2,027 research outputs found
Magnetic Field Seeding by Galactic Winds
The origin of intergalactic magnetic fields is still a mystery and several
scenarios have been proposed so far: among them, primordial phase transitions,
structure formation shocks and galactic outflows. In this work we investigate
how efficiently galactic winds can provide an intense and widespread "seed"
magnetisation. This may be used to explain the magnetic fields observed today
in clusters of galaxies and in the intergalactic medium (IGM). We use
semi-analytic simulations of magnetised galactic winds coupled to high
resolution N-body simulations of structure formation to estimate lower and
upper limits for the fraction of the IGM which can be magnetised up to a
specified level. We find that galactic winds are able to seed a substantial
fraction of the cosmic volume with magnetic fields. Most regions affected by
winds have magnetic fields in the range -12 < Log B < -8 G, while higher seed
fields can be obtained only rarely and in close proximity to wind-blowing
galaxies. These seed fields are sufficiently intense for a moderately efficient
turbulent dynamo to amplify them to the observed values. The volume filling
factor of the magnetised regions strongly depends on the efficiency of winds to
load mass from the ambient medium. However, winds never completely fill the
whole Universe and pristine gas can be found in cosmic voids and regions
unaffected by feedback even at z=0. This means that, in principle, there might
be the possibility to probe the existence of primordial magnetic fields in such
regions.Comment: 14 pages, 5 figures. Accepted for publications by MNRAS. A high
resolution version of the paper is available at
http://astronomy.sussex.ac.uk/~sb207/Papers/bb.ps.g
Relativistic formulation of coordinate light time, Doppler and astrometric observables up to the second post-Minkowskian order
Given the extreme accuracy of modern space science, a precise relativistic
modeling of observations is required. In particular, it is important to
describe properly light propagation through the Solar System. For two decades,
several modeling efforts based on the solution of the null geodesic equations
have been proposed but they are mainly valid only for the first order
Post-Newtonian approximation. However, with the increasing precision of ongoing
space missions as Gaia, GAME, BepiColombo, JUNO or JUICE, we know that some
corrections up to the second order have to be taken into account for future
experiments. We present a procedure to compute the relativistic coordinate time
delay, Doppler and astrometric observables avoiding the integration of the null
geodesic equation. This is possible using the Time Transfer Function formalism,
a powerful tool providing key quantities such as the time of flight of a light
signal between two point-events and the tangent vector to its null-geodesic.
Indeed we show how to compute the Time Transfer Functions and their derivatives
(and thus range, Doppler and astrometric observables) up to the second
post-Minkowskian order. We express these quantities as quadratures of some
functions that depend only on the metric and its derivatives evaluated along a
Minkowskian straight line. This method is particularly well adapted for
numerical estimations. As an illustration, we provide explicit expressions in
static and spherically symmetric space-time up to second post-Minkowskian
order. Then we give the order of magnitude of these corrections for the
range/Doppler on the BepiColombo mission and for astrometry in a GAME-like
observation.Comment: 22 pages, 5 figures, accepted in Phys. Rev.
ST-HM design section: Strategy and working methods
The design section of the ST-HM group is devoted to realise all studies for new transport and handling equipment to be procured and installed according to the needs of CERN. In year 2002, the design section has gradually passed from strong engagement in few huge projects to a multitude of âless expensiveâ projects that require a fast solution and procurement for the LHC installation. The future tasks of our section will be feasibility studies of transport and handling manoeuvres as required for the installation of LHC components and the calculation of lifting tools. In addition the procurement of not yet defined items to solve problems that will occur during the LHC installation. This document gives an overview of the organisation of the design section, the projects and will also underline some problems, such as the incompatibility between the urgency of the users and long CERN purchasing procedures, the work overload, the increasing design requirements for handling tools and operations that were originally not foreseen
Frequency shift up to the 2-PM approximation
A lot of fundamental tests of gravitational theories rely on highly precise
measurements of the travel time and/or the frequency shift of electromagnetic
signals propagating through the gravitational field of the Solar System. In
practically all of the previous studies, the explicit expressions of such
travel times and frequency shifts as predicted by various metric theories of
gravity are derived from an integration of the null geodesic differential
equations. However, the solution of the geodesic equations requires heavy
calculations when one has to take into account the presence of mass multipoles
in the gravitational field or the tidal effects due to the planetary motions,
and the calculations become quite complicated in the post-post-Minkowskian
approximation. This difficult task can be avoided using the time transfer
function's formalism. We present here our last advances in the formulation of
the one-way frequency shift using this formalism up to the
post-post-Minkowskian approximation.Comment: 4 pages, submitted to proceedings of SF2
Light propagation in the field of a moving axisymmetric body: theory and application to JUNO
Given the extreme accuracy of modern space science, a precise relativistic
modeling of observations is required. We use the Time Transfer Functions
formalism to study light propagation in the field of uniformly moving
axisymmetric bodies, which extends the field of application of previous works.
We first present a space-time metric adapted to describe the geometry of an
ensemble of uniformly moving bodies. Then, we show that the expression of the
Time Transfer Functions in the field of a uniformly moving body can be easily
derived from its well-known expression in a stationary field by using a change
of variables. We also give a general expression of the Time Transfer Function
in the case of an ensemble of arbitrarily moving point masses. This result is
given in the form of an integral easily computable numerically. We also provide
the derivatives of the Time Transfer Function in this case, which are mandatory
to compute Doppler and astrometric observables. We particularize our results in
the case of moving axisymmetric bodies. Finally, we apply our results to study
the different relativistic contributions to the range and Doppler tracking for
the JUNO mission in the Jovian system.Comment: 17 pages, 4 figures, submitted to Phys. Rev. D, some corrections
after revie
Transport and handling LHC components: A permanent challenge
The LHC project, collider and experiments, is an assembly of thousands of elements, large or small, heavy or light, fragile or robust. Each element has its own transport requirements that constitute a real challenge to handle. Even simple manoeuvres could lead to difficulties in integration, routing and execution due to the complex environment and confined underground spaces. Examples of typical LHC elements transport and handling will be detailed such as the 16-m long, 34-t heavy, fragile cryomagnets from the surface to the final destination in the tunnel, or the delicate cryogenic cold-boxes down to pits and detector components. This challenge did not only require a lot of imagination but also a close cooperation between all the involved parties, in particular with colleagues from safety, cryogenics, civil engineering, integration and logistics
Ponts roulants du LHC: Lot 3
Cette prĂ©sentation traitera du lot 3 des ponts roulants "lourds" du LHC, qui est constituĂ© de sept appareils (plus un en option). Ils doivent Ă©quiper les zones d'expĂ©riences ATLAS, ALICE et la tĂȘte de puits du PMI2. La mise en place de ces ponts roulants est prĂ©vue pour les annĂ©es 2002 et 2003. Cinq ponts sont destinĂ©s Ă l'expĂ©rience ATLAS, un portique pour l'expĂ©rience ALICE et le dernier pont pour la tĂȘte de puits du PMI 2 avec un pont en option ne devant servir que dans le hall de montage. La capacitĂ© de ces ponts s'Ă©tend sur une gamme de 16t Ă 2x140t et des hauteurs de levage de 6,5 m Ă 102 m. Les points forts de ces ponts seront la descente des bobines pour l'expĂ©rience ATLAS (deux ponts - trois chariots synchronisĂ©s) et la descente de 1200 cryodipoles et tous les aimants du LHC dans PMI2
Construction et installation du LHC
Lâinstallation du LHC a pris, en 2002, une nouvelle dimension avec le dĂ©but des travaux de la machine en parallĂšle Ă ses futures expĂ©riences. La premiĂšre phase des travaux pour le LHC a Ă©tĂ© la mise en place des services gĂ©nĂ©raux Ă©lectriques et des tuyauteries dâeau de refroidissement dans le secteur 7/8. Les premiĂšres lignes cryogĂ©niques (QRL) seront acheminĂ©es au mois de juin. Pour les expĂ©riences ALICE et LHC B, aprĂšs les derniĂšres opĂ©rations de dĂ©montage, les premiers travaux dâinstallation des futurs dĂ©tecteurs ont pu commencer. ATLAS a pu Ă©galement dĂ©buter lâinstallation de son infrastructure au point 1 avec le dĂ©but des travaux de charpente dans USA 15. Pour CMS au point 5, la rĂ©alisation dâune grande partie de son spectaculaire aimant, est lâobjet de nombreuses visites. Le montage des autres parties de son dĂ©tecteur reprĂ©sentant Ă©galement une large part dâactivitĂ© sur dâautres sites
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