several problems isotropy,
 accidental  similarity in yields of different
components... Signature pp ? p0 X

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Example Gamma Cygni
Cygnus loop, HST

• EGRET several sources compatible with
• SNRs but
• persistent location problems
• absence of clear p0 peaks

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Supernova Remnant RX J1713.7-3946 (HESS, Egt800
GeV)
Supernova remnants shine at TeV energy, but
whether this emission is due to p0 decays
remains unclear. GLAST will help sort out this
issue.
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Active Galaxy Nuclei (AGNs) - Blazars
A few of all galaxies are active, Lnucleus gt
Lstar 95 are radio-quietSeyfert 5 are
radio-loud quasars or blazars Blazars huge
luminosity (up to 1049 erg /s), high
polarisation, high variability (Tlt 1h),
superluminal motion High luminosity ?
gravitational energy high efficiency, up to 42
for accreation onto a maximally rotating black
hole 100 blazars were detected by EGRET as
powerful g -ray sources.
Cen A
1 M? 1054 erg
Cyg A
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Superluminal motion
beaming v?c, a?0
VLBI observation vapp4 c!
vapp up to 20 c observed!
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Jets in AGNs
Problem Compact sources, high luminosity High
optical thickness for pair production Only
known solution relativistic beaming

Current understanding of
blazars Supermassive black hole (108-109 M? )
accreting mass through a disk, emitting a
relativistic jet aligned with the observers
direction.
M87
t?t/d4
relativistic aberrations!
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Blazar morphology
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How to explain rapid variability
J.Kataoka
Modulation of relativistic flows - faster shell
(G1) catches up with the slower one (G2)
e-e (and possibly smaller fraction of p ) are
accelerated in the shock, and emit Synchrotron/
Inverse Compton radiation.
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Blazar Spectral Energy Distributions
Spectra exhibit two humps, corresponding to
synchrotron emission and IC scattering Emission
over 17 decades in energy! Variability studies
provide a wealth of information time lags between
bands ? acceleration/cooling competition
Mrk421
Spada et al.
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Open issues about blazars

• mechanism of extraction of energy from the BH and
  production of jet
• mechanism and sites of particle acceleration
• identification of the physical parameters driving
  the observational
• properties (LBL vs HBL) accretion rate?
• environment inducing the high-energy component
• Synchrotron Self-Compton vs External Compton
• Jet contents (leptonic or hadronic)
• luminosity function

EGRET 100 Blazars (0.03lt z
lt2.3) GLAST gt 4000 Blazars
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Extragalactic Background Light
Hubble Deep Sky Survey
dust
stars
Direct measurement difficult due to large
foreground components
E. Bloom
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Astroparticle Physics
Extragalactic Diffuse Background
Comes from non-resolved AGNs but a component
could correspond to the decay of relic particles
e.g. WIMPS
GLAST
GLAST
Stable supersymmetric candidate neutralino with
50 GeV lt Mc lt 100 GeV cc ? g X or cc ?
g g
EGRET
EGRET
The large number of blazars detected by GLAST
will enable to pin down the (non-accounted for)
contribution. Galaxy center Presence of a line
at EgMc?
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Gamma-Ray Bursts (GRBs)

• First detected in 1967, disclosed in 1973
• burst 100ms-100s (ms substructure) bimodal
• afterglow a few days
• g flux 103-104 times higher than AGN
• isotropy large galactic halo or cosmological
  distances?
• Galactic origin long-favored because of energy
  requirements...
• More than 100 models, some very exotic

• 1997 Major breakthrough
• BeppoSax enables the discovery
• of an optical counterpart
• host galaxy z0.695!
• GRBs are the most powerful
• phenomena since the Big Bang
• up to 1054erg/s (unbeamed)

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Light curves and energy spectra
Band et al.
Great variety of light curves!
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Long GRBs counterparts (afterglow)
X-rays (Beppo-Sax)
Optical (HST)
Finding the optical counteract enables the
distance to be inferred (emission or absorption
lines) and thus the absolute luminosity to be
determined.
redshift
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Progenitors (disentangled by positions in host
galaxies, light curve)
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The Fireball Model

• Explosion of stellar origin (galaxy z1)
  expanding fireball  of e,e-,g and a few
  baryons. Collimated jet with 10-4 M? and G
  100-1000
• Break in the afterglow s light curve proves the
  beaming (qjet1/Gbreak)
• Beaming alleviates the energetics problem by a
  factor W /4p
• Shocks between colliding  shells  acceleration
  of e, baryons (UHECRs)?
• g -ray emission via synchrotroninverse Compton
  scattering

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GRBs as seen by EGRET
30 (long) GRBs including 4 with Egt100
MeV EGRET hampered by long dead time (100ms)
18 GeV
occultation by the Earth
Energy spectrum for EGRETs 4 high-energy GRBs
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Studying GRBs with GLAST 
LATGRM coverage from 20 keV to 300 GeV
200 GRBs per year! Strong contraint on G
via the highest energy measured
Test of Quantum Gravity vc (1-x Eg / EQG) EQG
1019GeV
Other programs all other wavelengths (HETE2,
SWIFT, ECLAIR, TAROT)  neutrinos bursts 
probe hadronic interactions Ultra High Energy
Cosmic Rays? GRBs may solve the   energetics
Eloss  problem gravitational waves coalescence
of binary stars
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Other new windows on the High-Energy Universe
Neutrino astronomy (Ice Cube, Antares)
UHECR (Auger)
Gravitational waves (Ligo, Virgo, Lisa)
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Happy Birthday, John!
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Backup slides
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Unidentified EGRET sources
60 (170 out of 271) of EGRET sources, most lying
in the Galactic Plane have no visible (radio,
optical, X) counterparts in their error boxes and
are thus  unidentified . One of GLAST s major
goals is to clarify the situation thanks to
- a better localisation
- the accurate measure of the spectrum up to
Ecutoff - variability
studies Candidates - Radio-quiet pulsars (ex
Geminga) associated with the Gould belt - Shock
between binary systems - Microquasars (Galactic
version of blazars) - Unexpected objects??

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Pulsars, Plerions
Endpoints of stellar evolution Neutron stars R10
km, M1 M? , nuclear density, B1012G, superfluid
interior cosmic  lighthouses , T10ms -
3s gt1000 known in radio, 8 in g 150 SNR, 12
associations
The wind  of young pulsars may energize the
ejecta (nebula)  plerions  e.g. the Crab Nebula
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M1, the Crab Nebula (plerion)
T33ms
Inverse Compton component up to TeV!
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Diffuse Galactic Emission
Origin interactions of CR electrons (Brems,IC)
and protons (p0) with the ISM
and the ambiant photons Anomaly observed by
EGRET beyond 300 MeV harder CR (hadrons)
spectrum at the Galactic center or greater IC
component (leptons)? Determination of the CO/H2
ratio for molecular clouds
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Solar flares
Proton and electron acceleration beyond 1 GeV
during violent flares by reconnection of
magnetic field lines in solar loops
p0 bump clearly apparent in energy spectrum
Better understanding of high-energy phenomena in
the Sun
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GLASTs CsI calorimeter
Mass 1492 kg Consumption 91 Watt

• 8 layers comprising 12 CsI crystal each,
  amounting to 96 crystals per tower
• the layers are interlaced (x-y localisation)

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GLASTs crystals
Length 333 0.0 0.6 mm Width 26.7 0.0 0.4
mm Thickness19.9 0.0 0.4 mm (1.08 X0) 0.7 mm
bevel at 45 deg. Energy resolution (1.275 MeV)
lt 13 FWHM Crystal-to-crystal dispersion lt
10 Light yield after 50 krad gt 50
initial Monotonous attenuation, 60
overall. Two PIN diodes at both ends ? dynamic
range from 2 MeV to 60 GeV
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In-orbit calibration
Use of the ionisation energy loss of cosmic-ray
heavy ions C,N,O,Mg,Ne,Fe within the crystals
energies close to minimum of ionisation
(2GeV/nucleon)? well-defined peaks Goals of
the GSI experiment The light function L(E,Z)
must be experimentally determined. Efficient
algorithms enabling the rejection of reaction
events must be devised.
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Broad-band spectra

• Power peaked in g-rays
• No pulsed emission above 100 GeV?
• High-energy turnover
• Increase in hardness with age

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Predicted populations and cutoff energies
GLAST Many more radio pulsars detect Blind
pulsation searches ? radio-quiet
pulsars High-energy spectra