104 research outputs found
Embedded star clusters as sources of high-energy cosmic rays: Modelling and constraints
Massive stars are mainly found in stellar associations. These massive star
clusters occur in the heart of giant molecular clouds. The strong stellar wind
activity in these objects generates large bubbles and induces collective
effects that could accelerate particles up to high energy and produce gamma
rays. The best way to input an acceleration origin to the stellar wind
interaction in massive stellar cluster is to observe young massive star
clusters in which no supernova explosion has occurred yet.
This work aims to constrain the part of stellar wind mechanical energy that
is converted into energetic particles using the sensitivity of the ongoing
Fermi/LAT instrument. This work further provides detailed predictions of
expected gamma-ray fluxes in the view of the on-set of the next generation of
imaging atmospheric Cherenkov telescopes.
A one-zone model where energetic particles are accelerated by repeated
interactions with strong supersonic shocks occurring in massive star clusters
was developed. The particle escape from the star cluster and subsequent
interaction with the surrounding dense material and magnetic fields of the HII
region was computed. We applied this model to a selection of eight embedded
star clusters constricted by existing observations. We evaluated the gamma-ray
signal from each object, combining both leptonic and hadronic contributions. We
searched for these emissions in the Fermi/LAT observations in the energy range
from 3 to 300 GeV and compared them to the sensitivity of the Cherenkov
Telescope Array.
No significant gamma-ray emission from these star clusters has been found.
Less than 10% of stellar wind luminosities are supplied to the relativistic
particles. Some clusters even show acceleration efficiency of less than 1%. The
CTA would be able to detect gamma-ray emission from several clusters in the
case of an acceleration efficiency of close to 1%.Comment: accepted for publication in Astronomy&Astrophysic
Legislative Change and Economic Impact on the UK Economy after Brexit
Bakalářská práce se zabývá vztahy Spojeného království a EU po brexitu. Popisuje historický vývoj vztahů těchto subjektů se zaměřením na legislativní aspekty a celkovou integraci Spojeného království v rámci Evropy. Dále rozebírá a porovnává nové vztahy vyplývající z dohody uzavřené mezi těmito subjekty se vztahy předchozími. Věnuje se také rozboru ekonomických dopadů brexitu na hospodářství Spojeného království.
Z porovnání nově dohodnutých mezinárodních vztahů s předchozími vztahy vyplývá, že zůstala mezi subjekty zachována těsná spolupráce. Zároveň setrvává tendence, aby byl rozsah této spolupráce nadále prohlubován. Proto jsou v některých oblastech předpokládány dílčí změny nebo doplnění těchto vztahů. Ekonomické dopady brexitu byly v rámci této práce sledovány v několika oblastech. Jak z daných ukazatelů vyplývá, Brexit negativně ovlivnil ekonomiku Spojeného království, protože byl prokazatelně zpomalen hospodářský růst v rámci sledovaných oblastí, ve kterých byly zaznamenány negativní trendy a výkyvy. Zároveň je ale v celkovém ekonomickém vývoji Spojeného království sledován trend postupného růstu, který již dosahuje v určitých oblastech hodnot z období před propady způsobenými brexitem, ale také světovou pandemií.
V závěru práce je nabídnuto shrnutí rozdílů ve vztazích těchto subjektů a zhodnocení těchto vztahů. Dále jsou ohodnoceny samotné ekonomické dopady brexitu na hospodářství Spojeného království a také celkový negativní dopad brexitu. Popsán je také možný budoucí vývoj vztahů subjektů a ekonomiky Spojeného království. Pro hodnocení brexitu z dlouhodobého hlediska však bude zapotřebí dalších výzkumů.The bachelor thesis deals with UK-EU relations after Brexit. It covers the historical development of the relationship between these entities, focusing on the legislative aspects and the overall integration of the UK within Europe. It also analyses and compares the new relationships resulting from the agreement between these entities and previous relationships. It also analyses the economic impact of Brexit on the UK economy.
A comparison of the newly agreed international relationships with previous relationships shows that close cooperation between the entities has been maintained. At the same time, the trend remains for the scope of this cooperation to be further deepened. Therefore, partial changes or additions to these relationships are foreseen in some areas. This thesis has pursued the economic impact of Brexit in several areas. As the indicators given show, Brexit has negatively affected the UK economy as economic growth has been shown to have slowed down within the areas studied where negative trends and fluctuations have been noted. At the same time, however, there is a trend of gradual growth in the overall UK economy, which in certain areas is already reaching the levels of the pre-Brexit slump but also the global pandemic.
The paper concludes by offering a summary of the differences in the relationships between these entities and an assessment of these relationships. Furthermore, the actual economic impact of Brexit on the UK economy is assessed, as well as the overall negative impact of Brexit. Possible future developments in the relationships between the entities and the UK economy are also described. However, further research will be needed to assess Brexit in the long term.119 - Katedra právavýborn
Young star clusters as gamma ray emitters and their detection with Cherenkov Telescopes
Young massive star clusters as sites of strong stellar winds and supernova explosions may accelerate charged particles at high energies and produce gamma-rays. These sources may also contribute to the production of cosmic rays in our galaxy. At TeV energies several candidates have already been detected: Cygnus OB2, Westerlund 1 \& 2, W43, Pismis 22 and W49A. Our study addresses the issue of very young star clusters where no supernova has occurred yet. During the lifetime of a massive star (M), supersonic stellar winds do indeed release as much energy as a supernova explosion. As supernova remnants are already known as gamma-ray emitters our purpose is to avoid any ambiguity on the origin of a possible gamma ray emission and to fully assume a stellar wind contribution. In this work we first present a catalogue of potential gamma-ray emitting clusters and discuss the criteria used to built the catalogue. We hence model the expected energetic particle spectrum including escapes and losses. We deduce gamma-ray luminosities produced by Inverse Compton and pion decay emission of each cluster and their associated HII regions. We finally compare these gamma-ray luminosities with HESS-II and CTA Cherenkov telescopes sensitivities
Simulation and analysis of exotic non-specular phenomena
Le journal, Open Access, demande à ce qu'on ne dépose pas d'exemplaire du papier sur un autre serveur. https://www.jeos.org/index.php/jeos_rp/article/view/10025International audienceWhen coupled modes are excited in a multilayered structure, the profile of the reflected beam presents exotic characteristics like unexpectedly large lateral shifts or beam enlargment. These results are surprising because they are not accounted for by classical approaches (Artmann's formula or Tamir's description of the reflected beam's profile). Studying such situations requires reliable numerical tools - that is why our programmes are published with this paper. Such tools can be used to understand the behaviour of any multi-layered structure
Improved sensitivity of H.E.S.S.-II through the fifth telescope focus system
The Imaging Atmospheric Cherenkov Telescope (IACT) works by imaging the very
short flash of Cherenkov radiation generated by the cascade of relativistic
charged particles produced when a TeV gamma ray strikes the atmosphere. This
energetic air shower is initiated at an altitude of 10-30 km depending on the
energy and the arrival direction of the primary gamma ray. Whether the best
image of the shower is obtained by focusing the telescope at infinity and
measuring the Cherenkov photon angles or focusing on the central region of the
shower is a not obvious question. This is particularly true for large size IACT
for which the depth of the field is much smaller. We address this issue in
particular with the fifth telescope (CT5) of the High Energy Stereoscopic
System (H.E.S.S.); a 28 m dish large size telescope recently entered in
operation and sensitive to an energy threshold of tens of GeVs. CT5 is equipped
with a focus system, its working principle and the expected effect of focusing
depth on the telescope sensitivity at low energies (50-200 GeV) is discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil
Large negative lateral shifts due to negative refraction
When a thin structure in which negative refraction occurs (a
metallo-dielectric or a photonic crystal) is illuminated by a beam, the
reflected and transmitted beam can undergo a large negative lateral shift. This
phenomenon can be seen as an interferential enhancement of the geometrical
shift and can be considered as a signature of negative refraction
The camera of the fifth H.E.S.S. telescope. Part I: System description
In July 2012, as the four ground-based gamma-ray telescopes of the H.E.S.S.
(High Energy Stereoscopic System) array reached their tenth year of operation
in Khomas Highlands, Namibia, a fifth telescope took its first data as part of
the system. This new Cherenkov detector, comprising a 614.5 m^2 reflector with
a highly pixelized camera in its focal plane, improves the sensitivity of the
current array by a factor two and extends its energy domain down to a few tens
of GeV.
The present part I of the paper gives a detailed description of the fifth
H.E.S.S. telescope's camera, presenting the details of both the hardware and
the software, emphasizing the main improvements as compared to previous
H.E.S.S. camera technology.Comment: 16 pages, 13 figures, accepted for publication in NIM
Detailed spectral and morphological analysis of the shell type SNR RCW 86
Aims: We aim for an understanding of the morphological and spectral
properties of the supernova remnant RCW~86 and for insights into the production
mechanism leading to the RCW~86 very high-energy gamma-ray emission. Methods:
We analyzed High Energy Spectroscopic System data that had increased
sensitivity compared to the observations presented in the RCW~86 H.E.S.S.
discovery publication. Studies of the morphological correlation between the
0.5-1~keV X-ray band, the 2-5~keV X-ray band, radio, and gamma-ray emissions
have been performed as well as broadband modeling of the spectral energy
distribution with two different emission models. Results:We present the first
conclusive evidence that the TeV gamma-ray emission region is shell-like based
on our morphological studies. The comparison with 2-5~keV X-ray data reveals a
correlation with the 0.4-50~TeV gamma-ray emission.The spectrum of RCW~86 is
best described by a power law with an exponential cutoff at TeV and a spectral index of ~. A static
leptonic one-zone model adequately describes the measured spectral energy
distribution of RCW~86, with the resultant total kinetic energy of the
electrons above 1 GeV being equivalent to 0.1\% of the initial kinetic
energy of a Type I a supernova explosion. When using a hadronic model, a
magnetic field of ~100G is needed to represent the measured data.
Although this is comparable to formerly published estimates, a standard
E spectrum for the proton distribution cannot describe the gamma-ray
data. Instead, a spectral index of ~1.7 would be required, which
implies that ~erg has been transferred into
high-energy protons with the effective density cm^-3. This
is about 10\% of the kinetic energy of a typical Type Ia supernova under the
assumption of a density of 1~cm^-3.Comment: accepted for publication by A&
Characterizing the gamma-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT
Studying the temporal variability of BL Lac objects at the highest energies
provides unique insights into the extreme physical processes occurring in
relativistic jets and in the vicinity of super-massive black holes. To this
end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in
the high (HE, 100 MeV 200 GeV)
gamma-ray domain. Over the course of ~9 yr of H.E.S.S observations the VHE
light curve in the quiescent state is consistent with a log-normal behavior.
The VHE variability in this state is well described by flicker noise
(power-spectral-density index {\ss}_VHE = 1.10 +0.10 -0.13) on time scales
larger than one day. An analysis of 5.5 yr of HE Fermi LAT data gives
consistent results ({\ss}_HE = 1.20 +0.21 -0.23, on time scales larger than 10
days) compatible with the VHE findings. The HE and VHE power spectral densities
show a scale invariance across the probed time ranges. A direct linear
correlation between the VHE and HE fluxes could neither be excluded nor firmly
established. These long-term-variability properties are discussed and compared
to the red noise behavior ({\ss} ~ 2) seen on shorter time scales during
VHE-flaring states. The difference in power spectral noise behavior at VHE
energies during quiescent and flaring states provides evidence that these
states are influenced by different physical processes, while the compatibility
of the HE and VHE long-term results is suggestive of a common physical link as
it might be introduced by an underlying jet-disk connection.Comment: 11 pages, 16 figure
The exceptionally powerful TeV gamma-ray emitters in the Large Magellanic Cloud
The Large Magellanic Cloud, a satellite galaxy of the Milky Way, has been
observed with the High Energy Stereoscopic System (H.E.S.S.) above an energy of
100 billion electron volts for a deep exposure of 210 hours. Three sources of
different types were detected: the pulsar wind nebula of the most energetic
pulsar known N 157B, the radio-loud supernova remnant N 132D and the largest
non-thermal X-ray shell - the superbubble 30 Dor C. The unique object SN 1987A
is, surprisingly, not detected, which constrains the theoretical framework of
particle acceleration in very young supernova remnants. These detections reveal
the most energetic tip of a gamma-ray source population in an external galaxy,
and provide via 30 Dor C the unambiguous detection of gamma-ray emission from a
superbubble.Comment: Published in Science Magazine (Jan. 23, 2015). This ArXiv version has
the supplementary online material incorporated as an appendix to the main
pape
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