The Galactic sky through H.E.S.S. eyes
The High Energy Stereosopic System (H.E.S.S.) is an array of five imaging atmospheric Cherenkov telescopes. Since 2003 it has been operating in the configuration of four 12 m telescopes complemented in 2012 by a much bigger 28 m telescope in the centre of the array. It is designed to detect very hig...
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| Cite this: | The Galactic sky through H.E.S.S. eyes / I. Sushch // Advances in Astronomy and Space Physics. — 2015. — Т. 5., вип. 2. — С. 59-67. — Бібліогр.: 79 назв. — англ. |
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Sushch, I. 2017-06-10T06:05:38Z 2017-06-10T06:05:38Z 2015 The Galactic sky through H.E.S.S. eyes / I. Sushch // Advances in Astronomy and Space Physics. — 2015. — Т. 5., вип. 2. — С. 59-67. — Бібліогр.: 79 назв. — англ. 2227-1481 DOI: 10.17721/2227-1481.5.59-67 https://nasplib.isofts.kiev.ua/handle/123456789/119827 The High Energy Stereosopic System (H.E.S.S.) is an array of five imaging atmospheric Cherenkov telescopes. Since 2003 it has been operating in the configuration of four 12 m telescopes complemented in 2012 by a much bigger 28 m telescope in the centre of the array. It is designed to detect very high energy (VHE) gamma-rays in the range of ∼ 20 GeV to ∼ 50 TeV. Over the past decade it performed extremely sucessful observations of the Galactic plane, which led to the discovery of about 70 sources amongst which the most numerous classes are pulsar wind nebulae, supernova remnants and binary systems. Recently H.E.S.S. also discovered the VHE emission from the Vela pulsar, which became the second pulsar detected at TeV energies after the Crab pulsar. An overview of the main H.E.S.S. discoveries in our Galaxy and their implications on the understanding of physical processes is discussed in this paper. en Головна астрономічна обсерваторія НАН України Advances in Astronomy and Space Physics The Galactic sky through H.E.S.S. eyes Article published earlier |
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The Galactic sky through H.E.S.S. eyes Sushch, I. |
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The Galactic sky through H.E.S.S. eyes |
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The Galactic sky through H.E.S.S. eyes |
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The Galactic sky through H.E.S.S. eyes |
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galactic sky through h.e.s.s. eyes |
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The High Energy Stereosopic System (H.E.S.S.) is an array of five imaging atmospheric Cherenkov telescopes. Since 2003 it has been operating in the configuration of four 12 m telescopes complemented in 2012 by a much bigger 28 m telescope in the centre of the array. It is designed to detect very high energy (VHE) gamma-rays in the range of ∼ 20 GeV to ∼ 50 TeV. Over the past decade it performed extremely sucessful observations of the Galactic plane, which led to the discovery of about 70 sources amongst which the most numerous classes are pulsar wind nebulae, supernova remnants and binary systems. Recently H.E.S.S. also discovered the VHE emission from the Vela pulsar, which became the second pulsar detected at TeV energies after the Crab pulsar. An overview of
the main H.E.S.S. discoveries in our Galaxy and their implications on the understanding of physical processes is discussed in this paper.
|
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2227-1481 |
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https://nasplib.isofts.kiev.ua/handle/123456789/119827 |
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The Galactic sky through H.E.S.S. eyes / I. Sushch // Advances in Astronomy and Space Physics. — 2015. — Т. 5., вип. 2. — С. 59-67. — Бібліогр.: 79 назв. — англ. |
| work_keys_str_mv |
AT sushchi thegalacticskythroughhesseyes AT sushchi galacticskythroughhesseyes |
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2025-11-26T04:52:04Z |
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2025-11-26T04:52:04Z |
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| fulltext |
The Gala
ti
sky through H.E.S.S. eyes
I. Sush
h
1,2∗
, for the H.E.S.S. Collaboration
Advan
es in Astronomy and Spa
e Physi
s, 5, 59-67 (2015)
I. Sush
h, for the H.E.S.S. Collaboration, 2015
1
Centre for Spa
e Resear
h, North-West University, 11 Ho�man Street, 2531, Pot
hefstroom, South Afri
a
2
Astronomi
al Observatory of Ivan Franko National University of L'viv, vul. Kyryla i Methodia, L'viv, Ukraine
The High Energy Stereos
opi
System (H.E.S.S.) is an array of �ve imaging atmospheri
Cherenkov teles
opes.
Sin
e 2003 it has been operating in the
on�guration of four 12m teles
opes
omplemented in 2012 by a mu
h
bigger 28m teles
ope in the
entre of the array. It is designed to dete
t very high energy (VHE) gamma-rays in
the range of ∼ 20GeV to ∼ 50TeV. Over the past de
ade it performed extremely su
essful observations of the
Gala
ti
plane, whi
h led to the dis
overy of about 70 sour
es amongst whi
h the most numerous
lasses are pulsar
wind nebulae, supernova remnants and binary systems. Re
ently H.E.S.S. also dis
overed the VHE emission from
the Vela pulsar, whi
h be
ame the se
ond pulsar dete
ted at TeV energies after the Crab pulsar. An overview of
the main H.E.S.S. dis
overies in our Galaxy and their impli
ations on the understanding of physi
al pro
esses is
dis
ussed in this paper.
Key words: H.E.S.S., gamma-ray astronomy, Gala
ti
sour
es
introdu
tion
Over the past de
ade, very high energy (VHE;
E > 100GeV) gamma-ray astronomy has be
ome
one of the most popular and fast-developing bran
hes
of the observational s
ien
e and a driver of the-
oreti
al models in several topi
al areas of mod-
ern astrophysi
s and
osmology. The
urrent gen-
eration of imaging atmospheri
Cherenkov tele-
s
opes (H.E.S.S., VERITAS and MAGIC) represent
a breakthrough, opening up a window to the previ-
ously largely unexplored VHE Universe and its mys-
teries. An in
redibly su
essful period of operation of
these ground-based instruments resulted in dete
tion
of more than 100 VHE gamma-ray sour
es
1
. This re-
markable s
ienti�
breakthrough would not be pos-
sible without the High Energy Stereos
opi
System
(H.E.S.S.), whi
h played a major role in the opening
of the �eld of gamma-ray astronomy, be
oming the
main instrument in the southern hemisphere.
H.E.S.S. is an array of �ve imaging atmospheri
Cherenkov teles
opes lo
ated in the Khomas High-
land of Namibia at an altitude of 1800m above sea
level [24℄. During the summer of 2012 the array of
the �rst four 12m teles
opes was
ompleted with the
addition of a mu
h larger 28m teles
ope in the
en-
tre of the array. This upgrade expanded the energy
overage of the instrument down to ∼ 20GeV and
in
reased the system's sensitivity.
For the epo
h of the H.E.S.S. I observations (four
teles
ope array), the sour
es in our Galaxy
an be
summarised in the H.E.S.S. Gala
ti
Plane Survey
(HGPS; Fig. 1)
ombining the data
olle
ted during
the period starting from 2004 to 2013 [42℄. The
total of roughly 2800 hours of high quality obser-
vations in the Gala
ti
longitude range of 250◦ to
65◦ and Gala
ti
latitude range |b| < 3.5◦ are in-
luded in the survey. The HGPS reveals the diverse
population of
osmi
a
elerators in the Galaxy re-
sulting in the
atalogue of 77 VHE sour
es. This
in
ludes 13
omplex sour
es (Supernova Remnants,
SNRs, and Gala
ti
entre region) whi
h were ex-
luded from the analysis pipeline. Only sour
es
with TS > 25 were in
luded in the
atalogue
2
. The
data analysis was performed for the energy range
of 0.2− 100TeV. The
atalogue
omprises 12 pulsar
wind nebulae (PWNe), 6 SNRs, 6
omposite ob-
je
ts and 3 binary systems (Fig. 2). Fifty sour
es
remain unidenti�ed, mainly due to multiple asso
i-
ations, but also due to the la
k of
ounterparts at
other wavebands. Five new sour
es were dis
overed
in the HGPS: HESS J1813−126, HESS J1826−130,
HESS J1828−099, HESS J1832−085, and
HESS J1844−030. Most of these sour
es were not
dete
ted before due to their proximity to the other,
more extended H.E.S.S. sour
es, and only a highly
in
reased amount of data allowed one to dis
rimi-
nate them from their
ompanions. Some of these new
sour
es are
oin
ident with the known pulsars, whi
h
suggests that they might be PWNe. HESS J1844-030
is
oin
ident with the
atalogued SNR G29.4+0.1.
∗
iurii.sush
h�nwu.a
.za
1
For the
urrent status of the population of the VHE gamma-ray sour
es
he
k TeVCat, an online TeV gamma-ray
atalogue, at
http://tev
at.u
hi
ago.edu
2
Test Statisti
s � the likelihood ratio of a model with the additional sour
e at a spe
i�ed lo
ation and a model without the additional
sour
e
59
Advan
es in Astronomy and Spa
e Physi
s I. Sush
h, for the H.E.S.S. Collaboration
Fig. 1: The TS map of the H.E.S.S. Gala
ti
Plane Survey.
Beyond our Galaxy, more than 30 sour
es were
dis
overed and asso
iated with a
tive gala
ti
nu-
lei (AGNi). This population is dominated by the
blazars of the BL La
ertae type.
Fig. 2: Sour
e
lassi�
ation of the HGPS.
The observational strategy of H.E.S.S. has been
evolving over the years. During the �rst years of op-
eration, H.E.S.S. was opening a new �eld of gamma-
ray astronomy, dis
overing many new sour
es. Ea
h
new dete
tion was treated as a major dis
overy,
but in many
ases limited exposure did not allow
deep studies of the spe
trum and morphology of the
sour
e. However, in re
ent years with the
ontinu-
ously growing population of gamma-ray sour
es, s
i-
enti�
priorities had shifted towards the better un-
derstanding of the nature of the gamma-ray emis-
sion from these sour
es. This led to deeper obser-
vations of spe
i�
obje
ts in attempt to reveal and
explain the physi
al pro
esses generating VHE emis-
sion. The dete
tion of numerous sour
es belonging
to one
lass of obje
ts (PWNe, SNRs, AGNi) allowed
for population studies, whi
h led to the investigation
of
ommon properties of sour
es of the same
lass.
The goal of this paper is to give an overview of
the
urrent status of the Gala
ti
sky as seen with
H.E.S.S. with an emphasis on the re
ent results ob-
tained during the last few years. Note that some
of the results dis
ussed here are preliminary results
whi
h were presented for the �rst time at the 34th
Internation Cosmi
Ray Conferen
e in summer of
2015. For the most re
ent review of the H.E.S.S.
Gala
ti
sky please
onsult [44℄, and for the most re-
ent reviews of the VHE gamma-ray astronomy see,
e. g., [59, 69℄.
supernova remnants
Supernova remnants are the remains of the su-
pernova explosions of massive stars at the end of
their evolution. As a result of this explosion the
outer layers of the star are blown o� into the sur-
rounding medium, heating it up. The expansion of
the SNR into the medium
reates a sho
k wave at
60
Advan
es in Astronomy and Spa
e Physi
s I. Sush
h, for the H.E.S.S. Collaboration
whi
h parti
les (ele
trons and protons)
an be a
el-
erated to extremely high energies. The theory of dif-
fusive sho
k a
eleration at sho
k fronts [47℄ predi
ts
the generation of a
elerated parti
le populations in
SNRs whi
h, intera
ting in turn with ambient pho-
ton �elds (ele
trons) or ambient matter (protons),
an produ
e VHE gamma rays (see e. g. [39℄).
Fig. 3: The surfa
e brightness map of HESS J1534−571.
The green ellipse indi
ate the position of the radio SNR
G323.7−1.0. The �gure is taken from [42℄.
SNRs are the se
ond most numerous
lass of
VHE gamma-ray Gala
ti
sour
es. Approximately
50 gamma-ray sour
es dete
ted in the H.E.S.S.
Gala
ti
Plane Survey (HGPS) are spatially
o-
in
ident with SNRs dete
ted in radio and higher
frequen
y observations. However, the VHE emis-
sion
an �rmly be asso
iated with SNRs for only
7 H.E.S.S. sour
es: RXJ0852-4622 (Vela Jr.) [20,
28℄, RXJ1713.7-3946 [14, 23, 26, 51℄, RCW86 [30℄,
SN 1006
3
[11℄, G323.7-01.0
4
[42℄, G353.6−0.7 [4℄,
and W28 [29℄. All ex
ept the last one are shell-
type SNRs with a resolved shell-like TeV morphol-
ogy. For RCW86 the TeV shell was resolved only
re
ently in a detailed morphology study whi
h ben-
e�ted from signi�
antly improved statisti
s
om-
pared with the dis
overy paper [61℄. G353.6−0.7
(or HESS J1731−347) is the �rst SNR dis
overed
serendipitously in VHE gamma-rays and only later
on�rmed by radio and X-ray observations [77, 78℄.
A new TeV shell-like sour
e HESS J1534−571 (Fig. 3)
was dete
ted re
ently in the HGPS [42℄
oin
ident
with the radio SNR G323.7-1.0 and thus �rmly iden-
ti�ed as SNR [68℄. Moreover, there are several other
SNR
andidates (with resolved TeV shell-like mor-
phology) with the most prominent example being
HESS J1912+101 [18℄ (Fig. 4). These, however,
an-
not be �rmly identi�ed as SNRs due to the la
k of
SNR
ounterparts at other wavelengths [68℄. Unfor-
tunately, the TeV data alone are unable to �rmly
identify the shell-like sour
e as an SNR as there are
other astrophysi
al obje
ts that potentially may ap-
pear shell-like while being potential TeV gamma-ray
emitters, su
h as superbubbles or wind-blown
avi-
ties into whi
h hadroni
parti
les are di�using [68℄.
Fig. 4: The surfa
e brightness map of HESS J1912+101.
The �gure is taken from [42℄.
The list of shell-type SNRs dete
ted at TeV en-
ergies
an be
ompleted by the sour
es dete
ted
in the northern sky by VERITAS and MAGIC �
CasA [7, 19, 33℄, Ty
ho [8℄, and IC 443 [6, 34℄. How-
ever, only for IC 443
ould the TeV shell re
ently be
resolved [60℄.
The most important question, whi
h studies of
the VHE emission from SNRs are expe
ted to be
able to answer, pertains to the origin of the Gala
-
ti
osmi
rays. Gala
ti
osmi
rays are believed
to be mainly produ
ed at the sho
ks of SNRs via
a
eleration of protons and ele
trons. When a
el-
erated to very high energies, ele
trons and protons
an in turn generate VHE gamma-rays via inverse
Compton s
attering on ambient photon �elds and
the bremsstrahlung pro
ess (ele
trons) and proton-
proton intera
tions (protons). Cosmi
rays
onsist
of 99% protons, thus any eviden
e of hadroni
na-
ture of the dete
ted gamma-ray emission from SNRs
an be treated as an indire
t
on�rmation of the hy-
pothesis that the Gala
ti
osmi
rays originate in
SNRs.
The spe
tral shape of the most of TeV SNRs
an
be des
ribed with both leptoni
and hadroni
s
e-
narios with a slight preferen
e for the leptoni
one.
3
Lo
ated outside the Gala
ti
Plane and, thus, not in
luded in the HGPS
4
HESS J1534−571 � a new sour
e dete
ted in the HGPS
61
Advan
es in Astronomy and Spa
e Physi
s I. Sush
h, for the H.E.S.S. Collaboration
This is not quite surprising as, usually SNRs ex-
pand into a rather rare�ed medium
reated by their
progenitor stars. However, for several examples in
ases where SNRs intera
t with mole
ular
louds
with mu
h higher matter density, the hadroni
s
e-
nario is mu
h more preferable. This appear to be
the
ase for su
h sour
es as e. g. IC 443 [6, 12, 34℄,
H.E.S.S. SNR W28 [29℄, and the GeV SNR de-
te
ted by Fermi-LAT W44 [12℄. Usually these are
middle-aged SNRs whi
h feature an es
ape of high-
energy parti
les whi
h then intera
t with mole
ular
louds produ
ing VHE gamma-rays. The spatial o�-
set of the gamma-ray emission region
ompared to
the emission region at lower energies provides ev-
iden
e for parti
le es
ape. Su
h dete
tions of the
SNRs intera
ting with mole
ular
louds are the �rst
dire
t indi
ations of the e�e
tive proton a
eleration
at SNR sho
ks.
Dete
tion of TeV SNRs (or SNR
andidates)
whi
h do not have
ounterparts at X-ray energies
(su
h as HESS J1912+101) be
omes an important
method to tra
e hadroni
dominated SNRs. La
k
of non-thermal X-ray emission suggests insu�
ient
amounts of high energy ele
trons, and thus the
gamma-ray emission from su
h sour
es
an hardly
be explained in the leptoni
s
enario.
Fig. 5: The magneti
�eld map of RXJ1713.7-3946. The
�gure is taken from [50℄.
The fo
us of H.E.S.S. observations on spe
i�
sour
es led to very detailed studies of a number of
obje
ts in
luding some SNRs. One of the most re-
markable examples is RXJ1713.7-3946. New mea-
surements based on ∼ 150h of observations bene�t
from improvement in the exposure by fa
tors of 2
(sky maps) to 4 (spe
tra) over the previous mea-
surements [50℄. This improvement allows spe
tral
and morphology studies of unpre
edented pre
ision,
leading to detailed, spatially dependent studies of the
SNR. The angular resolution of better than 0.05◦ al-
lows one to perform a detailed investigation of the
morphologi
al di�eren
es between the TeV and X-
ray emission, yielding remarkable results. For the
�rst time in TeV gamma-ray astronomy, VHE data
allow one to
onstru
t the maps of physi
al parame-
ters su
h as magneti
�eld. The magneti
�eld map
obtained for RXJ1713.7-3946 (Fig. 5) shows that this
quantity is very variable a
ross the remnant. A
om-
parison of the TeV and X-ray radial pro�les shows
that the TeV shell is extended beyond the X-ray shell
whi
h may point either to parti
le es
ape or to a
ompli
ated
on�guration of the magneti
�eld [50℄.
In any
ase these high-pre
ision measurements show
how the VHE gamma-ray astronomy
an probe a
-
eleration regions and open up new prospe
ts for
studies.
More than 300 SNRs are dete
ted at radio fre-
quen
ies, of whi
h about 250 fall into the region of
the HGPS, but only ∼ 50 are
oin
ident with VHE
sour
es. This underdete
tion of SNRs at TeV ener-
gies motivated an SNR population study [53℄ pro-
viding �ux upper limits for 124 sour
es. The study
showed a
lear
orrelation between VHE �ux to radio
�ux ratio and sour
e age. This kind of study might
be very useful for future observations of SNR with
the Cherenkov Teles
ope Array (CTA).
pulsars
Pulsars are rapidly rotating and highly magne-
tised neutron stars
reated as a result of supernovae
explosions. They are surrounded by a rotating mag-
netosphere and feature relativisti
out�ows. Pulsars
emit pulsed emission at all wavelengths and although
they were primarily dete
ted at radio frequen
ies
(with ∼ 2500 radio pulsars dete
ted so far), most
of their radiation is believed to be emitted at high
energies via
urvature radiation of
harged parti
les
(ele
trons and positrons) a
elerated in the ele
tro-
magneti
�eld of the pulsar. This is supported by
a rapid in
rease of pulsar dete
tions at GeV ener-
gies in re
ent years thanks to the new sensitive in-
struments Fermi-LAT and AGILE, with numbers
rea
hing now more than 150 obje
ts [3℄. The en-
ergy spe
tra of most of the gamma-ray pulsars
an be
well des
ribed by an exponentially
ut-o� power law,
E−Γ exp
[
− (E/Ecut)
b
]
, with b ≤ 1 and
ut-o� en-
ergy Ecut typi
ally between 1 and 10GeV [3℄. A sub-
exponential
ut-o� supports models of gamma-ray
produ
tion in the outer magnetopshere, ex
luding a
polar
ap model for whi
h a super-exponential
ut-
o� (b > 1) is expe
ted. The extrapolation of pulsar
spe
tra dete
ted by Fermi-LAT to higher energies
reveals a dramati
de
rease of the gamma-ray �ux
beyond 10GeV, whi
h makes the dete
tion of pulsars
at energies & 100GeV with
urrent ground-based in-
struments very unlikely. However, quite surprisingly,
the �rst dete
tion of the Crab pulsar above 25GeV
62
Advan
es in Astronomy and Spa
e Physi
s I. Sush
h, for the H.E.S.S. Collaboration
by MAGIC [36℄ with the �ux
onsistent with the
extrapolation of Fermi-LAT spe
trum was followed
by further dete
tions of pulsed gamma-ray emission,
�rst up to 250GeV by VERITAS [79℄, and later up to
400GeV by MAGIC [35℄. The nature of this emission
is still not understood with several explanations be-
ing suggested, su
h as inverse Compton ups
attering
of the magnetospheri
X-ray emission by the pulsar
wind ele
trons [31℄ or the IC emission of se
ondary
ele
trons in the outer magnetosphere [66, 74℄. Re-
ently 320 h of observations allowed MAGIC to ex-
tend the spe
trum up to ∼ 2TeV [45℄, providing a
further support for the IC models. However, VER-
ITAS (with 194 h of observations) did not
on�rm
this result, revealing a �rm dete
tion of the pulsed
emission only up to 400GeV [67℄.
One of the major s
ien
e obje
tives for the new
28m H.E.S.S. teles
ope was to pursue the pulsar ob-
servation program, providing more information for
the understanding of the nature of the pulsed VHE
radiation. The prin
ipal sour
e
hosen for this pur-
pose was the Vela pulsar, the brightest sour
e in the
high energy gamma-ray sky with a hint of pulsed
emission above 20GeV observed using the Fermi-
LAT data. The data was taken only with the 28m
teles
ope, in the monos
opi
way, providing a �rm
dete
tion of the pulsed radiation in the energy band
from 20GeV to 120GeV, establishing a se
ond VHE
pulsar [52℄.
pulsar wind nebulae
The ele
tron-positron plasma eje
ted from ener-
geti
pulsars in the form of relativisti
winds
arries
most of the rotational energy of the pulsars. The
pulsar wind intera
ting with the ambient medium
terminates at a standing sho
k where parti
les
an
be e�
iently a
elerated. A
elerated leptons
an in-
tera
t with the magneti
�eld and low-energy photon
�elds, generating non-thermal emission from radio
frequen
ies to energies as high as 100TeV. This re-
sults in the formation of a syn
hrotron nebula around
the pulsar seen in radio to X-rays and more extended
IC nebula at GeV and TeV energies.
Pulsar wind nebulae appear to be the most e�e
-
tive gamma-ray emitters in the Galaxy, forming the
most numerous
lass of VHE gamma-ray Gala
ti
obje
ts. The list of 12 �rmly identi�ed VHE PWNe
dete
ted in the HGPS
an be
ompleted by 6 PWNe
outside the HGPS and about one third of the 50
unidenti�ed sour
es whi
h are
oin
ident with young
powerful pulsars. TeV PWNe dete
ted by H.E.S.S.
an naturally be divided into two
lasses based on
their morphology, whi
h in turn serves as an indi
a-
tion of the pulsar age. Young PWNe su
h as the
Crab Nebula [24℄, G0.9+0.1 [15℄, G21.5−0.9 [46℄,
et
., are generally dete
ted as
ompa
t and un-
resolved obje
ts. In su
h systems the TeV emis-
sion region is
oin
ident with the asso
iated young
high spindown luminosity pulsar and is
ompati-
ble with the X-ray emission region. Older PWNe,
su
h as VelaX [5, 22℄, HESS J1825-137 [21, 25℄ and
HESS J1303-631 [16, 55℄, show mu
h more
ompli-
ated morphologies, with the TeV emission regions
mu
h larger than the X-ray emission regions and pul-
sars signi�
antly o�set from the
entre of the nebula.
The larger size of the VHE PWN
omparing to the
X-ray one
an be explained by syn
hrotron
ooling
of very energeti
ele
trons. Very energeti
ele
trons
produ
ing the X-ray emission via syn
hrotron radi-
ation undergo strong radiative losses and lose their
energy relatively fast. At the same time ele
trons
need less energy to produ
e TeV emission via IC
s
attering. These ele
trons su�er less from radia-
tive
ooling and therefore
an survive longer and
in greater number. This s
enario was supported by
the detailed energy-dependent morphology studies in
HESS J1825-137 and HESS J1303-631. Steepening of
the spe
trum with the distan
e from the pulsar de-
te
ted in HESS J1825-137 (Fig. 6)
learly indi
ates
the radiative
ooling of ele
trons and gives an in-
sight into the PWN evolution, allowing one to look
into older epo
hes. Similarly, the energy-dependent
morphology study of HESS J1303-631 showed that
the emission region �shrinks� towards the position of
the pulsar with the in
rease of the energy threshold
(Fig. 7).
The o�set of the pulsar
an be explained by the
proper motion of the pulsar due to the initial ki
k ob-
tained in the supernova explosion and/or by the de-
stru
tion of a part of the nebula by the reverse sho
k
of an SNR. The latter is believed to be the
ase for
the PWN of Vela pulsar, VelaX. North-eastern and
south-western sides of the Vela SNR are believed to
be expanding into the media with di�erent parti
le
densities [71, 72℄ whi
h leads to faster formation of
the reverse sho
k on the side with higher density [38℄.
Therefore it is possible that on one side the reverse
sho
k has already rea
hed the PWN while on the
other side they still did not intera
t.
From the beginning of the VHE astronomy era,
H.E.S.S. dete
ted many so-
alled �dark� sour
es.
These are the sour
es dete
ted only at TeV ener-
gies without
ounterparts at radio or X-ray ener-
gies. A majority of these sour
es are signi�
antly
extended, and a lot of them are
oin
ident with en-
ergeti
pulsars. It has been suggested re
ently that
a substantial fra
tion of these �dark a
elerators�
might be the evolved PWNe [43℄. It was shown that
the magneti
�eld in PWNe de
reases with time,
hen
e leading to the suppression of the syn
hrotron
emission, while the IC emission in
reases with time
until most of the pulsar spindown energy is trans-
ferred to the nebula. One of the best examples of
previously �dark� emitters whi
h were identi�ed as
PWNe is HESS J1303−631. Its identi�
ation as a
PWN was based on the energy-dependent morphol-
ogy whi
h indi
ated the asso
iation with the pulsar
PSRJ1301−6305 and on the subsequent dete
tion
63
Advan
es in Astronomy and Spa
e Physi
s I. Sush
h, for the H.E.S.S. Collaboration
of the X-ray
ounterpart [55℄. The sour
e was sub-
sequently dete
ted at GeV energies by Fermi-LAT,
exhibiting a similar morphology as at TeV energies
with a larger emission region [10℄. Re
ent dedi
ated
radio observations with ATCA did not reveal any sig-
ni�
ant extended emission asso
iated with the pul-
sar, but a shell-like stru
ture, possibly an SNR, was
dete
ted in the �eld of view [73℄. In
ase it is an
SNR, it might be the birth pla
e of the pulsar.
Fig. 6: Energy spe
tra of HESS J1825-137 in radial bins,
re�e
ting the steepening of the spe
trum with the dis-
tan
e from the pulsar. The HESS ex
ess map is shown
in the inset. The wedges show the radial regions with
radii in steps of 0.1◦ at whi
h the energy spe
tra were de-
termined. The innermost region is
entred on the pulsar
PSR J1826-1334. The di�erential energy spe
tra for the
regions illustrated in the inset are s
aled by powers of 10
for the purposes of
larity. The spe
trum for the analysis
at the pulsar position is shown with the dashed line as
a referen
e along with the other spe
tra. The �gure is
taken from [25℄.
A large sample of identi�ed TeV PWNe and a
omparable amount of PWN
andidates dete
ted
only in the TeV range motivated the population
study of these obje
ts [63, 64℄. For the �rst time
the HGPS also allows for the extra
tion of �ux up-
per limits from the regions around pulsars without
dete
ted TeV emission. All this information allows
for a systemati
investigation of the evolution of pa-
rameters su
h as luminosity and extension over ∼ 105
years after the birth of the pulsar. Population studies
reveal some trends in the evolution of PWNe, su
h as
a de
rease of spin-down luminsity with age, expan-
sion of PWNe with time and the fading of old PWNe,
but there are also sour
es whi
h exhibit large vari-
ations from the average behaviour, whi
h are likely
due to the diversity of ambient media and intrinsi
initial
onditions.
07:00.0 06:00.0 05:00.0 04:00.0 03:00.0 02:00.0 01:00.0 13:00:00.0
-62:55:00.0
-63:00:00.0
05:00.0
10:00.0
15:00.0
20:00.0
25:00.0
IRAS 13010-6254
E > 10 TeV
E 2-10 TeV
E < 2 TeV
PSR J1301-6305
Fig. 7: Energy mosai
of HESS J1303−631. The hori-
zontal axis is the Right As
ension and the verti
al axis
is the De
lination in J2000.0
oordinates. Red, green
and blue
olours indi
ate di�erent energy ranges: E1 =
(0.8−2)TeV, E2 = (2−10)TeV and E3 > 10TeV, respe
-
tively. The highest energy photons originate nearest to
the pulsar, PSR J1301-6305 (marked by the green dot).
The visible red
orresponds roughly to the 10σ signi�-
an
e
ontour of the entire sour
e. XMM-Newton X-ray
ontours are shown in bla
k. The �gure is taken from [55℄.
gamma-ray binaries
Gamma-ray binaries
omprise a relatively small
lass of VHE sour
es
onsisting only of 5 obje
ts.
Binary systems are variable sour
es
onsisting of a
massive star and a
ompa
t obje
t su
h as a bla
k
hole or a pulsar. The TeV emission in these systems
is believed to originate from the intera
tion between
the two obje
ts, either in the a
retion-powered jet,
or in the sho
k between the pulsar wind and the stel-
lar wind.
The �ve binaries dete
ted at TeV energies
are PSRB1259−63/LS 2883 [13, 17, 56℄, LS 5039,
HESS J0632+057 [27, 37℄, LSI+61 303 [9, 32℄,
and HESS J1018−589 (1FGL1018.6−5856) [54, 57℄.
HESS J0632+057 is the �rst binary primarily dis
ov-
ered at TeV energies [58℄ and the only one whi
h
an be observed in both the northern and south-
ern sky. HESS J1018−589 is a new member of the
lass of the TeV gamma-ray binaries. Re
ently, re-
64
Advan
es in Astronomy and Spa
e Physi
s I. Sush
h, for the H.E.S.S. Collaboration
observations of this point-like sour
e,
oin
ident with
the high-energy binary 1FGL1018.6−5856 dete
ted
by Fermi-LAT [1℄, revealed its variability at TeV en-
ergies [57℄. The
onsisten
y of the TeV light
urve
of the sour
e with the GeV and X-ray light
urves
(Fig. 8)
on�rms the asso
iation of HESS J1018−589
with 1FGL1018.6−5856.
Fig. 8: Gamma-ray and X-ray �uxes of
1FGLJ1018.6−5856 folded with the orbital period
of P = 16.58 d. Two orbits are shown for
larity. Top:
VHE integral �ux above 0.35TeV measured by H.E.S.S.
(red
ir
les). For
omparison, a s
aled light
urve from
the nearby bright sour
e HESS J1023−589 is shown in
grey. Middle top and middle bottom: Fermi-LAT light
urve between 1 and 10 GeV (solid blue squares) and
between 0.1 and 1 GeV (open blue squares). Bottom:
X-ray 0.3−10 keV
ount-rate light
urve from Swift-XRT
observations in 2011 (green), 2012 (blue), and 2013
(red). The �gure is taken from [57℄.
The only TeV binary for whi
h the na-
ture of the
ompa
t obje
t is well known is
PSRB1259−63/LS 2883. It
onsists of a pulsar or-
biting a Be star in a very e
entri
orbit (e = 0.87)
with a period of 3.4 years. Sin
e the start of H.E.S.S.
operation, the periastron passage in the system has
o
urred four times, in 2004 [17℄, 2007 [13℄, 2010 [56℄,
and 2014 [70℄, thoroughly observed by H.E.S.S.ea
h
time. In 2014, the sour
e was visible for the �rst
time dire
tly at the periastron
rossing and also for
the �rst time it was possible to observe the sour
e
both before and after the periastron passage. This
allowed for the
on�rmation of the light
urve shape
obtained from the
ombined observation of three pre-
vious periastron passages, showing that it does not
hange from orbit to orbit. VHE observations show
no emission far from periastron and a
omplex light
urve at the periastron passage exhibiting two peaks,
before and after periastron (Fig. 9). The TeV �ux
variability has a similar shape as the X-ray and radio
emission, featuring peaks at the same orbital phases.
The nature of the VHE emission
an be explained
as IC radiation within the pulsar-wind stellar-wind
s
enario. The shape of the light
urve is not well un-
derstood yet, but it is believed that pre- and post-
periastron peaks are related to the lo
ation of the
equatorial
ir
umstellar disk of the Be star whi
h
the pulsar
rosses twi
e ea
h orbit.
At GeV energies, however, PSRB1259−63/LS 2883
shows a
ompletely di�erent behaviour, displaying
a remarkable post-periastron �are whi
h is time-
shifted with respe
t to the post-periastron peak at
other wavebands [2, 41, 75, 76℄. First dete
ted
around the 2010 periastron passage, the �are then
re-appeared with a slightly lower �ux at the same
orbital phase during the 2014 periastron passage,
revealing a periodi
behaviour of this phenomenon.
Apart from the �are, GeV observations around 2010
periastron passage also showed a faint dete
tion
lose to periastron [2, 75℄ whi
h, however, was not
on�rmed during the 2014 passage [41, 76℄. The
nature of the �are is still not understood. Sev-
eral explanations for this have been suggested (see
e. g. [40, 48, 49, 62, 65℄), but ea
h has its limitations.
In 2014, the sour
e was for the �rst time observed
in the H.E.S.S.II phase, exploiting the new 28m tele-
s
ope. The data
olle
ted with H.E.S.S.II allowed for
the extention of the spe
trum down to 200GeV, re-
sulting in a spe
trum harder than during previous
periastrons [70℄. New observations also revealed a
rather high �ux from the sour
e 50 days after pe-
riastron during the period overlaping with the GeV
�are. Although these results are still preliminary
and
areful data analysis is still ongoing, this new
information may inspire new e�orts towards a better
understanding of the unexpe
ted GeV �are.
summary
This paper dis
usses only those results from the
Gala
ti
VHE gamma-ray astronomy that are the
most re
ent and the most interesting a
ording to the
admittedly biased opinion of the author. Other VHE
gamma-ray astronomy dis
overies go far beyond our
own Galaxy � e. g. dete
ting gala
ti
sour
es in the
65
Advan
es in Astronomy and Spa
e Physi
s I. Sush
h, for the H.E.S.S. Collaboration
Large Magelani
Cloud and observing a
tive gala
ti
nu
lei up to z ∼ 1. Among the other targets of the
gamma-ray astronomy is the sear
h for the dark mat-
ter annihilation. The remarkable results a
hieved
over the last de
ade in the �eld of the VHE gamma-
ray astronomy ex
eeded all expe
tations, providing a
massive boost to the development of theoreti
al stud-
ies in the �elds of parti
le a
eleration and radiation
pro
esses. The importan
e of gamma-ray astronomy
was highly re
ognised by the s
ienti�
ommunity,
resulting in the development of new proje
ts, su
h
as CTA and HAWC, whi
h will further in
rease the
sensitivity and resolution of the gamma-ray observa-
tions thereby providing deep insights into a number
of physi
al problems.
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