Les fonctionnaires polonais de l’Union européenne à Bruxelles : analyse d’une communauté émergente

This article examines the process of emergence and consolidation of an expat community of over 2000 Polish EU Institutions officials in Brussels. This group appears quite alienated both from other Polish migrants in Belgium and from the local population. At the same time, they constitute a strongly integrated community. The objective of the present analysis is to explain the drivers and the dynamics of this process and to elucidate factors facilitating enclosure: anticipated rejection by local communities driven by national or professional stereotypes, identification with the EU Institutions, but also intensive networking activity. The study consists of two major parts. The theoretical part provides an insight to literature related to the social particularities of the EU officials in general. The empirical part examines processes observed specifically with regard to the Polish EU officials.U radu se istražuje proces nastajanja i konsolidacije trajno iseljene zajednice od oko dvije tisuće poljskih službenika u institucijama Europske unije u Bruxellesu. Ta se skupina doima potpuno otuđenom od ostalih poljskih migranata u Belgiji i lokalnog stanovništva, a istovremeno čini jako integriranu zajednicu. Cilj je ove analize objasniti pokretače i dinamiku tog procesa te rastumačiti faktore koji olakšavaju zatvaranje: očekivano neprihvaćanje od lokalnih zajednica potaknuto nacionalnim ili profesionalnim stereotipima, poistovjećivanje s institucijama Europske unije, ali i intenzivnu aktivnost umrežavanja. Rad se sastoji od dva dijela. U teorijskome se daje uvid u literaturu koja se odnosi na društvene osobitosti službenika Europske unije općenito, dok se u empirijskome istražuje proces koji se promatra posebno u odnosu na poljske službenike u Europskoj uniji.L’article se propose d’étudier le processus d’émergence et de consolidation d’une communauté d’expatriés de plus de 2000 fonctionnaires polonais travaillant au sein des institutions de l’Union européenne à Bruxelles. Ce groupe semble s’être complètement détaché tant du reste des immigrés polonais en Belgique que de la population locale et constitue, dans le même temps, une communauté fortement intégrée. La présente analyse a pour objectif d’expliquer le moteur et la dynamique de ce processus ainsi que d’éclairer les facteurs facilitant cette réclusion, à savoir le rejet prévisible de ce groupe par les communautés locales, induit par les stéréotypes nationaux ou professionnels, l’identification avec les institutions de l’Union européenne ainsi qu’une activité de réseautage intensif. L’étude se compose de deux parties. Le volet théorique fournit un aperçu sur la littérature se rapportant aux particularités sociales des fonctionnaires européens de façon générale. Le volet empirique se consacre, quant à lui, à l’examen du processus spécifique observé parmi les fonctionnaires polonais de l’Union européenne

Response of the warm absorber cloud to a variable nuclear flux in active galactic nuclei

Recent modeling of the warm absorber in active galactic nuclei has proved the usefulness of constant total (gas plus radiation) pressure models, which are highly stratified in temperature and density. We explore the consistency of those models when the typical variation of the flux from the central source is taken into account. We perform a variability study of the warm absorber response, based on timescales and our photoionization code TITAN. We show that the ionization and recombination timescales are much shorter than the dynamical timescale. Clouds very close to the central black hole will maintain their equilibrium since the characteristic variability timescales of the nuclear source are longer than cloud timescales. For more distant clouds, the density structure has no time to vary, in response to the variations of the temperature or ionization structure, and such clouds will show the departure from the constant pressure equilibrium. We explore the impact of this departure on the observed properties of the transmitted spectrum and soft X-ray variability: (i) non uniform velocities, of the order of sound speed, appear due to pressure gradients, up to typical values of 100 km/s. These velocities lead to the broadening of lines. This broadening is usually observed and very difficult to explain otherwise. (ii) Energy-dependent fractional variability amplitude in soft X-ray range has a broader hump around ~ 1-2 keV, and (iv) the plot of the equivalent hydrogen column density vs. ionization parameter is steeper than for equilibrium clouds. The results have the character of a preliminary study and should be supplemented in the future with full time-dependent radiation transfer and dynamical computations.Comment: 9 pages, 7 figures, accepted for publication by Astronomy & Astrophysic

X-ray-binary spectra in the lamp post model

[Abridged] Context. The high-energy radiation from black-hole binaries may be due to the reprocessing of a lamp located on the black hole axis, emitting X-rays. The observed spectrum is made of 3 components: the direct spectrum; the thermal bump; and the reflected spectrum made of the Compton hump and the iron-line complex. Aims. We aim at computing accurately the complete reprocessed spectrum (thermal bump + reflected) of black-hole binaries over the entire X-ray band. We also determine the strength of the direct component. Our choice of parameters is adapted to a source showing an important thermal component. Methods. We compute in full GR the illumination of a thin disk by a lamp along the rotation axis. We use the ATM21 radiative transfer code to compute the spectrum emitted along the disk. We ray trace this local spectrum to determine the reprocessed spectrum as observed at infinity. We discuss the dependence of the local and ray-traced spectra on the emission angle and spin. Results. We show the importance of the angle dependence of the total disk specific intensity spectrum emitted by the illuminated atmosphere when the thermal disk emission if fully taken into account. High spin implies high temperature in the inner regions, so the emitted thermal disk spectrum covers the iron-line complex. As a result we locally observe absorption lines produced in the hot disk atmosphere. Absorption lines are narrow and disappear after ray tracing the local spectrum. Conclusions. Our results mainly highlight the importance of considering the angle dependence of the local spectrum when computing reprocessed spectra, as was already found in a recent study. The main new result of our work is to show the importance of computing the thermal bump of the spectrum, as this feature can change considerably the observed iron-line complex.Comment: 12 pages, 10 figures, accepted in A&A; 2 paragraphs added in section 2 wrt version

Tau longitudinal polarization in B -> D tau nu and its role in the search for charged Higgs boson

We study the longitudinal polarization of the tau lepton in B -> D tau nu decay. After discussing possible sensitivities of tau decay modes to the tau polarization, we examine the effect of charged Higgs boson on the tau polarization in B -> D tau nu. We find a relation between the decay rate and the tau polarization, and clarify the role of the tau polarization measurement in the search for the charged Higgs boson.Comment: 17 pages, 3 figures. Tau -> l nu nu is included in Sec. II. Accordingly the title is changed. References are adde

Disk emission and atmospheric absorption lines in black hole candidate 4U 1630-472

We re-analyzed SUZAKU data of the black hole candidate 4U 1630-472 being in the high/soft state. We show that the continuum X-ray spectrum of 4U 1630-472 with iron absorption lines can be satisfactorily modeled by the spectrum from an accretion disk atmosphere. Absorption lines of highly ionized iron originating in hot accretion disk atmosphere can be an alternative or complementary explanation to the wind model usually favored for these type of sources. We model continuum and line spectra using a single model. Absorption lines of highly ionized iron can origin in upper parts of the disk atmosphere which is intrinsically hot due to high disk temperature. Iron line profiles computed with natural, thermal and pressure broadenings match very well observations. We showed that the accretion disk atmosphere can effectively produce iron absorption lines observed in 4U 1630-472 spectrum. Absorption line arising in accretion disk atmosphere is the important part of the observed line profile, even if there are also other mechanisms responsible for the absorption features. Nevertheless, the wind theory can be an artifact of the fitting procedure, when the continuum and lines are fitted as separate model components.Comment: 10 pages, 4 figures, accepted by Astronomy and Astrophysic

Vertical Structure of Accretion Discs with Hot Coronae in AGN

We study vertical structure of radiation pressure dominated disc with a hot corona. We include all the relevant processes like bound--free opacity and convection. We show that the presence of the corona modifies considerably the density and the opacity of the disc surface layers which are important from the point of view of spectrum formation. The surface of the disc with a corona is much denser and less ionized than the surface of a bare disc. Such a disc is likely to produce a neutral reflection and a local spectrum close to a black body. This effect will help to reconcile the predictions of accretion disc models with the observational data since a neutral reflection and a lack of Lyman edge are generally seen in AGN.Comment: 10 pages, 9 figures, submitted to MNRA

The existence of warm and optically thick dissipative coronae above accretion disks

In the past years, several observations of AGN and X-ray binaries have suggested the existence of a warm T around 0.5-1 keV and optically thick, \tau ~ 10-20, corona covering the inner parts of the accretion disk. These properties are directly derived from spectral fitting in UV to soft-X-rays using Comptonization models. However, whether such a medium can be both in radiative and hydrostatic equilibrium with an accretion disk is still uncertain. We investigate the properties of such warm, optically thick coronae and put constraints on their existence. We solve the radiative transfer equation for grey atmosphere analytically in a pure scattering medium, including local dissipation as an additional heating term in the warm corona. The temperature profile of the warm corona is calculated assuming it is cooled by Compton scattering, with the underlying dissipative disk providing photons to the corona. Our analytic calculations show that a dissipative thick, (\tau_{cor} ~ 10-12) corona on the top of a standard accretion disk can reach temperatures of the order of 0.5-1 keV in its upper layers provided that the disk is passive. But, in absence of strong magnetic fields, the requirement of a Compton cooled corona in hydrostatic equilibrium in the vertical direction sets an upper limit on the Thomson optical depth \tau_{cor} < 5 . We show this value cannot be exceeded independently of the accretion disk parameters. However, magnetic pressure can extend this result to larger optical depths. Namely, a dissipative corona might have an optical depth up to ~ 20 when the magnetic pressure is 100 times higher that the gas pressure. The observation of warm coronae with Thomson depth larger than ~ 5 puts tights constraints on the physics of the accretion disk/corona systems and requires either strong magnetic fields or vertical outflows to stabilize the system.Comment: 9 pages 6 figure, submitted to A&A, comments are welcom

X-ray Variability of AGN and the Flare Model

Short-term variability of X-ray continuum spectra has been reported for several Active Galactic Nuclei. Significant X-ray flux variations are observed within time scales down to 10^3-10^5 seconds. We discuss short variability time scales in the frame of the X-ray flare model, which assumes the release of a large hard X-ray flux above a small portion of the accretion disk. The resulting observed X-ray spectrum is composed of the primary radiation and of a reprocessed Compton reflection component that we model with numerical radiative transfer simulations. The incident hard X-rays of the flare will heat up the atmosphere of the accretion disk and hence induce thermal expansion. Eventually, the flare source will be surrounded by an optically thick medium, which should modify the observed spectra.Comment: 4 pages, 3 figures, accepted proceedings for a talk at the conference "AGN variability from the X-rays to the radio", June 2004, Crimean Observator