Groundbased GammaRay Astronomy

1
Ground-based Gamma-Ray Astronomy
Bernard Degrange
LLR,
Ecole polytechnique CNRS/IN2P3
2
Twenty years ago, the Crab nebula was the first
?-ray source firmly detected (9s) at very high
energies

• Whipple observatory 10 m telescope T.C.
  Weekes, et al. ApJ 342 (1989) 379
• The advent of the Imaging Atmospheric-Cherenkov
  technique with a 37 pixel camera
• Image analysis A.M. Hillas, 1985, 19th ICRC, La
  Jolla, USA

In 2009, a very different landscape

• Several arrays of ground-based detectors achieve
  a sensitivity 1/100 of the flux of the Crab
  nebula above 100 GeV
• Almost 100 sources of Very-High-Energy (VHE)
  ?-rays have been discovered.
• Ground-based detectors currently participate to
  multi-wavelength observations
• The VHE domain (Egt100 GeV) has joined mainstream
  Astrophysics

3
The last spectral domain in photonic astrophysics

• Gamma-ray fluxes (Egt1 TeV) are generally less
  than 2 10-7 m-2 s-1 , the flux of
  the Crab nebula, the standard candle of VHE
  astrophysics, ? large effective
  detection area (gt 105 m2) required
  ? ground-based instruments
  detecting the electromagnetic cascade produced in
  the atmosphere by the primary ?-ray.
• Physics aims are essentially the same as those of
  the 30 MeV-300 GeV spectral region (Fermi LAT,
  AGILE)
• Understand the high-energy cosmic phenomena and
  the astrophysical objects in which they are at
  work
• Identify the main cosmic accelerators, both
  galactic and extra-galactic
• Search for exotic phenomena implying new
  physics
• but the dominant processes and the strongest
  sources are not always the same as at lower
  energies.

4
Detected at Egt100 GeV
Only detected at Elt100 GeV
X-ray binary system OB Association
Supermassive black hole
(Active Galactic Nucleus)
Stellar explosion (supernova)
Stellar wind shock or pulsar wind terminal shock
Shocks in the interstellar medium and ejecta
Stellar-mass black hole
Pulsar
Accretion-ejection disk and jets
Shock waves in the jet
Shell-type supernova remnants
Shock wave close to the horizon
?-ray pulsar
Pulsar Wind Nebula
Microquasars Blazars
Cosmic-ray diffusion and interactions in the
Galaxy
?-ray bursts
?-ray binary systems and OB associations
Diffuse ?-rays Molecular clouds
Radiogalaxies
5
Outline

• New powerful ground-based detectors
• Galactic sources (selected results)
• Extragalactic sources (selected results)
• Next-generation instruments
• Conclusion

6
1. New powerful ground-based detectors

• Gamma-rays interact in the higher atmosphere
  ?
  electromagnetic cascade (secondary e and ?-rays)
• Two possible detection methods from the ground
• Detect Cherenkov light emitted by the
  ultra-relativistic es of the shower ?
  Atmospheric Cherenkov Telescopes
• Detect charged particles and secondary ?-rays
  reaching the ground at high altitude ? Synoptic
  detectors (field of view 1 steradian) ? MILAGRO
  (LosAlamos, USA), Tibet ARGO (Yang Ba Jing,
  Tibet)
• But one has to face the huge background of
  showers generated by charged cosmic rays, mainly
  protons and nuclei above 100 GeV ? sensitivity is
  mainly determined by the capability of background
  rejection
• Up to now, Imaging Atmospheric Cherenkov
  Telescopes have achieved
• The highest sensitivity
• The best angular resolution
• The lowest energy thresholds

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Imaging Atmospheric Cherenkov Telescopes

• Cherenkov light is emitted on a very narrow cone
  (? lt 1) illuminating an area of about 300 m
  diameter at 1800 m a.s.l.on the ground.
• A telescope located within the light pool detects
  the shower if it collects enough Cherenkov
  photons ? effective detection area 105 m2
• Build up the image of the shower in Cherenkov
  light in the focal plane of the telescope with a
  high-definition camera (many fast phototubes)
• With an array of several telescopes, the shower
  can be reconstructed in 3D (stereoscopy)
• ? total number of Cherenkov photons (energy
  estimator)
• ? better angular resolution

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Imaging Atmospheric Cherenkov Telescopes
Main problem discriminate between ?-ray-induced
and hadronic showers
? sensitivity is mainly determined by the
background rejection power
Cherenkov Imaging Technique

• Record the image of the shower in Cherenkov light
  with a fine-grain camera in the focal planes
  use image shape (?-ray showers have smaller
  width) and direction ?
• High gamma-hadron discrimination power
  particularly for stereoscopic arrays
• Angular resolution 4' to 6' better than gamma-ray
  satellites
• BUT limited field of view (5 diameter for
  H.E.S.S.) ? follow a given source during its
  apparent motion in the sky.
• BUT require moonless clear nights (10 duty
  cycle)

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Main stereoscopic systems of Imaging Atmospheric
Cherenkov Telescopes
CANGAROO III, Australia
MAGIC II, Canary islands, Spain
H.E.S.S., Namibia
VERITAS, Arizona, USA
10
MAGIC I camera
H.E.S.S. camera
Courtesy of J. Hinton (arXiv0803.1609)
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Present Imaging Atmospheric-Cherenkov Telescopes
have a sensitivity of about 0.01 flux of the
Crab nebula
Energy thresholds at zenith
Fermi LAT

• MAGIC 30 GeV
• H.E.S.S. 100 GeV
• VERITAS 100 GeV
• CANGAROO 400 GeV

Other active IACTs Whipple Obs. 10m (USA),
Shalon (Khazakstan), TACTIC (India)
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2. Galactic sources (selected results)
Most results come from the H.E.S.S. galactic
plane survey
Sun

• Inner Galaxy (2004) extension (2005-2008) from
  l -85 (or 275) to l 60 and blt3
• Survey is complete for fluxes gt 0.09
  Crab ( deeper observations)
• Low diffuse flux (? Fermi/AGILE) ? Individual
  sources appear clearly
• Most of the revealed sources are mildly extended
  (D gt 3 ' to 4 ')

G.C.
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2.1 Shell-type supernova remnants

• Supernova remnants (SNRs) are suspected to be
  the main accelerators of galactic cosmic-rays up
  to the knee
• Diffusive shock acceleration
• 10 of the mechanical energy of supernovae can
  account for the injection of CRs in the Galaxy.
• Thin filaments seen in non-thermal (synchrotron)
  X-rays prove that electrons are accelerated up to
  100 TeV and indicate the shock position.
• SNRs are often extended sources ? no clear
  detection by EGRET (the source does not
  stand out over the diffuse ?-ray flux)
• H.E.S.S. provided resolved images of several
  large shell-type SNRs

SN1006, X-rays Chandra
ApJ 589 (2003) 827
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Detected young (Tlt104 years) shell-type
supernova remnants
Morphologies in VHE ?-rays are compatible with
those in non-thermal X-rays
15
ApJ 661 (2007) 236
AA 464 (2007) 235
ApJ 692 (2009) 1500
RX J0852.0-4622
RCW 86
RX J1713-3946
SN 1006, X-rays, XMM Newton
SN 1006, VHE ?-rays
M. Naumann-Godó, Moriond 2009
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Particle acceleration in supernova remnants
up to which energy ?

• Two possible processes
• Inverse Compton scattering of VHE electrons on
  ambient photons (mainly CMB)
• Hadronic interactions of accelerated protons or
  ions in the interstellar medium
• The ?-ray energy spectrum of RX J1713-3946 as
  measured by H.E.S.S. suffers at cut-off above
  10TeV.
• There is a significant ?-ray flux (4.8 s) above
  30 TeV
• In the hadronic scenario, this implies efficient
  proton acceleration up to 200 TeV

AA 464 (2007) 235
G 2.040.04 E0 17.9 3.3 TeV
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Some older (Tgt104 years) supernova remnants are
detected by their interaction with a nearby
molecular cloud

• The VHE ?-ray signal coincides with the molecular
  cloud (CO map), not with the radio shell.
• W28 H.E.S.S. AA 481(2008)401
• IC 443
• MAGIC ApJ 664(2007)87
• VERITAS arXiv0905.3291
• In the preceding cases, OH
  masers show that molecular clouds are perturbed
  by SNR shocks
• Hadronic origin of ?-rays likely

W28 (CO map)
W28 (H.E.S.S.)
CO contours
W28 (radio)
IC443 (MAGIC)
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2.2 Pulsar-Wind Nebulae
HESS J1825-137
Vela X
Courtesy of P. Slane

• Above 100 GeV, no periodic signals from pulsars
  have been detected (MAGIC could detect the Crab
  pulsar by lowering its threshold down to 25 GeV)
• Some supernova remnants include a pulsar whose
  strong wind termination shock accelerates
  particles and thus powers a nebula within the
  ejecta
• In the Crab nebula, the source of VHE ?-rays is
  point-like. The X-ray synchrotron nebula
  (figure) is not resolved by Cherenkov telescopes.
• Some pulsar-wind nebulae are extended, sometimes
  more than the corresponding X-ray nebula (older
  objects). Sometimes, the nebula is displaced with
  respect to the pulsar (HESS J1825, Vela X)

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2.3 Other galactic sources

• A few High-Mass X-ray Binary systems are VHE
  ?-rays emitters
• Massive bright star Compact object (pulsar or
  black hole)
• ?-ray signal exhibits the orbital period in 3 of
  them
• Young stellar clusters (OB associations) O and B
  stars are those which end their lives in
  supernovae ? particle acceleration can be due by
  strong stellar winds or by super-bubbles
  (coalescence of wind-blown bubbles)
  Cyg OB2 (TeV
  J20324130 detected by HEGRA), Westerlund 2 (HESS
  J 1023-575), W43 (HESS J1848-018) also detected
  by MAGIC

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Unidentified sources

• Dark sources (no radio or X-ray counterparts).
  They could be due to old pulsar-wind nebulae in
  which the magnetic field has decreased (weak
  synchrotron component, but Inverse Compton still
  active).
• Several possible counterparts, but the
  point-spread function or the intrinsic extension
  of a few arc min does not permit a clear
  identification e.g. the Galactic Centre (HESS
  J1745-290) is compatible
• with the central black hole Sgr A
  (3.6 106 Msun),
• but also with the pulsar-wind nebula
  G359.95-0.04. The supernova remnant Sgr A East
  is now excluded.

Galactic Centre
red square expected position of the
centroid if the ?-ray flux followed the radio
flux of Sgr A East (C.van Eldik
et al. 2008)
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The Galactic Centre HESS J1745-290
Phys. Rev. Lett 97 (2006) 221102

• Source is found stable over 3 years with a flux
  10 of that of the Crab nebula.
• The central black hole SgrA exhibits flares in
  IR and in X-rays which are not observed in VHE
  ?-rays on HESS J1745-290 (simultaneous
  Chandra-H.E.S.S. observations arXiv0812.3762).
• Power-law spectrum with an exponential cut-off
  above 10 TeV ? annihilation of dark matter
  particles is excluded as the main component of
  the spectrum.

G2.100.04sta0.1sysE015.73.4sta2.5sys
H.E.S.S. arXiv0906.1247
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3. Extra-galactic sources (selected results)
Active Galactic Nuclei (AGN)

• Ground-based detectors have discovered gt 20
  AGN emitting VHE ?-rays.
• All these sources are radio-loud AGN (5 of all
  AGN) the radio emision is due to the
  relativistic jets ejected from the central
  region.
• All VHE ?-ray emitting AGN but 2 belong to the
  blazar class, whose characteristics result from
  an observation effect jets are emitted at small
  angle with respect to the line of sight.
• The 2 exceptions are radio-galaxies M87
  (HEGRA, H.E.S.S., MAGIC),
  and Centaurus A (H.E.S.S.)

Courtesy of C.M. Urry and P. Padovani
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Two radio-galaxies emission region compatible
with radio core
Cen A arXiv0903.1582
Cen A flux (Egt250 GeV) 0.8 Crab
Science 314 (2006) 1424
M87 flux variable (day-scale) 1 to 5
Crab
HESS 95 CL limit
HESS 99.9 CL limit
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Most VHE-emitting AGN are blazars
PKS2155-304 light curve (July 2006)
ApJ 664 (2007) L71

• Blazars are variable sources which exhibit
• Very high ?-ray luminosity during flaring
  periods
  (gt 10 times the Crab flux
  for PKS2155-304 in July 2006)
• Short timescale variability ( few minutes for
  PKS2155-304 in July 2006) ? constraint on the
  size of the emission zone
• In order to avoid opacity (?? ? ee-)
  
  ? constraint on the Lorentz factor of the
  jet Ggt or 10

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Energy-dependent photon velocity ? v c(1E/MQG)

• Blazars are located at cosmological distances and
  show rapid variability
• Whipple Observatory Mkn 421 in May 1996,
• MAGIC Mkn 501 from May to July 2005,
• H.E.S.S. PKS2155-304 in July 2006
• Search for differences (time-lags) between
  light-curves in different energy ranges

PKS2155-304 cross-correlation function vs. time
lag
Phys. Rev. Lett. 101(2008)170402
PKS2155-304 200ltElt800 GeV
?t/?E (s TeV-1) vs redshift
MQG gt 7 Planck mass
PKS2155-304 Egt800 GeV
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Simultaneous multi-wavelength observation
campaign Optical (ATOM)-X-ray
(Swift, RXTE)-Fermi LAT-H.E.S.S. on the blazar
PKS 2155-304 (z0.117) in a low state
E2 dF/dE two-bump spectrum
August 25 to September 6, 2008

• In the low state, no correlation between X-rays
  and VHE ?-rays (? flares)
• The most energetic electrons (responsible for
  X-rays) do not significantly contribute to the
  Inverse Compton bump (extreme Klein-Nishina
  regime)

arXiv 0903.2924
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2 radio-galaxies (M87, Cen A), 1 FSRQ (3C279), 22
BL Lac
Most redshifts lt 0.25
Courtesy of R. Wagner
On their way to he Earth, ?-rays from distant
sources may interact with background photons ?
(TeV) photon (Optical/Infrared) ? e e-

Blazar spectra bring constraints on
extra-galactic background light
? upper bounds close to the lower bounds
obtained from galaxy counts.
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4. Next-generation instruments
MAGIC and H.E.S.S. upgrades
H.E.S.S. II a very large telescope (28 m
diameter) in the centre of the present array
(2010) camera comprising 2048 pixels?

• 30 GeV threshold with the very large telescope
• 80 GeV threshold with the very large telescope
  one of the other 4
• Sensitivity 2 for Egt200 GeV

MAGIC II a second 17 m telescope (since end
2008) ? stereoscopy will improve sensitivity (
3) and angular resolution
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30 GeV-300 GeV, a spectral domain in which
satellite and ground-based detectors have
complementary assets

• For the first time, a common spectral domain will
  be accessible to satellites (Fermi LAT/AGILE) and
  to low-threshold ground-based detectors (MAGIC
  II, H.E.S.S. II)
• Take advantage of the complementarity of both
  types of instruments for observing variable
  sources (AGN) or transient events (gamma-ray
  bursts)
• satellites (large field of view) give the alert
  in case of a flare
• ground-based detectors (large effective area) can
  measure the light curve with a high temporal
  resolution (e.g. minute-sale variations).
• More extragalactic sources at higher redshifts
  will be accessible to ground-based detectors (as
  shown by MAGIC with 3C279).
• Another single-dish project MACE (Major
  Atmospheric Cherenkov Experiment) 21 m diameter
  telescope at 4200 m a.s.l. in Hanle (India)
  expected for 2011.

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Towards large arrays of Imaging Atmospheric
Cherenkov Telescopes the CTA AGIS projects

• 3 types of Cherenkov telescopes
• a few very large telescopes for lower energies 30
  GeV 100 GeV
• about 40 telescopes (HESS I type) spread over
  1 km2 ? a milli-Crab sensitivity in the TeV range
• about 25 smaller telescopes spread over a larger
  area (10 km2) should explore the energy domain E
  gt 10 TeV
• Angular resolution 2 arc minutes

• Two projects
• Collaboration including HESSMAGIC other
  groups ? the CTA project Cherenkov Telescope
  Array ( start building in 2014 ? full array in
  2018 ?)
• Similar project in the USA AGISAdvanced
  Gamma-ray Imaging System
• Such arrays should work as observatories

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Large arrays of Cherenkov telescopes
Crab flux
Flux (gtE) cm-2 s-1
10-3 Crab flux
K. Bernlöhr 2008
arXiv0810.5722
32

• An angular resolution of 2 arc-minutes ? better
  separation of sources in the Galactic Plane ?
  easier identification with a radio or X-ray
  source
• The expected sensitivity of the array will result
  in a catalogue of about 1000 sources, both
  galactic and extra-galactic ? population studies
• New types of sources of VHE ?-rays are likely to
  be detected, e.g.
• Starburst galaxies a large number of supernovae
  explosions generating superbubbles
• Ultra-Luminous InfraRed Galaxies  ULIRG  (star
  formation, strong stellar winds)
• Galaxy clusters
•  Exotic  sources (e.g. annihilation of Dark
  Matter particles)

S.Funk, J.Hinton, G.Hermann S. Digel, arXiv
0901.1885
33
Conclusion

• Imaging Atmospheric Cherenkov Telescopes have now
  produced a catalogue of almost 100 sources
• gt 60 galactic sources shell-type supernova
  remnants, pulsar-wind nebulae, binary systems, OB
  associations
• gt 24 extra-galactic sources blazars,
  radio-galaxies
• In the following years, H.E.S.S. II and MAGIC II
  should usefully complement satellite observations
  in the GeV range (Fermi LAT and AGILE),
  particularly on variable sources (e.g. AGN).
• In the long term, observatories based on large
  arrays of Cherenkov Telescopes (CTA, AGIS),
  should reach the milli-Crab sensitivity and still
  improve the angular resolution, opening the way
  to population studies and to the discovery of new
  types of cosmic accelerators.

34
Complements
35
A limitation due to pair production ? photon ?
e e-

• For E gt 100 GeV, ?-rays can interact with
  background photons and produce e e- pairs.
• In the Galaxy, this occurs in very radiative
  environments, e.g. close to a massive bright star
  ( 1016 photons cm-3 in LS
  5039)
• In the extragalactic space, ?-rays from very
  distant objects can be absorbed
• by the cosmic infrared or optical background
  radiation at TeV energies
• by the Cosmic Microwave Background (CMB)
  radiation above 100 TeV

Optical and infrared background light
Cosmic Microwave Background
Mpc
PeV
TeV
Absorption length (Mpc) as a function of ?-ray
energy
36
SN 1006 H.E.S.S. results (morphology and spectra)
37
Origin of VHE ?-rays ?

The case of RX J1713-3946

• Two possible processes
• Inverse Compton scattering of VHE electrons on
  ambient photons (mainly CMB)
• Hadronic interactions of accelerated protons or
  ions in the interstellar medium
• Purely leptonic scenario are more constrained
  ? determine the magnetic field but
  cannot be excluded.
• Both processes are probably contributing

Broad-band spectrum of RX J1713-3946 from radio
waves to VHE ?-rays
Curves show a fit according to a
hadronic scenario red contributions from
electrons blue
contributions from protons
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Pulsar magnetospheres No periodic
signals above 100 GeV
but the Crab pulsar (MAGIC at 25 GeV)
Upper limits at very-high energies
MAGIC detection at 25 GeV
Science 322(2008)1221
39
Pulsar-Wind Nebulae
Crab nebula (X-rays, Chandra)
20
Courtesy of P. Slane

• Above 100 GeV, no periodic signals from pulsars
  have been detected (MAGIC could detect the Crab
  pulsar by lowering its threshold down to 25 GeV)
• Some supernova remnants include a pulsar whose
  strong wind termination shock accelerates
  particles and thus powers a nebula within the
  ejecta
• synchrotron nebula from radio waves to hard
  X-rays or low-energy ?-rays
• VHE ?-rays can be produced through Inverse
  Compton scattering of electrons on ambient
  photons (including synchrotron photons)
• In the Crab nebula, the source of VHE ?-rays is
  point-like. The X-ray synchrotron nebula
  (figure) is not resolved by Cherenkov telescopes.

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Pulsar Wind Nebulae (PWNe)

• PWNe seem to be the most frequent type of
  sources revealed by the H.E.S.S. galactic plane
  survey. Some of them are extended.
• Young PWNe have a morphology in VHE ?-rays
  similar to that of the synchrotron (X-ray) nebula
  (e.g. MSH 15-52 associated with the pulsar PSR
  B1509-58)
• Older objects (age gt 104 years) have a larger
  extension in VHE ?-rays (e.g.
  HESS J 1825-137, Vela X)
• Sometimes, the nebula is
  displaced from the pulsar by
  distances comparable to the
  nebula size (HESS J 1825-137,
  Vela X).

MSH 15-52
HESS J1825-137
Vela X
41
A few High-Mass X-ray Binary systems (HMXB)
are VHE ?-rays emitters

• Massive bright star Compact object (pulsar or
  black hole)
• ?-ray signal exhibits the orbital period in 3 of
  them
• Two potential candidates
• Cyg X1 (21 Msun black hole O9.7Iab star)
  MAGIC found a signal in coincidence with X-ray
  flare on September 24, 2006 (4.9s after trials
  for 79 min.)
• HESS J0632057 unidentified but point-like and
  variable (VERITAS 2009) same location as the
  massive star (B0pe) MWC148

42
LS 5039 orbital modulation of the ?-ray flux
AA 460 (2006) 743

• Very different spectra in different orbital
  phases harder spectrum in the inferior
  conjunction.
• ?-ray absorption in the intense photon field in
  the vicinity of the star could partially explain
  the modulation.

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Galactic Centre (HESS J1745-290)
Flare (MJD 53581) X-rays and VHE ?-rays
Broad-band spectrum E2 dF/dE
radio, IR, X-rays, low-energy ?-rays, VHE
?-rays
44
The Galactic Centre Ridge
Nature,439(2006)695

• Subtracting the brightest two sources HESS
  J1745-290 and G0.90.1 ? significant diffuse flux
  coinciding with central molecular clouds (white
  radio contours CS emission)
• The photon index of the ?-ray spectrum of the
  diffuse flux ( index of the proton spectrum) is
  close to that of HESS J1745-290 (G2.1)? the
  cosmic-ray spectrum at the Galactic Centre is
  harder than at the Earth (GEarth 2.7)

45
Centaurus A broad-band spectrum
46
From EGRET/Fermi blazars to TeV blazars

• Broad-band spectra E2 dF/dE of blazars seem to
  follow a quasi-continuous sequence
  (Fossati et al. MNRAS 299(1998)433
• Two broad bumps
• Synchrotron bump.
• Gamma-ray bump (probably due to Inverse Compton
  scattering)
• Flat Spectrum Radio-Quasars (FSRQ)
• Satellites explore the decreasing part of the
  gamma-ray bump
• Only one FSRQ detected by MAGIC during a flare
  above 60 GeV 3C279
• BL Lac (from the name of the prototype source BL
  Lacertæ)
• Satellites explore the rising part of the
  gamma-ray bump.
• Cherenkov telescopes explore the decreasing part

E2 dF/dE
100 GeV
50 MeV
47
Blazar spectra bring constraints on
extra-galactic background light

• H.E.S.S. used the spectra of two remote blazars
  1ES1102-232 et H2356-309 to set bounds on ?-ray
  absorption by extragalactic background light
  (near-infrared and optical)
• After correcting for the absorption, the spectrum
  at the source must have a spectral index gt 1.5 ?
  for a given shape of the spectrum of background
  light, one finds that its intensity is close to
  the lower bounds obtained from galaxy counts.
• Bounds confirmed by the detection of 3C279
  (z0.536) by MAGIC (Egt75 GeV), during an
  optical high state in early 2006 (arXiv
  0901.3275)

Infrared background light spectrum
Nature 440 (2006) 1018
48
Search for Quantum Gravity effects

• Some quantum gravity theories predict Lorentz
  invariance violation (LIV) at the scale of the
  Planck mass leading to
• modified dispersion relations at E ltlt EP
• ? distorted reaction thresholds (invoked for
  suppressing pion photoproduction if the GZK
  cutoff were not present)
• ? energy-dependent photon velocity e.g. for n1

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Gamma-ray bursts at very-high energies ?
GRB 050713a ?t 40 s

• No detection yet, but
• Cherenkov telescopes have a small field of view
  
  ? Alert broadcast by satellites
  Telescope re-positioning ? Delay
• Gamma-ray bursts are submitted to absorption by
  the extragalactic background light, and only
  those produced at redshits lt 0.5 can be detected
  by ground-based instruments.
• MAGIC is presently the best-suited VHE instrument
  for detecting gamma-ray bursts
• Capability of fast slewing (repositioning delay lt
  100 s)
• Lowest energy threshold (50 GeV at zenith) lower
  energy ?-rays are less absorbed (cf. detection of
  3C279 at z0.5)
• Whipple/VERITAS, MAGIC and H.E.S.S. have
  published late-time flux limits of the order of a
  few of the Crab.

50
?-rays from the annihilation of Dark Matter
particles

• Galactic Centre but the spectrum of HESS
  J1745-290 is mainly due to a conventional
  astrophysical source.
• M87 centre of the Virgo supercluster but the
  variability of the signal (HEGRA,H.E.S.S.,MAGIC)
  excludes DM as the main source
• Nearby dwarf spheroidal galaxies are interesting
  targets
• High Mass/Luminosity ratio inferred from the
  radial velocity dispersion of stars
• No warm or hot gas, little dust and no cosmic
  rays
• Possible intermediate mass black holes (from 20
  Msun to 106 Msun) may produce local overdensities
  of DM (minispikes)

51
Indirect search for Dark Matter
Dwarf spheroidal
galaxies

• Flux upper limits of the order of 10-12 cm-2 s-1
  from several dwarf spheroidal galaxies
• Sagittarius 24 kpc (H.E.S.S.),
• Draco 82 kpc (VERITAS, MAGIC),
• Ursa Minor 66 kpc (VERITAS),
• Willman-1 38 kpc (VERITAS)
• and on the overdensity Canis major (8 kpc) which
  could be a dwarf galaxy (H.E.S.S.)
• Constraints in the ltsvgt, m? plane depend on the
  assumed density profile.

52
Synoptic detectors
Detect charged particles and secondary ?-rays
reaching the ground (high
altitude) ? high energy threshold
Tibet
Scintillators
Los Alamos (USA)
Water pool Water Cherenkov detectors
Large field of view and high duty cycle but low
?/hadron discrimination with MILAGRO,
3 months are needed to detect the Crab nebula (5s)
MILAGRO 2p sky survey (20 TeV, 25 Crab)
53
New synoptic detectors should provide large
surveys, albeit with a lower
sensitivity

• The american project High Altitude Water
  Cherenkov (HAWC) should be installed at 4100 m
  a.s.l. in Sierra Negra (Mexico) . It consists of
• 900 water Cherenkov detectors (water tanks, 5 m
  diameter, 4.3 m height) on a 3030 square grid
  over 22500 m2. They detect charged particles
  reaching the ground and electrons from the
  conversion of secondary ?-rays.
• At the bottom of each tank, an 8 phototube is
  directed towards the top and detects the
  Cherenkov light produced by charged particles
  reaching the ground and electrons from the
  conversion of secondary ?-rays.
• The sensitivity of HAWC should be 10 times better
  than that of MILAGRO the Crab nebula could be
  detected in one day
  (instead of 3 months) and the angular resolution
  should be 0.2 to 0.3.
• Not in competition with Cherenkov telescopes, but
  should provide a complete survey of 2p steradians
  at a sensitivity of 1/30 Crab.
• Also sensitive to transient phenomena in the TeV
  range (e.g. AGN flares and gamma-ray bursts)

54
Cosmic electron spectrum (e) the anomaly
claimed by ATIC (Nature 456 (2008) 362,
expérience en ballon) is not confirmed
Fermi LAT arXiv 0905.0025
H.E.S.S. arXiv 0905.0105
The slight deviation with respect to the diffuse
emission model can be due to pulsars (D lt 1 kpc)