The Very High Energy Gamma Ray Astronomy (VHE) is a rapidly evolving branch
of modern astronomy, which covers the range from about 50 GeV to several tens
of TeV from the ground. In the past years, the second generation instruments
firmly established a growing and varied list of sources including plerions,
supernova remnants and active galactic nuclei, and started to study some
fundamental questions such as the origin of cosmic rays or the emission
mechanisms of the active galactic nuclei.
  New results now include the first VHE unidentified sources as well as more
puzzling sources such as the Galactic center. The arrival of new generation
instruments (HESS, CANGAROO III, VERITAS, MAGIC) already gives a impressive
look at the near future. Here we attempt to summarize the current status of the
field. We briefly describe the instruments and analysis techniques, and give an
outlook on the sources detected sofar.Comment: Invited talk at the XXIV Physics in Collisions Conference (PIC04),
  Boston, USA, June 2004, 10 pages, LaTeX, 13 eps figures. PSN TUET0

de Naurois, Mathieu

English

arXiv

Invited talk at the XXIV Physics in Collisions Conference (PIC04), Boston, USA, June 2004, 10 pages, LaTeX, 13 eps figures. PSN TUET07, complete proceedings available at http://www.slac.stanford.edu/econf/C0406271/The Very High Energy Gamma Ray Astronomy (VHE) is a rapidly evolving branch of modern astronomy, which covers the range from about 50 GeV to several tens of TeV from the ground. In the past years, the second generation instruments firmly established a growing and varied list of sources including plerions, supernova remnants and active galactic nuclei, and started to study some fundamental questions such as the origin of cosmic rays or the emission mechanisms of the active galactic nuclei. New results now include the first VHE unidentified sources as well as more puzzling sources such as the Galactic center. The arrival of new generation instruments (HESS, CANGAROO III, VERITAS, MAGIC) already gives a impressive look at the near future. Here we attempt to summarize the current status of the field. We briefly describe the instruments and analysis techniques, and give an outlook on the sources detected sofar

De Naurois, M.

Hal-Diderot

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XXIV Physics in Collision - Boston, June 27-29, 2004
HIGH ENERGY GAMMA RAYS
Mathieu de Naurois
LPNHE IN2P3 - CNRS - Universities Paris VI-VII,
4 place Jussieu, 75252 Paris Cedex 05 FRANCE
ABSTRACT
The Very High Energy Gamma Ray Astronomy (VHE) is a rapidly evolving branch
of modern astronomy, which covers the range from about 50 GeV to several tens of
TeV from the ground. In the past years, the second generation instruments firmly
established a growing and varied list of sources including plerions, supernova rem-
nants and active galactic nuclei, and started to study some fundamental questions
such as the origin of cosmic rays or the emission mechanisms of the active galactic
nuclei.
New results now include the first VHE unidentified sources as well as more
puzzling sources such as the Galactic center. The arrival of new generation instru-
ments (HESS, CANGAROO III, VERITAS, MAGIC) already gives a impressive look
at the near future. Here we attempt to summarize the current status of the field.
We briefly describe the instruments and analysis techniques, and give an outlook on
the sources detected sofar.
1
1 The universe in gamma-rays
1.1 A little bit of history
Since their discovery by Victor Hess in 1912, one of the most puzzling (and still
not completely solved) problems of astronomy is the origin of Cosmic Rays. These
high energy particles mostly consist of charged nuclei and are spread over more
than 10 orders of magnitude in energy up to 1020 eV (and possibly above). The
search for the corresponding cosmic accelerators motivated the developement of
gamma-ray astronomy, first from space and then from the ground. In contrast to
the charged component of the cosmic rays, the gamma-rays are not deviated by the
Galactic and extra-galactic magnetic fields and thus point back to their emission
source. They are emitted by particle physics processes (non-thermal synchrotron
radiation of accelerated electrons, inverse Compton scattering of cosmic rays off
ambient photons, pion decay,. . . ) occuring in high electromagnetic field acceleration
regions, shocks in astrophysical plasmas or interaction of the cosmic rays with the
interstellar medium. In the active galactic nuclei or other compact objects, they
are thought to be emitted by the very central engine (instead of being secondary
products) and can therefore probably provide the most valuable constraints on the
emission models. Finally, they could also possibly originate from annihilation of
neutralinos in dark matter clumps.
+90
-90
-180+180
Third EGRET Catalog
E > 100 MeV
Active Galactic Nuclei
Unidentified EGRET Sources
Pulsars
LMC
Solar FLare
Figure 1: Third Egret Catalog.
After the pionneering work of SAS II and COS-B, a breakthrough was
achieved in the 1990’s by the EGRET detector onboard the CGRO satellite: this
spark-chamber detector made the first map of the diffuse γ-ray emission from the
2
Galactic plane and detected 271 point-like source between 100 MeV and 10 GeV.
The third EGRET catalogue [2] (Figure 1) contains 66 AGNs, 8 pulsars and 170
still-unidentified sources. A important fraction of these sources should be Galactic,
and probably contribute to the bulk of cosmic-ray sources. The identification of
these sources is one of the major chalenges of the 21th century astronomy.
1.2 Atmospheric Cerenkov Imaging and other techniques
Above 10 GeV the rapid fall in of the flux limits the sensitivity of space detectors.
The ground base detection relies on the sampling of the Cerenkov light emitted by
the charged particles in the extensive air showers (Figure 2). The opening angle
of the Cerenkov light makes the showers visible at great distances (up to 300 m)
away from the detector and thus permits huge detection area (> 109 cm2). The
shape of the image is then used to discriminate between the high cosmic ray back-
ground (several hundreds Hz) and the γ candidates (representing at most 0.2% of
the background for the strongest sources).
Figure 2: The Atmospheric Cerenkov Imaging technique. Left: detection principle.
Right: Typical images for muons, hadron and γ candidates.
Between roughly 20 GeV and 100 GeV remains a quasi-unexplored region,
where the Cerenkov light is too faint for existing Imaging Cerenkov Telescopes.
Several Solar Farm experiments (Solar II, Stacee, Celeste) took the opportunity
of the huge light collection area provided by the existing solar plants to lower the
threshold down to 50 GeV, but they could never reach high-enough hadron rejection
capabilities to be competitive.
3
The VHE catalogue (figure 3) was opened in 1989 by the discovery of
the Crab Nebula [1] (see section 2.1) by the WHIPPLE collaboration. HEGRA
introduced in 1995 the stereoscopic technique, which, by looking at the same shower
from different points of view, gave a dramatic improvement in hadron and local
muon rejection as well as in angular and energy resolution. After 15 years the VHE
catalogue now consists of about 20 sources with very varied properties.
+180 -180
+90
-90Active Galactic Nuclei
Supernovae Remnant
Plerion
RadioGalaxy
Starbust Galaxy
Binary System
Galactic center
Other
Mkn 421
Mkn 501
PKS 2155-304
1ES 2344+514
1ES 1959+650
3C 66A
M 87
H 1426+428
BL Lac
SN 1006
RXJ 1713.7-3946Cassiopeia A
RCW 86
Vela Junior
Vela SNR
PSR B1706-44 Crab NebulaPSR B1259-63
Sgr A
Cen X-3
TeV J2032+4131
NGC 253
Figure 3: The VHE sky c. 2004
1.3 New instruments
Table 1: Third generation instruments.
Name Location # Telescopes Diameter FoV Opening
CANGAROO III Australia 4 10 m 4◦ Completed
HESS Namibia 4 12 m 5◦ Completed
MAGIC La Palma 1 17 m 4◦ Commissionning
VERITAS USA 4→7 12 m 3.5◦ 2006
After the success of the previous generation (WHIPPLE, HEGRA, CAT,
CANGAROO-II), third generation instruments are now coming online. Their key
properties are summarized in the table 1. Three of these projects are multi-telescope
arrays combining the advantages of a big reflector inherited from WHIPPLE, a
fine pixelization camera first developed by CAT, and stereoscopic observation pi-
onneered by HEGRA. Their detection threshold lies around 100 GeV at zenith,
and increases with zenith angle due to bigger atmospheric thickness. The latest
experiment, MAGIC, is a bigger dish single telescope experiment which comprises
a number of advanced technologies in the design of the mirror and the signal trans-
mission, and aims to lower the detection threshold closer to the 20 GeV domain.
Complementary to this, survey instrument with poor point-like sensitivity but nearly
full sky coverage and duty cycle (Milagro, Tibet III) are providing the firsts VHE
surveys at higher energies (above 2 TeV).
4
2 Galactic source
Nearly half of the known VHE sources belongs to our Galaxy. We will briefly recall
the properties of some of them, focusing on the most recent results.
2.1 The Crab Nebula
This first discovered VHE source is a plerion, that is a synchrotron nebula fed by
the electron wind of a central pulsar. A high resolution image of the Crab Nebula by
the telescope Chandra is show in figure 4. Since its first discovery in 1989, the Crab
Nebula showed no evidence for variability of any kind. It is therefore considered
as the standard candle of high energy gamma-ray astronomy and can be used to
compare and intercalibrate the instruments.
Figure 4: The Crab Nebula. Left: high resolution X-rays image from Chandra, with
confidence region from HESS. Right: Distribution of squared angular distance of the
events to the direction of the Crab nebula as recorded by HESS, showing a highly
significant excess in the direction of the source (θ2 = 0).
New observation carried out by HESS using an uncomplete array of 3
telescopes yielded a highly significant signal, as show in the figure 4. The supreme
achieved angular resolution gives a position compatible with the central pulsar,
however a definite conclusion on the emission region will be possible only in a near
future, when the systematic uncertainties will be reduced. The spectrum derived
by HESS is found to be compatible with previously published results. MAGIC also
reported a 10 σ detection of the Crab Nebula during the comissionning phase.
2.2 Galactic Pevatrons: the Supernova remnants
The supernova remnants are though to be the site of acceleration of the Galactic
cosmic rays up to 1015 eV. However the naive picture according to which the TeV
5
spectrum would give a clear signature of π◦ decay has not turned true, and the
situation is still under debate.
Figure 5: Left: The SN 1006 supernova remnant seen by Rosat. Middle: zoom on
the north west region by Chandra [3]. Right: section of one of the filaments showing
the shock thinckness.
The detection of the supernova remnant SN 1006 by CANGAROO-II in
1996 and 1997 was originally attributed to an inverse Compton emission of acceler-
ated electrons, without the need of nuclear cosmic rays. The situation changed in
2003 with the obervation by Chandra [3] of very thin filaments of intense non ther-
mal synchrotron X-ray emission. This has been interpreted as the effect of magnetic
field amplification made possible by a accelerated population of nuclear cosmic rays
[4]. In this scheme of high magnetic field (≥ 100µG), the TeV emission would be
mainly due to π◦ decay.
HESS reported a non detection of SN 1006 and derived an upper limit at
the level of 10% of the published CANGAROO-II flux. This discrepancy raises a lot
of questions: As the emission of a SNR is not expected to vary significantly on so
short timescales, could the emission come rather from a extragalactic background
source? Only new observations will solved this point.
CANGAROO-II recently reported the detection of two other supernova
remnants, RCW 86 and RX J0852.0-4622. The supernova remnants are now firmly
established as sources of 100 TeV electrons, but the debate concerning the nuclear
cosmic rays acceleration is not closed yet. We expect the situation to improve very
quickly with the results of the third generation instruments.
2.3 An exotic system: PSR B1259-63
PSR B1259-63 is a very unique system in our Galaxy, consisting of a 47.7 ms pulsar
in highly (ǫ ≈ 0.87) excentric orbit around a massive star SS 2883 (figure 6). Every
6
3.4 years at the periastron, the distance of the pulsar to the star is only 23R⋆, R⋆
being the radius of the star. An interaction between the pulsar wind and the stellar
disk surrounding SS 2883 could, according to several models (e.g. [5]), lead to the
production of high energy gamma-rays.
]2 [deg2θ∆squared angular distance   
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
ev
en
ts
0
204
60
80
100
120
140
ba
ck
gr
ou
nd
 (n
or
ma
liz
ed
)
0
200
400
600
800
1000PSR B1259−63/SS 2883 (H.E.S.S., preliminary)
(Feb/Mar 2004, pre−periastron)
signal region
background region
Figure 6: Left: schematics of the PSR B1259-63 binary system. Right: signal
observed by HESS
HESS observed PSR B1259-63 during 10 hours around its periastron (March,
7th 2004). An significant excess at the 8.2 σ has been detected, and further work
on the modeling is under way.
2.4 The Galactic center
The CANGAROO-II experiment first announced the discovery of a TeV gamma-ray
signal from the Galactic center [6]. Soon after, WHIPPLE brought a marginal
confirmation [7]. The very soft and somewhat unusual spectrum measured by
CANGAROO-II, ∝ E−4.6 and the compatibility of the position with the center
of the Galaxy led many authors to interpret this signal in terms of neutralinos an-
nihilation [8]. For a very massive WIMP (of mass mχ) annihilating mostly into
W+W−, Z◦Z◦ and qq¯ pairs, a reasonable description of the decay spectrum is given
by the parametrisation of the form:
dN
dE
=
0.73
mχ
×
e−7.8E/mχ
E0 + (E/mχ)1.5
(1)
If the signal observed by CANGAROO-II was due to annihilation of neu-
tralinos, the mass of the later would be in the range 1− 3 TeV.
As shown in the figure 7, recent results obtained by HESS [9] with a much
better angular resolution (better than 0.1◦) are incompatible with CANGAROO-II,
7
Energy (TeV)10
-1 1 10
 
Te
V)
-
1
 
s
-
2
 
dN
/d
E 
 (m
2 E
10
-9
10
-8
10
-7
10
-6
HESS June/July
HESS July/August
CANGAROO-II
Whipple
10 -5 10 -4 10 -3 10 -2 10 -1 1 10
10-9
10-8
10-7
10-6
10
-5
10
-4
10-3
EGRET
Figure 7: CANGAROO-II and HESS spectra of the Galactic Center.
and would give, if the signal was due to neutralinos, a lower limit of 7.5 TeV on
the neutralino mass at 99% CL. However, the Galactic center is a very crowded
region, with several potential sources in the CANGAROO-II and even in the HESS
confidence level region. New observations with the complete HESS array should for
instance clearly exclude the supernova remnant SgrA East which lies only 1.7′ away
from the Galactic center.
More conventional model for the emission of central black hole Sgr A⋆
include Advection Dominated Accretion Flow (ADAF), or diffuse emission due to
interaction of accelerated protons and nuclei with the high density ambient matter.
2.5 TeV 2032+42 : First TeV unidentifed source
During the year 2002 the first unidentified TeV source, TeV 2032+42 was detected
by HEGRA [10]. This extended and faint source (3% of the Crab flux), having no
radio or X-rays counterparts, exhibits a hard spectrum and has been confirmed by
WHIPPLE analysis of archival data at a slighlty higher level. There are now some
indications that this is not an isolated case, but rather a hint of a bright future.
3 A quick look outside the Galaxy
Almost all known extragalactic TeV sources are active galactic nuclei (AGN) be-
longing to the class of Blazars. They consist of a super-massive (≈ 109M⊙) black
holes surrounded by an accretion disk, from which two giant ultrarelativistic jets of
plasma escape up to Megaparsec distances. For the blazars, one of theses jets points
towards the earth and completely outshines the rest of the AGN.
Amongst other properties, the TeV emission of the Blazars is characterized
8
by a dramatic variability on all time scales, the flux been sometimes multiplied by
a factor 2 in times as short as 20 minutes.
Two principle classes of models aim to explain the emission mechanism
of the Blazars: The leptonic SSC (Synchroton Self Compton) and EC (Extrernal
Compton) models attribute the TeV emission to inverse Compton emission of accel-
erated electrons respectively on the synchroton X photons or on the environment
photons. The second class of models is based on proton acceleration and hadronic
cascades in the jets.
Very detailed observations of the correlation and timelag between the X-
ray and γ-ray emissions are required to discriminate between the models. Recently
organized multi-wavelength observation campains, combining the radio, optical, X-
rays and γ-rays observations, tend to give a more comprehensive picture in favour
of the leptonic models.
Very recent observations of giant flares of Markarian 421 by HESS and
MAGIC confirm that the new generation instruments now reached the required
sensitivity to study the time-variability of the Blazars at the minute timescale, and
will probably solve this acceleration problem in a short time-scale.
4 Conclusion
year
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
So
ur
ce
 C
ou
nt
1
10
210
310
410
510
X-rays
-rays (MeV-GeV)γ
VHE (TeV)
?
Figure 8: Moore’s law of astonomy: Source count vs time for X-ray, γ-ray and high
energy γ-ray astronomy.
In the last years, the VHE astronomy left the field of experiment to become
a real branch of astonomy. The VHE catalogue already contains a big variety of
sources and steadily increases. The similar evolution of the source count versus time
for X-ray, γ-ray and high energy γ-ray astronomy (Figure 8) gives hints about a very
bright future. The increase of statistics about extragalactic sources will also make
9
new measurements possible: The interaction of the γ-rays with intergalactic infra-
red and visible photons (through electron-positron pair creation) can be exploited
to do a tomography of the ambiant star light at cosmological distances. This very
delicate measurement would require a significance number of AGN with similar
spectral features to disentangle the intrinsic properties of each source from the effect
of comic absorption.
5 Acknowledgements
I would like to thank the organizers of the conference for their very warm welcome.
References
1. T.C. Weekes et al, Observation of TeV gamma rays from the Crab nebula using
the atmospheric Cerenkov imaging technique, ApJ 342, 379 (1989).
2. R.C. Hartman et al,The third catalog of High-Energy Gamma-Ray Sources,
ApJS 123, 79 (1999).
3. A. Bamba et al, Fine Structure of Shock of SN 1006 with the Chandra Obser-
vation, ApJ 589, 827 (2003)
4. E.G. Berezhko et al, Confirmation of strong magnetic field amplification and
nuclear cosmic ray acceleration in SN 1006, A&A 412, L11 (2003)
5. J.G. Kirk et al, Inverse Compton Emission of TeV Gamma Rays from
PSR B1259-63, Astropart. Phys. 10, 31 (1999)
6. K. Tsichiya et al, Detection of sub-TeV Gamma-rays from the Galactic Center
Direction by CANGAROO-II, Astrophys. J. Lett. 606, L115 (2004)
7. K. Kosack et al, TeV Gamma-Ray Observations of the Galactic Center, ApJ
608, 97 (2004)
8. D. Hooper et al, Have Atmospheric Cerenkov Telescopes Observed Dark Mat-
ter?, JCAP 09, 002 (2004) and astro-ph/0404205
9. F.A. Aharonian, et al, Very high energy gamma rays from the direction of Sagit-
tarius A*, accepted for publication in A&A Letters (2004) and astro-ph/0408145
10. F.A. Aharonian, et al, An unidentified TeV source in the vicinity of Cygnus
OB2, A&A 393 L37-L40 (2002)
10


High Energy Gamma Rays

de Naurois, M.

HAL-IN2P3

arXiv:astro-ph/0409361v1  15 Sep 2004XXIV Physics in Collision - Boston, June 27-29, 2004HIGH ENERGY GAMMA RAYSMathieu de NauroisLPNHE IN2P3 - CNRS - Universities Paris VI-VII,4 place Jussieu, 75252 Paris Cedex 05 FRANCEABSTRACTThe Very High Energy Gamma Ray Astronomy (VHE) is a rapidly evolving branchof modern astronomy, which covers the range from about 50 GeV to several tens ofTeV from the ground. In the past years, the second generation instruments firmlyestablished a growing and varied list of sources including plerions, supernova rem-nants and active galactic nuclei, and started to study some fundamental questionssuch as the origin of cosmic rays or the emission mechanisms of the active galacticnuclei.New results now include the first VHE unidentified sources as well as morepuzzling sources such as the Galactic center. The arrival of new generation instru-ments (HESS, CANGAROO III, VERITAS, MAGIC) already gives a impressive lookat the near future. Here we attempt to summarize the current status of the field.We briefly describe the instruments and analysis techniques, and give an outlook onthe sources detected sofar.11 The universe in gamma-rays1.1 A little bit of historySince their discovery by Victor Hess in 1912, one of the most puzzling (and stillnot completely solved) problems of astronomy is the origin of Cosmic Rays. Thesehigh energy particles mostly consist of charged nuclei and are spread over morethan 10 orders of magnitude in energy up to 1020 eV (and possibly above). Thesearch for the corresponding cosmic accelerators motivated the developement ofgamma-ray astronomy, first from space and then from the ground. In contrast tothe charged component of the cosmic rays, the gamma-rays are not deviated by theGalactic and extra-galactic magnetic fields and thus point back to their emissionsource. They are emitted by particle physics processes (non-thermal synchrotronradiation of accelerated electrons, inverse Compton scattering of cosmic rays offambient photons, pion decay,. . . ) occuring in high electromagnetic field accelerationregions, shocks in astrophysical plasmas or interaction of the cosmic rays with theinterstellar medium. In the active galactic nuclei or other compact objects, theyare thought to be emitted by the very central engine (instead of being secondaryproducts) and can therefore probably provide the most valuable constraints on theemission models. Finally, they could also possibly originate from annihilation ofneutralinos in dark matter clumps.+90-90-180+180Third EGRET CatalogE > 100 MeVActive Galactic NucleiUnidentified EGRET SourcesPulsarsLMCSolar FLareFigure 1: Third Egret Catalog.After the pionneering work of SAS II and COS-B, a breakthrough wasachieved in the 1990’s by the EGRET detector onboard the CGRO satellite: thisspark-chamber detector made the first map of the diffuse γ-ray emission from the2Galactic plane and detected 271 point-like source between 100 MeV and 10 GeV.The third EGRET catalogue [2] (Figure 1) contains 66 AGNs, 8 pulsars and 170still-unidentified sources. A important fraction of these sources should be Galactic,and probably contribute to the bulk of cosmic-ray sources. The identification ofthese sources is one of the major chalenges of the 21th century astronomy.1.2 Atmospheric Cerenkov Imaging and other techniquesAbove 10 GeV the rapid fall in of the flux limits the sensitivity of space detectors.The ground base detection relies on the sampling of the Cerenkov light emitted bythe charged particles in the extensive air showers (Figure 2). The opening angleof the Cerenkov light makes the showers visible at great distances (up to 300 m)away from the detector and thus permits huge detection area (> 109 cm2). Theshape of the image is then used to discriminate between the high cosmic ray back-ground (several hundreds Hz) and the γ candidates (representing at most 0.2% ofthe background for the strongest sources).Figure 2: The Atmospheric Cerenkov Imaging technique. Left: detection principle.Right: Typical images for muons, hadron and γ candidates.Between roughly 20 GeV and 100 GeV remains a quasi-unexplored region,where the Cerenkov light is too faint for existing Imaging Cerenkov Telescopes.Several Solar Farm experiments (Solar II, Stacee, Celeste) took the opportunityof the huge light collection area provided by the existing solar plants to lower thethreshold down to 50 GeV, but they could never reach high-enough hadron rejectioncapabilities to be competitive.3The VHE catalogue (figure 3) was opened in 1989 by the discovery ofthe Crab Nebula [1] (see section 2.1) by the WHIPPLE collaboration. HEGRAintroduced in 1995 the stereoscopic technique, which, by looking at the same showerfrom different points of view, gave a dramatic improvement in hadron and localmuon rejection as well as in angular and energy resolution. After 15 years the VHEcatalogue now consists of about 20 sources with very varied properties.+180 -180+90-90Active Galactic NucleiSupernovae RemnantPlerionRadioGalaxyStarbust GalaxyBinary SystemGalactic centerOtherMkn 421Mkn 501PKS 2155-3041ES 2344+5141ES 1959+6503C 66AM 87H 1426+428BL LacSN 1006RXJ 1713.7-3946Cassiopeia ARCW 86Vela JuniorVela SNRPSR B1706-44 Crab NebulaPSR B1259-63Sgr ACen X-3TeV J2032+4131NGC 253Figure 3: The VHE sky c. 20041.3 New instrumentsTable 1: Third generation instruments.Name Location # Telescopes Diameter FoV OpeningCANGAROO III Australia 4 10 m 4◦ CompletedHESS Namibia 4 12 m 5◦ CompletedMAGIC La Palma 1 17 m 4◦ CommissionningVERITAS USA 4→7 12 m 3.5◦ 2006After the success of the previous generation (WHIPPLE, HEGRA, CAT,CANGAROO-II), third generation instruments are now coming online. Their keyproperties are summarized in the table 1. Three of these projects are multi-telescopearrays combining the advantages of a big reflector inherited from WHIPPLE, afine pixelization camera first developed by CAT, and stereoscopic observation pi-onneered by HEGRA. Their detection threshold lies around 100 GeV at zenith,and increases with zenith angle due to bigger atmospheric thickness. The latestexperiment, MAGIC, is a bigger dish single telescope experiment which comprisesa number of advanced technologies in the design of the mirror and the signal trans-mission, and aims to lower the detection threshold closer to the 20 GeV domain.Complementary to this, survey instrument with poor point-like sensitivity but nearlyfull sky coverage and duty cycle (Milagro, Tibet III) are providing the firsts VHEsurveys at higher energies (above 2 TeV).42 Galactic sourceNearly half of the known VHE sources belongs to our Galaxy. We will briefly recallthe properties of some of them, focusing on the most recent results.2.1 The Crab NebulaThis first discovered VHE source is a plerion, that is a synchrotron nebula fed bythe electron wind of a central pulsar. A high resolution image of the Crab Nebula bythe telescope Chandra is show in figure 4. Since its first discovery in 1989, the CrabNebula showed no evidence for variability of any kind. It is therefore consideredas the standard candle of high energy gamma-ray astronomy and can be used tocompare and intercalibrate the instruments.Figure 4: The Crab Nebula. Left: high resolution X-rays image from Chandra, withconfidence region from HESS. Right: Distribution of squared angular distance of theevents to the direction of the Crab nebula as recorded by HESS, showing a highlysignificant excess in the direction of the source (θ2 = 0).New observation carried out by HESS using an uncomplete array of 3telescopes yielded a highly significant signal, as show in the figure 4. The supremeachieved angular resolution gives a position compatible with the central pulsar,however a definite conclusion on the emission region will be possible only in a nearfuture, when the systematic uncertainties will be reduced. The spectrum derivedby HESS is found to be compatible with previously published results. MAGIC alsoreported a 10 σ detection of the Crab Nebula during the comissionning phase.2.2 Galactic Pevatrons: the Supernova remnantsThe supernova remnants are though to be the site of acceleration of the Galacticcosmic rays up to 1015 eV. However the naive picture according to which the TeV5spectrum would give a clear signature of π◦ decay has not turned true, and thesituation is still under debate.Figure 5: Left: The SN 1006 supernova remnant seen by Rosat. Middle: zoom onthe north west region by Chandra [3]. Right: section of one of the filaments showingthe shock thinckness.The detection of the supernova remnant SN 1006 by CANGAROO-II in1996 and 1997 was originally attributed to an inverse Compton emission of acceler-ated electrons, without the need of nuclear cosmic rays. The situation changed in2003 with the obervation by Chandra [3] of very thin filaments of intense non ther-mal synchrotron X-ray emission. This has been interpreted as the effect of magneticfield amplification made possible by a accelerated population of nuclear cosmic rays[4]. In this scheme of high magnetic field (≥ 100µG), the TeV emission would bemainly due to π◦ decay.HESS reported a non detection of SN 1006 and derived an upper limit atthe level of 10% of the published CANGAROO-II flux. This discrepancy raises a lotof questions: As the emission of a SNR is not expected to vary significantly on soshort timescales, could the emission come rather from a extragalactic backgroundsource? Only new observations will solved this point.CANGAROO-II recently reported the detection of two other supernovaremnants, RCW 86 and RX J0852.0-4622. The supernova remnants are now firmlyestablished as sources of 100 TeV electrons, but the debate concerning the nuclearcosmic rays acceleration is not closed yet. We expect the situation to improve veryquickly with the results of the third generation instruments.2.3 An exotic system: PSR B1259-63PSR B1259-63 is a very unique system in our Galaxy, consisting of a 47.7 ms pulsarin highly (ǫ ≈ 0.87) excentric orbit around a massive star SS 2883 (figure 6). Every63.4 years at the periastron, the distance of the pulsar to the star is only 23R⋆, R⋆being the radius of the star. An interaction between the pulsar wind and the stellardisk surrounding SS 2883 could, according to several models (e.g. [5]), lead to theproduction of high energy gamma-rays.]2 [deg2θ∆squared angular distance   0 0.02 0.04 0.06 0.08 0.1 0.12 0.14events02046080100120140background (normalized)02004006008001000PSR B1259−63/SS 2883 (H.E.S.S., preliminary)(Feb/Mar 2004, pre−periastron)signal regionbackground regionFigure 6: Left: schematics of the PSR B1259-63 binary system. Right: signalobserved by HESSHESS observed PSR B1259-63 during 10 hours around its periastron (March,7th 2004). An significant excess at the 8.2 σ has been detected, and further workon the modeling is under way.2.4 The Galactic centerThe CANGAROO-II experiment first announced the discovery of a TeV gamma-raysignal from the Galactic center [6]. Soon after, WHIPPLE brought a marginalconfirmation [7]. The very soft and somewhat unusual spectrum measured byCANGAROO-II, ∝ E−4.6 and the compatibility of the position with the centerof the Galaxy led many authors to interpret this signal in terms of neutralinos an-nihilation [8]. For a very massive WIMP (of mass mχ) annihilating mostly intoW+W−, Z◦Z◦ and qq¯ pairs, a reasonable description of the decay spectrum is givenby the parametrisation of the form:dNdE=0.73mχ×e−7.8E/mχE0 + (E/mχ)1.5(1)If the signal observed by CANGAROO-II was due to annihilation of neu-tralinos, the mass of the later would be in the range 1− 3 TeV.As shown in the figure 7, recent results obtained by HESS [9] with a muchbetter angular resolution (better than 0.1◦) are incompatible with CANGAROO-II,7Energy (TeV)10-1 1 10 TeV)-1 s-2 dN/dE  (m2 E10-910-810-710-6HESS June/JulyHESS July/AugustCANGAROO-IIWhipple10 -5 10 -4 10 -3 10 -2 10 -1 1 1010-910-810-710-610-510-410-3EGRETFigure 7: CANGAROO-II and HESS spectra of the Galactic Center.and would give, if the signal was due to neutralinos, a lower limit of 7.5 TeV onthe neutralino mass at 99% CL. However, the Galactic center is a very crowdedregion, with several potential sources in the CANGAROO-II and even in the HESSconfidence level region. New observations with the complete HESS array should forinstance clearly exclude the supernova remnant SgrA East which lies only 1.7′ awayfrom the Galactic center.More conventional model for the emission of central black hole Sgr A⋆include Advection Dominated Accretion Flow (ADAF), or diffuse emission due tointeraction of accelerated protons and nuclei with the high density ambient matter.2.5 TeV 2032+42 : First TeV unidentifed sourceDuring the year 2002 the first unidentified TeV source, TeV 2032+42 was detectedby HEGRA [10]. This extended and faint source (3% of the Crab flux), having noradio or X-rays counterparts, exhibits a hard spectrum and has been confirmed byWHIPPLE analysis of archival data at a slighlty higher level. There are now someindications that this is not an isolated case, but rather a hint of a bright future.3 A quick look outside the GalaxyAlmost all known extragalactic TeV sources are active galactic nuclei (AGN) be-longing to the class of Blazars. They consist of a super-massive (≈ 109M⊙) blackholes surrounded by an accretion disk, from which two giant ultrarelativistic jets ofplasma escape up to Megaparsec distances. For the blazars, one of theses jets pointstowards the earth and completely outshines the rest of the AGN.Amongst other properties, the TeV emission of the Blazars is characterized8by a dramatic variability on all time scales, the flux been sometimes multiplied bya factor 2 in times as short as 20 minutes.Two principle classes of models aim to explain the emission mechanismof the Blazars: The leptonic SSC (Synchroton Self Compton) and EC (ExtrernalCompton) models attribute the TeV emission to inverse Compton emission of accel-erated electrons respectively on the synchroton X photons or on the environmentphotons. The second class of models is based on proton acceleration and hadroniccascades in the jets.Very detailed observations of the correlation and timelag between the X-ray and γ-ray emissions are required to discriminate between the models. Recentlyorganized multi-wavelength observation campains, combining the radio, optical, X-rays and γ-rays observations, tend to give a more comprehensive picture in favourof the leptonic models.Very recent observations of giant flares of Markarian 421 by HESS andMAGIC confirm that the new generation instruments now reached the requiredsensitivity to study the time-variability of the Blazars at the minute timescale, andwill probably solve this acceleration problem in a short time-scale.4 Conclusionyear1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010Source Count110210310410510X-rays-rays (MeV-GeV)γVHE (TeV)?Figure 8: Moore’s law of astonomy: Source count vs time for X-ray, γ-ray and highenergy γ-ray astronomy.In the last years, the VHE astronomy left the field of experiment to becomea real branch of astonomy. The VHE catalogue already contains a big variety ofsources and steadily increases. The similar evolution of the source count versus timefor X-ray, γ-ray and high energy γ-ray astronomy (Figure 8) gives hints about a verybright future. The increase of statistics about extragalactic sources will also make9new measurements possible: The interaction of the γ-rays with intergalactic infra-red and visible photons (through electron-positron pair creation) can be exploitedto do a tomography of the ambiant star light at cosmological distances. This verydelicate measurement would require a significance number of AGN with similarspectral features to disentangle the intrinsic properties of each source from the effectof comic absorption.5 AcknowledgementsI would like to thank the organizers of the conference for their very warm welcome.References1. T.C. Weekes et al, Observation of TeV gamma rays from the Crab nebula usingthe atmospheric Cerenkov imaging technique, ApJ 342, 379 (1989).2. R.C. Hartman et al,The third catalog of High-Energy Gamma-Ray Sources,ApJS 123, 79 (1999).3. A. Bamba et al, Fine Structure of Shock of SN 1006 with the Chandra Obser-vation, ApJ 589, 827 (2003)4. E.G. Berezhko et al, Confirmation of strong magnetic field amplification andnuclear cosmic ray acceleration in SN 1006, A&A 412, L11 (2003)5. J.G. Kirk et al, Inverse Compton Emission of TeV Gamma Rays fromPSR B1259-63, Astropart. Phys. 10, 31 (1999)6. K. Tsichiya et al, Detection of sub-TeV Gamma-rays from the Galactic CenterDirection by CANGAROO-II, Astrophys. J. Lett. 606, L115 (2004)7. K. Kosack et al, TeV Gamma-Ray Observations of the Galactic Center, ApJ608, 97 (2004)8. D. Hooper et al, Have Atmospheric Cerenkov Telescopes Observed Dark Mat-ter?, JCAP 09, 002 (2004) and astro-ph/04042059. F.A. Aharonian, et al, Very high energy gamma rays from the direction of Sagit-tarius A*, accepted for publication in A&A Letters (2004) and astro-ph/040814510. F.A. Aharonian, et al, An unidentified TeV source in the vicinity of CygnusOB2, A&A 393 L37-L40 (2002)10

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