Bibliografie tematiche / Accretion phenomena

Letteratura scientifica selezionata sul tema "Accretion phenomena"

Autore: Grafiati

Pubblicato: 8 giugno 2024

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Articoli di riviste sul tema "Accretion phenomena":

Walker, Mark. "Accretion-driven nonthermal phenomena". Astrophysical Journal 330 (luglio 1988): L47. http://dx.doi.org/10.1086/185202.

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Bailey, Jeremy. "Accretion Phenomena in Cataclysmic Variables". Publications of the Astronomical Society of Australia 13, n. 1 (gennaio 1996): 75–80. http://dx.doi.org/10.1017/s1323358000020579.

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AbstractThe processes of accretion onto the white dwarf in a cataclysmic variable are reviewed. These systems provide the most easily observable examples of accretion disks, and enable the structure of the accretion region to be studied in detail. The properties of column accretion onto magnetic cataclysmic variables are also described.

KUNCIC, ZDENKA, e GEOFFREY V. BICKNELL. "TOWARDS A NEW STANDARD THEORY FOR ASTROPHYSICAL DISK ACCRETION". Modern Physics Letters A 22, n. 23 (30 luglio 2007): 1685–700. http://dx.doi.org/10.1142/s0217732307024243.

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We briefly review recent developments in black hole accretion disk theory, placing new emphasis on the vital role played by magnetohydrodynamic (MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong indicator that vertical transport of angular momentum by large-scale MHD torques is important and may even dominate radial transport by small-scale MHD turbulence. This leads to an enhanced overall rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue that when vertical transport is important, the radial structure of the accretion disk is modified and this affects the disk emission spectrum. We present a simple model demonstrating that energetic, magnetically-driven outflows give rise to a disk spectrum that is dimmer and redder than a standard accretion disk accreting at the same rate. We briefly discuss the implications of this key result for accreting black holes in different astrophysical systems.

Lee, Umin, e Tod E. Strohmayer. "Thermonuclear Excitation of R-modes in Neutron Stars". International Astronomical Union Colloquium 155 (1995): 445–46. http://dx.doi.org/10.1017/s0252921100037921.

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AbstractNeutron stars display phenomena which result from the burning of accreted nuclear fuel in their surface layers. In addition to producing X-ray bursts and other temporal phenomema, such burning can excite oscillation modes of the neutron star. Here we investigate the stability of R-modes in uniformly rotating neutron stars using nonadiabatic pulsation calculations of steady burning envelopes on accreting neutron stars. We find that the fundamental R-modes are unstable for a large range of accretion rates, and thus might manifest themselves observationally in the power spectra of accreting sources above about 1 kHz.

Ferreira, J. "Accretion-ejection phenomena from young stars". EAS Publications Series 9 (2003): 33. http://dx.doi.org/10.1051/eas:2003083.

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Kim, Hongsu, e Uicheol Jang. "Effect of Radiation Pressure Formed at the Inner Region of the Accretion Disk on the Accretion Flow in the Outer Region". Journal of Astronomy and Space Sciences 40, n. 4 (dicembre 2023): 247–58. http://dx.doi.org/10.5140/jass.2023.40.4.247.

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Studying the accretion phenomena provides a window into understanding most heavenly bodies, from the birth of stars to active galactic nuclei (AGN). We would adopt the effect of the radiation pressure, which reduces accretion rates ( M ˙ ), on the accretion phenomena. The Shakura-Sunyaev α-disk model of disk accretion is a good candidate theory of advection dominated accretion flow (ADAF). Reduction in the angular velocity leads to the suppression the disk luminosity and surface temperature, essentially indicating the transition of the standard accretion disk model from convection dominated accretion flow (CDAF) to ADAF.

Yokosawa, M. "Dynamical Evolution of Accretion Flow onto a Black Hole". Symposium - International Astronomical Union 188 (1998): 455–56. http://dx.doi.org/10.1017/s0074180900116006.

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Active galactic nuclei(AGN) produce many type of active phenomena, powerful X-ray emission, UV hump, narrow beam ejection, gamma-ray emission. Energy of these phenomena is thought to be brought out binding energy between a black hole and surrounding matter. What condition around a black hole produces many type of active phenomena? We investigated dynamical evolution of accretion flow onto a black hole by using a general-relativistic, hydrodynamic code which contains a viscosity based on the alpha-model. We find three types of flow's pattern, depending on thickness of accretion disk. In a case of the thin disk with a thickness less than the radius of the event horizon at the vicinity of a marginally stable orbit, the accreting flow through a surface of the marginally stable orbit becomes thinner due to additional cooling caused by a general-relativistic Roche-lobe overflow and horizontal advection of heat. An accretion disk with a middle thickness, 2rh≤h≤ 3rh, divides into two flows: the upper region of the accreting flow expands into the atmosphere of the black hole, and the inner region of the flow becomes thinner, smoothly accreting onto the black hole. The expansion of the flow generates a dynamically violent structure around the event horizon. The kinetic energy of the violent motion becomes equivalent to the thermal energy of the accreting disk. The shock heating due to violent motion produces a thermally driven wind which flows through the atmosphere above the accretion disk. A very thick disk, 4rh≤h,forms a narrow beam whose energy is largely supplied from hot region generated by shock wave. The accretion flowing through the thick disk,h≥ 2rh, cannot only form a single, laminar flow falling into the black hole, but also produces turbulent-like structure above the event horizon. The middle disk may possibly emit the X-ray radiation observed in active galactic nuclei. The thin disk may produce UV hump of Seyfert galaxy. Thick disk may produce a jet observed in radio galaxy. The thickness of the disk is determined by accretion rate, such ashκ κes/cṁf(r) κ 10rhṁf(r), at the inner region of the disk where the radiation pressure dominates over the gas pressure. Here, Ṁ is the accretion rate and ṁ is the normarized one by the critical-mass flux of the Eddington limit. κesandcare the opacity by electron scattering and the velocity of light.f(r) is a function with a value of unity far from the hole.

Tanaka, Y. "Outburst Phenomena in X-Ray Binaries". Symposium - International Astronomical Union 151 (1992): 215–24. http://dx.doi.org/10.1017/s0074180900122211.

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Three distinctly different types of outburst phenomenon in X-ray binaries are discussed: (1) the so-called type 1 burst which is a thermonuclear flash of matter accreted on the surface of neutron stars, (2) the so-called type 2 burst which is a burst-like X-ray emission caused by a spasmodic accretion, and (3) the nova-like outburst which is triggered by a sudden commencement of accretion.

Iijima, T. "Rapid Mass Accretion in the Symbiotic Star AG Dra". International Astronomical Union Colloquium 93 (1987): 759–62. http://dx.doi.org/10.1017/s0252921100105640.

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AbstractThe mass accretion process onto the hot component of AG Dra and its explosive phenomena are discussed. The hot component seems to be a massive white dwarf (M > 1 M⊙). The mass accretion rate is estimated to be about 10−7M⊙/year. Many properties of the explosive phenomena agree with those of mild hydrogen flashes expected from this rapid mass accretion.

de Gouveia Dal Pino, Elisabete M., e Alex C. Raga. "JD7 -Astrophysical Outflows and Associated Accretion Phenomena". Proceedings of the International Astronomical Union 5, H15 (novembre 2009): 235–36. http://dx.doi.org/10.1017/s1743921310009002.

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Highly collimated supersonic jets and outflows are very frequent in several astrophysical environments. They are seen in young stellar objects (YSOs), proto-planetary nebulae, compact objects (like galactic black holes or microquasars, and X-ray binary stars), active galactic nuclei, and are also possibly associated to gamma-ray bursts (GRBs) and to ultra-high energy cosmic rays sources (UHECRs). Despite their different physical scales, all these outflow classes have strong morphological similarities, but questions such as - what physics do they share? - or - can we find a universal mechanism of acceleration and collimation that operates in all classes? - remain matters of debate. The most accepted mechanism for their origin relies on a rotating accretion disk threaded by perpendicular large-scale magnetic fields and, though most of the systems producing jets contain an accretion disk around the central source, the real role that rotation and magnetic fields play in these processes is still not fully understood, nor are the highly non-linear physical processes connected to these jet-disk systems in the large parameter space involved.

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Tesi sul tema "Accretion phenomena":

Hickinbotham, Simon John. "S-Gabor filters for line accretion phenomena". Thesis, University of York, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341113.

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Rolfe, Daniel James. "Accretion flow and precession phenomena in cataclysmic variables". Thesis, n.p, 2001. http://library7.open.ac.uk/abstracts/page.php?thesisid=52.

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Smith, Amanda Jane. "Accretion Disc Phenomena in Extreme Mass Ratio Cataclysmic Variables". Thesis, Open University, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486254.

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We study a variety of accretion phenomena under the umbrella of extreme mass ratio cataclysmic variables (CVs), employing both observation and numerical simulation. A primary focus is the superhump phenomenon and the underlying eccentric instability and accretion disc precession. We make a detailed examination of the relationship between superhump period excess, f, and system mass ratio, q. We conduct extensive high resolution three-dimensional smoothed particle hydrodynamic (SPH) simulations, improving upon the realism of previous work from both a numerical and physical perspective. We find a much improved match with observation. A complete compilation from the literature suggeSts an observed multi-valued f - q relation. Our simulations reveal the mechanism generating the superhump modulation, and we examine the distribution of eccentricity in the disc. The latter has bearing on disc precession rates. Other results include a quantifiable enhancement of tidal torques as the disc becomes eccentric, very low eccentricity growth rates particularly at high q, and damped eccentricity at very low q. We look for observational signatures of the underlying precessing accretion disc in AM CVn and HP Lib. Previous simulations predicted trailed spectra to evolve with precession phase, and we find this indeed to be the case. A long disc precession period S-wave is found in the line profiles, also foreseen by simulation. For VWHyi, we use the indirect imaging technique of Doppler tomography.to interpret the phase dependent optical emission line profiles during quiescence. We find an ionisation structure in the region of impact between the accretion stream and disc. Strongly phasedependent HeII emission further suggests a hot cavity at the initial impact site. Our observations feature prominent white dwarf (WD) spectral features in the optical, and we make a determination of the WD mass using the gravitational redshift of an MgII photospheric absorption line.

Symington, Neil H. "Observations and modelling of accretion phenomena in Classical T Tauri stars". Thesis, University of Exeter, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.421579.

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Finke, Justin David. "Monte Carlo/Fokker-Planck simulations of Accretion Phenomena and Optical Spectra of BL Lacertae Objects". Ohio University / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1181833339.

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De, Jong Sandra. "Accretion processes of radio galaxies at high energies". Phd thesis, Observatoire de Paris, 2013. http://tel.archives-ouvertes.fr/tel-00914365.

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Les NAG (Active galactic nuclei, ou noyaux actifs de galaxie) sont des noyaux galactiques lumineux (L>10^42 erg/s) au centre desquels se trouve un trou noir super massif. Leur énergie lumineuse est libérée lors de l'accrétion sur ce trou noir, soit via un disque d'accrétion, soit via un flux d'accrétion relativement inefficace. Certaines questions, notamment concernant l'origine des jets dans environ 10 % des NAG, n'ont pas encore de réponse. Le Fermi/LAT gamma-ray survey a soulevé une nouvelle question lorsqu'il a détecté un petit groupe de radio galaxies en addition à de nombreux blazars. Les radio galaxies sont séparées en deux catégories en fonction du de la luminosité de leurs jets dans le domaine radio. La catégorie Fanaroff-Riley I (FR-I) rassemble les sources dont les jets sont brillants près du noyau, tandis que les jets des sources de la catégorie FR-II sont brillants aux extrémités. Les sources FR-I sont apparentées aux BL Lacs, blazars de faible luminosité. Les FSRQ, blazars de forte luminosité, sont probablement enfantés par les sources de type FR-II. Cette thèse présente une étude de radio galaxies lumineuses dans le domaine gamma. Nous avons étudié deux exemples de ce nouveau type de sources en analysant des données X et gamma et en créant des distributions spectrales d'énergie (SED) large-bande. Pour 3C 111, de type FR -II, nous avons analysé des observations de Suzaku/XIS et PIN, ainsi que des observations d'INTEGRAL IBIS/ISGRI, pour créer un spectre dans le domaine X. Nous avons aussi utilisé un spectre mesuré par Swift/BAT lors de sa campagne d'observation de 58 mois. Le spectre résultant, qui couvre les énergies de 0,4 à 200 keV, met en évidence deux contributions : l'une thermale de type Seyfert montrant une raie de fer K-alpha, l'autre non thermale caractéristique d'un jet. Nous avons aussi analysé des données gamma de Fermi/LAT. Nous avons combiné les données X et gamma avec des données historiques dans les domaines radio, infrarouge et optiques, pour construire le SED. Ce SED est modélisé de manière satisfaisante par un jet non thermal. La luminosité bolométrique de 3C 111 est relativement faible, et le modèle SED correspond plus à une source de type BL Lac que de type FSRQ auquel nous nous attendions. La seconde source que nous avons étudiée est M87, de type FR-I. Cette source proche a été détectée dans les bandes gamma et TeV, mais pas encore en rayons X durs (> 10 keV). Nous avons concentré la première partie de notre analyse sur la limite supérieure de l'émission X de cette source en utilisant des observations d'INTEGRAL IBIS/ISGRI. En plus de la méthode habituelle, nous avons appliqué plusieurs techniques telles que "pointing selection" et "shadogram treatment" afin d'augmenter le rapport signal sur bruit. En utilisant 5,1 Ms de données ISGRI nous avons déterminé, avec une certitude de 3 sigma, une limite supérieure de f < 3x10-12 erg/cm2/s pour le flux de M87 dans la bande 20-60 keV. Notre analyse d'observations de Suzaku/PIN nous a permis d'effectuer la première détection de rayons X dur émis par M87. Celle-ci a un flux de f=1.3+0.1-0.2x 10-11 erg/cm2/s entre 20 et 60 keV. Cette détection suggeste une éruption; en effet ce flux est très supérieur à la limite supérieure que nous avons calculée. En combinant cette limite supérieure d'émission X avec des données de Fermi/LAT et données historiques radio, infrarouges et optiques, nous avons construit un SED. Celui-ci est correctement modélisé par une source de type BL Lac, conforme à nos attentes puisque M87 est de type FR-I. Nous avons alors examiné les aspects généraux des radio galaxies à forte émission gamma. La plupart de ces objets sont de type FR-I, et le noyau d'au moins une source FR-II (3C 111) est plus proche de BL Lac que de FSRQ. Il est possible que ce soit aussi le cas des autres sources FR-II. Comme dans le cas des blazars, leur émission gamma est originaire du jet. La source est trop inclinée pour que l'émission du jet apparaisse boostée. En revanche, puisque les rayons gammas sont émis à proximité du trou noir central, les observations peuvent être expliquées soit par un grand angle d'ouverture du jet, soit par réflection sur le disque. Fermi/LAT a observé un potentiel halo de matière sombre aux alentours de l'amas de la vierge. J'ai participé à l'étude de l'émission de cette source. Notre travail a mis en évidence qu'une collection de source ponctuelles contribue à cette émission. Je présente dans cette thèse le résultat de notre analyse. Pour terminer, nous rapportons la première détection de rayons X provenant de l'objet BL Lac BZB J1552+0850 et de la galaxie de Seyfert LSBC F727-V01. Nous les avons observées avec les instruments UVOT et XRT de Swift. Ces deux sources sont situées dans le rayon d'erreur de la source Fermi/LAT 2FGL J1551.9+0855. Puisque les galaxies de Seyfert émettent rarement des rayons gammas, nous avançons l'hypothèse que BZB J1552+0850 est la contrepartie UV et X de la source gamma Fermi/LAT 2FGL J1551.9+0855. L'étude du rayonnement X des radio galaxies à forte émission gamma aide à caractériser ces sources. La résolution spectrale de la nouvelle génération d'instruments tels que NuSTAR et ASTRO-H permettra de distinguer les composantes thermales et non-thermales des spectres X. Construire des spectres de distribution d'énergie à partir d'observation à plusieurs longueurs d'ondes aidera à contraindre les émissions large-bande. Cela facilitera l'assignation de contreparties visibles aux sources détectées par Fermi/LAT, tâche non triviale à cause des incertitudes de position.

Aimar, Nicolas. "Astrophysique extrême avec GRAVITY : sursauts énergétiques aux abords de l'horizon des événements du trou noir central de la Galaxie". Electronic Thesis or Diss., Université Paris sciences et lettres, 2023. http://www.theses.fr/2023UPSLO006.

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La Voie Lactée, comme a priori toutes les galaxies structurées, abrite en son cœur un trou noir supermassif d'environ 4,3 millions de masses solaires, nommé Sagittarius A* (Sgr A*). Sa taille, liée à sa masse, et sa proximité d'environ 8,3 kpc, en font le trou noir avec la plus grande taille angulaire dans le ciel (~50 μas), ce qui en fait la cible d'études idéales de ce genre d'objets. Les trous noirs sont les objets les plus compacts de l'Univers, avec un champ gravitationnel extrême proche de leur horizon. La description de ces objets et de leur environnement proche nécessite la prise en compte de la Relativité Générale, introduite en 1915 par Albert Einstein.Depuis plus de 20 ans, Sgr A* et son environnement sont la cible de nombreuses campagnes d'observations à différentes longueurs d'ondes (radio, IR, rayons X). Le suivi des orbites des étoiles-S contenues dans la seconde d'angle autour de Sgr A* a permis de prouver certains effets prédits par la Relativité Générale, tels que la précession de Schwarzschild. Les observations en rayons X et en NIR ont montré que Sgr A* présente une importante variabilité du flux émis par le flot d'accrétion, avec des sursauts dont le flux peut atteindre jusqu'à ~100 fois le flux médian. L'avènement de l'optique adaptative et de l'interférométrie optique, notamment avec les quatre grands télescopes du VLTI et l'instrument GRAVITY, ont permis de mettre en évidence un mouvement orbital de l'origine de trois sursauts observés en 2018.De nombreux modèles ont été envisagés pour expliquer les sursauts de Sgr A*, mais l'observation d'un mouvement orbital a fortement contraint ces modèles. Parmi eux, le modèle analytique de point chaud est largement utilisé avec différents degrés de complexité et d'hypothèses. En parallèle du développement des modèles analytiques, de nombreuses simulations d'accrétion autour de trous noirs ont été étudiées avec un intérêt particulier pour le phénomène de reconnexion magnétique qui apparaît comme un scénario plausible pour expliquer l'origine des sursauts de Sgr A*.Dans cette thèse, nous étudions différents modèles pour les sursauts de Sgr A* à l'aide du code de tracé de rayons Gyoto, allant d'un modèle de point chaud analytique avec une variabilité intrinsèque à un modèle semi-analytique basé sur la reconnexion magnétique.Le premier modèle est très utile pour comprendre les effets de la Relativité (Restreinte et Générale) sur les observables (astrométries et courbes de lumière), ainsi que l'influence de la variabilité intrinsèque sur celles-ci.Le second modèle est motivé par un phénomène physique particulier, la reconnexion magnétique, et est construit à partir des résultats des simulations numériques. Dans ce modèle, la vitesse azimutale est libre d'être super-Képlérienne, en raison de l'entraînement du site de reconnexion par les lignes de champ magnétique. Cette propriété constitue une contrainte observationnelle des sursauts de 2018 observés par GRAVITY que les modèles précédents ne parvenaient pas à expliquer.De plus, nous étudions également l'impact de la modélisation de l'état quiescent combiné aux sursauts sur les observables. La contribution de celui-ci dans les calculs d'astrométrie se traduit par un décalage entre la position du trou noir et le centre de l'orbite apparente, ce qui constitue une autre conclusion des observations des sursauts de 2018.En plus des astrométries et des courbes de lumière, GRAVITY a mesuré la polarisation des sursauts de 2018. Le code de tracé de rayons Gyoto est maintenant capable de calculer la polarisation des images. La nouvelle version du code a été validée en comparant les résultats avec un autre code de tracé de rayons, ipole. Le modèle basé sur la reconnexion magnétique montre des résultats très encourageants et peut être encore amélioré pour prendre en compte la polarisation, ainsi que les propriétés multi-longueurs d'onde des sursauts de Sgr A*
The Milky Way, like presumably all structured galaxies, harbors a supermassive black hole at its core, approximately 4.3 million times the mass of the Sun, named Sagittarius A* (Sgr A*). Its size, determined by its mass, and its proximity of about 8.3 kpc make it the black hole with the largest angular size in the sky (~50 μas), making it the ideal target for studying this type of object. Black holes are the most compact objects in the Universe, with an extreme gravitational field near their event horizon. Describing these objects and their immediate environment requires taking into account General Relativity, introduced in 1915 by Albert Einstein .For over 20 years, Sgr A* and its environment have been the subject of numerous observation campaigns at various wavelengths (radio, IR, X-rays). Tracking the orbits of S-stars within one arcsecond around Sgr A* has provided evidence for certain effects predicted by General Relativity, such as Schwarzschild precession. X-ray and NIR observations have shown that Sgr A* exhibits significant variability in the emitted flux from the accretion flow. The advent of adaptive optics and optical interferometry, particularly with the four large telescopes of the VLTI and the GRAVITY instrument, have revealed an orbital motion of the origin of three flares observed in 2018.Numerous models have been proposed to explain the flares of Sgr A*, but the observation of orbital motion has strongly constrained these models. Among them, the analytical hot spot model is widely used with varying degrees of complexity and assumptions. In parallel with the development of analytical models, numerous simulations of accretion around black holes have been studied, with a particular focus on the phenomenon of magnetic reconnection, which appears as a plausible scenario to explain the origin of the flares of Sgr A*.In this thesis, we study different models for the flares of Sgr A* using the ray-tracing code Gyoto, ranging from an analytical hot spot model with intrinsic variability to a semi-analytical model based on magnetic reconnection. The first model is very useful for understanding the effects of Relativity (Special and General) on observables (astrometry and light curves), as well as the influence of intrinsic variability on them. The second model is motivated by a specific physical phenomenon, magnetic reconnection, and is constructed based on the results of numerical simulations. In this model, the azimuthal velocity is free to be super-Keplerian, due to the dragging of the reconnection site by the magnetic field lines. This property constitutes an observational constraint of the 2018 flares observed by GRAVITY that previous models failed to explain. Additionally, we also study the impact of modeling the quiescent state combined with the flares on the observables. Its contribution in astrometry calculations results in a shift between the position of the black hole and the center of the apparent orbit, which is another conclusion from the observations of the 2018 flares.In addition to astrometry and light curves, GRAVITY has measured the polarization of the 2018 flares. The Gyoto ray-tracing code is now capable of calculating the polarization of images. The new version of the code has been validated by comparing the results with another ray-tracing code, ipole.The model based on magnetic reconnection shows very promising results and can be further improved to account for polarization, as well as the multi-wavelength properties of the flares of Sgr A*

Kersalé, Evy. "Etude analytique et numérique du développement d'instabilités MHD dans des structures d'accrétion-éjection magnétisées". Phd thesis, Université Joseph Fourier (Grenoble), 2000. http://tel.archives-ouvertes.fr/tel-00724447.

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La première partie de ce travail se propose de définir une nouvelle version du formalisme d'étude des instabilités MHD de pression dans les structures d'accrétion-éjection magnétisées. Ces processus se produisent dans des plasmas confinés magnétiquement et sont très contraignants dans le domaine de la fusion thermonucléaire mais leur influence est peu étudiée dans des contextes astrophysiques. Dans un cadre d'approximation éliminant les ondes magnétosoniques rapides nous avons développé un système d'équations général permettant de s'intéresser à la fois aux modes instables d'interchange et aux modes de ballooning. L'application de ce système à un jet cylindrique en rotation solide nous montre que le cisaillement magnétique conduit à la déstabilisation des parties internes de ces structures. En outre, tout en clarifiant cette problématique dans une certaine mesure, nous retrouvons que ces flots sont génériquement instables vis-à-vis des modes d'interchange. Par ailleurs, nous avons étudié les méthodes numériques de résolution des équations aux dérivées partielles et plus particulièrement celles de la MHD. A partir d'un algorithme d'intégration élémentaire, nous avons pu évaluer les effets de géométrie, de conditions aux limites et de dissipation artificielle sur le calcul numérique, à travers une série de tests classiques. L'étude de la production de rayons cosmiques de très haute énergie dans les gamma-ray bursts constitue la dernière partie du travail effectué. Dans ces objets, des processus de Fermi accélèrent des particules jusqu'à des énergies de 10 21 eV, lors du croisement de perturbations d'Alfvén relativistes. Une interaction de type faisceau-plasma, entre une coquille de plasma en mouvement relativiste et les baryons qui la traversent, génère ces fronts alfvéniques et un mécanisme de rétrodiffusion redistribue l'énergie disponible entre des perturbations alfvéniques progressive, régressive et des perturbations magnétosoniques.

Tomei, Niccolò. "GRMHD simulations of thick accretion disks in the Event Horizon Telescope era: the role of the mean-field dynamo mechanism". Doctoral thesis, 2022. http://hdl.handle.net/2158/1264722.

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The baryonic matter of the Universe is found almost entirely in the form of plasma, or ionized gas where the moving charges interact with each other through the self-generated currents and the related magnetic fields within a highly material conductive. The energy contained in the plasma in terms of the magnetic field is usually comparable to the kinetic and/or thermal energy of the plasma itself, and many violent observed phenomena are attributable to the sudden release of magnetic energy (e.g. flares and coronal mass ejection in the Sun, gamma ray bursts, gamma flare in magnetars, relativistic jets in active galactic nuclei). Since the Universe may not have formed magnetized, a natural question one needs to answer is what processes can lead to a weak magnetic field, from zero initial fields. Battery-like mechanisms are needed to create primordial extra- galactic fields, which may be amplified to higher values by plasma advection, rotation and collapse to values appropriate for stellar magnetism, up to B ∼ 1012 G, the field of a standard neutron star, a value required to power the surrounding young supernova remnant. Most of these batteries lead to field strengths much weaker than the observed field. So some way of amplifying the field is required. Magnetic fields of small scale and large scale components are observed in various astrophysical settings. This thesis work is focused on the investigation of the amplification mechanisms of magnetic fields in system of gas rotating around a supermassive black hole, following the renewed interest given by the recent imaging of the M87 black hole at millimetre wavelengths by the Event Horizon Telescope (EHT). In most cases an amplification of the magnetic fields may occur also by instabilities capable of converting kinetic energy into magnetic energy. A very efficient and ideal process is the magneto-rotational instability (MRI) that provided a local mechanism, effcient for a wide range of magnetic field strength, which leads to a growth on dynamical time-scales of linear perturbations and naturally develops MHD turbulence. The only necessary condition for its onset is the presence of a differentially rotating fluid threaded by a weak magnetic field. However, the amplification of the magnetic field is a non-ideal process due to the non-linear coupling of small-scale velocity and magnetic field fluctuations, possibly caused by the MRI. The result of this correlation leads to the creation of an electromotive force capable of amplifying magnetic fields. This process is known as mean-field dynamo and has been applied to a large number of astrophysical contexts. Currently, GRMHD simulations of MRI-induced accretion on to rotating black holes are being receiving considerable attention due to the success of the EHT collaboration, capable of imaging the emission and the shadow around the event horizon of a black hole for the very first time. The aim of this thesis work is to provide an alternative numerical accretion modelling to the ideal one in which the initial magnetic field has a well-defined poloidal structure and an intensity not exactly negligible. The mean-field dynamo allows us to investigate the possibility of producing poloidal field necessary for the development of MRI and the launch of jets even starting from the most unfavorable condition, that is an initial toroidal field with extremely lower magnetization than those used in ideal GRMHD simulations. In this work we have investigated, for the first time by means of non-ideal axisymmetric GRMHD simulations, the mean-field dynamo process operating in thick accretion disks around black holes, in the fully non-linear regime. Combined with the differential rotation of the disk, the dynamo process is able to produce an exponential growth of any initial seed magnetic field up to the values required to explain the observations,when the instability tends to saturate even in the absence of artificial quenching effects. Before reaching the final saturation stage we observe a secondary regime of exponential growing, where the magnetic field increases more slowly due to accretion, which is modifying the underlying equilibrium. In the stationary state characterized by the saturation of the magnetic field growth, the dynamo is able to remove the angular momentum and trigger the accretion. Finally, we show that it is possible to reproduce the main diagnostics present in the literature by starting from very unfavorable initial configurations, such as a purely toroidal magnetic field with negligible magnetization. In parallel, we present the contribution to the code Comparison Project that aims to compare ideal GRMHD solutions for the evolution of a magnetized accretion flow in two distinct regimes where turbulence is promoted by the magnetorotational instability.

Libri sul tema "Accretion phenomena":

Beskin, Vassily, Gilles Henri, François Menard, Guy Pelletier e Jean Dalibard, a cura di. Accretion discs, jets and high energy phenomena in astrophysics. Berlin, Heidelberg: Springer Berlin Heidelberg, 2003. http://dx.doi.org/10.1007/b80353.

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IAU Colloquium (163rd 1996 Port Douglas, Qld.). Accretion phenomena and related outflows: IAU Colloquium 163 : colloquium held in Port Douglas, Queensland, Australia, 15-19 July 1996. San Francisco, Calif: Astronomical Society of the Pacific, 1997.

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Ecole d'été de physique théorique (Les Houches, Haute-Savoie, France) (78th 2002). Accretion discs, jets, and high energy phenomena in astrophysics =: Disques d'accrétion, jets et phénomènes de haute énergie en astrophysique : Ecole d'été de physique des Houches, Session LXXVIII, 29 July-23 August 2002 : Nato Advanced Study Institute, Euro Summer School, Ecole thématique du CNRS. Les Ulis: EDP Sciences, 2003.

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(Editor), Vassily Beskin, Gilles Henri (Editor), Francois Menard (Editor), Guy Pelletier (Editor) e Jean Dalibard (Editor), a cura di. Accretion Disks, Jets and High-Energy Phenomena in Astrophysics (Les Houches - Ecole d'Ete de Physique Theorique). Springer, 2004.

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Pelletier, Guy, Jean Dalibard, Gilles Henri, Vassily Beskin e Francois Menard. Accretion Disks, Jets and High-Energy Phenomena in Astrophysics: Les Houches Session LXXVIII, July 29 - August 23, 2002. Springer, 2010.

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(Editor), Heon-Young Chang, Chang-Hwan Lee (Editor), Mannque Rho (Editor) e Insu Yi (Editor), a cura di. Explosive Phenomena in Astrophysical Compact Objects: First KIAS Astrophysics Workshop, Seoul, Korea, 24-27 May 2000 (AIP Conference Proceedings / Astronomy and Astrophysics). American Institute of Physics, 2001.

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Wickramasinghe, D. T. Accretion Phenomena and Related Outflows: Iau Colloquium 163: Colloquium Held in Port Douglas, Queensland, Australia, 15-19 July 1996 (Astronomical Society of the Pacific Conference Series). Astronomical Society of the Pacific, 1997.

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Tombesi, Francesco. X-ray probe of ultra-fast outflows in active galactic nuclei: An X-ray study of extreme ejection phenomena near accreting supermassive black holes in Seyfert and radio galaxies. LAP Lambert Academic Publishing, 2012.

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Capitoli di libri sul tema "Accretion phenomena":

Pringle, J. E. "Accretion Disc Phenomena". In Reviews in Modern Astronomy, 97–104. Berlin, Heidelberg: Springer Berlin Heidelberg, 1992. http://dx.doi.org/10.1007/978-3-642-77543-7_7.

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Mineshige, S. "Accretion Disk Instabilities". In Nonlinear Phenomena in Stellar Variability, 83–103. Dordrecht: Springer Netherlands, 1993. http://dx.doi.org/10.1007/978-94-011-1062-4_8.

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Kenyon, Scott J. "Accretion Disks and Eruptive Phenomena". In The Origin of Stars and Planetary Systems, 613–42. Dordrecht: Springer Netherlands, 1999. http://dx.doi.org/10.1007/978-94-011-4509-1_18.

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Rebetzky, A., H. Herold, U. Kraus, H. P. Nollert e H. Ruder. "Accretion Phenomena at Neutron Stars". In Reviews in Modern Astronomy, 74–85. Berlin, Heidelberg: Springer Berlin Heidelberg, 1990. http://dx.doi.org/10.1007/978-3-642-76238-3_7.

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Schulz, Norbert S. "Accretion Phenomena and Magnetic Activity in YSOs". In The Formation and Early Evolution of Stars, 183–216. Berlin, Heidelberg: Springer Berlin Heidelberg, 2012. http://dx.doi.org/10.1007/978-3-642-23926-7_8.

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Okuda, T., e S. Mineshige. "Pulsational Instability of Accretion Disks Around Compact Objects". In Nonlinear Phenomena in Stellar Variability, 361–64. Dordrecht: Springer Netherlands, 1993. http://dx.doi.org/10.1007/978-94-011-1062-4_59.

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Heyvaerts, Jean. "Course 1: Accretion and Ejection-Related MHD". In Accretion discs, jets and high energy phenomena in astrophysics, 3–82. Berlin, Heidelberg: Springer Berlin Heidelberg, 2004. http://dx.doi.org/10.1007/978-3-540-39932-2_1.

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Czerny, Bozena. "Course 9: Accretion around Active Galactic Nuclei". In Accretion discs, jets and high energy phenomena in astrophysics, 461–90. Berlin, Heidelberg: Springer Berlin Heidelberg, 2004. http://dx.doi.org/10.1007/978-3-540-39932-2_9.

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Honma, F., R. Matsumoto e S. Kato. "Numerical Simulations of Pulsationally Unstable Accretion Disks Around Supermassive Black Holes". In Nonlinear Phenomena in Stellar Variability, 365–67. Dordrecht: Springer Netherlands, 1993. http://dx.doi.org/10.1007/978-94-011-1062-4_60.

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Kudoh, T., e K. Shibata. "Magnetically Driven Jets From Accretion Disks: Nonsteady And Steady Solutions". In Magnetodynamic Phenomena in the Solar Atmosphere, 511–12. Dordrecht: Springer Netherlands, 1996. http://dx.doi.org/10.1007/978-94-009-0315-9_119.

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Atti di convegni sul tema "Accretion phenomena":

Vanbeveren, Dany. "Binaries, cluster dynamics and population studies of stars and stellar phenomena". In INTERACTING BINARIES: Accretion, Evolution, and Outcomes. AIP, 2005. http://dx.doi.org/10.1063/1.2130266.

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Meintjes, Pieter. "Magnetically driven transient phenomena in accretion driven systems: New breakthroughs with meerKAT and CTA?" In Accretion Processes in Cosmic Sources. Trieste, Italy: Sissa Medialab, 2018. http://dx.doi.org/10.22323/1.288.0039.

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Hayashi, Ryosuke, e Makoto Yamamoto. "Numerical Simulation on Ice Accretion Phenomena in Rotor-Stator Interaction Field". In ASME Turbo Expo 2013: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2013. http://dx.doi.org/10.1115/gt2013-95448.

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Abstract (sommario):

Ice accretion is a phenomenon where super-cooled water droplets impinge and accrete on a body. On a jet engine, ice accretion disturbs the inlet flow, and separated ice pieces can damage to the turbine component, which leads to the severe performance degradation. We simulated icing phenomena in a fan rotor-stator interaction field [1]. This study mainly focused on the icing property at the rotor blade, because the maximum ice thickness of the rotor blade is about seven times larger than that of the stator vane. Moreover, in the study, the effect of the tip clearance was neglected because of the computational simplicity. Therefore, in the present study, we simulate icing phenomena in a rotor-stator interaction field, considering the tip clearance, to clarify the effect of the tip clearance on the ice accretion of the stator vane. The numerical results indicate the tip leakage vortex affect the icing phenomenon at the stator vane.

Chattopadhyay, Indranil, Mukesh Vyas e Kuldeep Singh. "Relativistic Jets From Black Hole Accretion Disc". In High Energy Phenomena in Relativistic Outflows VII. Trieste, Italy: Sissa Medialab, 2020. http://dx.doi.org/10.22323/1.354.0007.

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Meintjes, Pieter, e No Co-authors. "Magnetic reconnection and transient phenomena in accretion driven systems". In 4th Annual Conference on High Energy Astrophysics in Southern Africa. Trieste, Italy: Sissa Medialab, 2017. http://dx.doi.org/10.22323/1.275.0032.

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Boula, Stella, Demosthenes Kazanas, e Apostolos Mastichiadis. "Mhd Accretion Disk Winds And The Blazar Sequence". In High Energy Phenomena in Relativistic Outflows VII. Trieste, Italy: Sissa Medialab, 2020. http://dx.doi.org/10.22323/1.354.0009.

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Tezok, F., M. Brahimi e I. Paraschivoiu. "Investigation of the physical processes underlying the ice accretion phenomena". In 36th AIAA Aerospace Sciences Meeting and Exhibit. Reston, Virigina: American Institute of Aeronautics and Astronautics, 1998. http://dx.doi.org/10.2514/6.1998-484.

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Furuta, Koharu, e Makoto Yamamoto. "Numerical Simulation on Ice Growth in High-Temperature Environment". In ASME Turbo Expo 2014: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/gt2014-25847.

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Abstract (sommario):

This paper discusses a fundamental study on icing phenomena in a high-temperature environment. Generally, ice accretion is a phenomenon to form ice layer on a body due to impingement of super-cooled water droplets. In recent years, it is known that ice accretion occurs in the engine core such as the low pressure compressor and the first stage of the high pressure compressor, where the temperature is about 30 degree C. The ice accretion in the engine core is called as “ice crystal accretion”. Some scenarios are given for the ice crystal accretion, but the mechanism has not been sufficiently clarified yet. Moreover, the current icing model is not available in the environment where the temperature is above the freezing point. In this paper, we develop a new icing code which is applicable to a warm environment. The new icing model consists of four iterative computations for turbulent flow, droplet/ice trajectory, thermodynamics of icing, and heat conduction within a wall. First, we validate our new icing model with a flat plate instead of a compressor stator blade as the fundamental study of ice crystal accretion. Then, we simulate ice accretion on a two-dimensional compressor stator blade in a high-temperature environment, in order to clarify the ice-crystal physics.

Fochi Wang, Yuzhen Lv, Qing Zhang, Zhou You e Chengrong Li. "Ice accretion on different aluminum cable steel reinforced". In 2010 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP 2010). IEEE, 2010. http://dx.doi.org/10.1109/ceidp.2010.5723984.

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Park, Myeong-Gu. "Interaction of radiation with matter in accretion flow". In The first KIAS astrophysics workshop: Explosive phenomena in astrophysical compact objects. AIP, 2001. http://dx.doi.org/10.1063/1.1368268.

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Rapporti di organizzazioni sul tema "Accretion phenomena":

Mayas, Magda. Creating with timbre. Norges Musikkhøgskole, agosto 2018. http://dx.doi.org/10.22501/nmh-ar.686088.

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Unfolding processes of timbre and memory in improvisational piano performance This exposition is an introduction to my research and practice as a pianist, in which I unfold processes of timbre and memory in improvised music from a performer’s perspective. Timbre is often understood as a purely sonic perceptual phenomenon. However, this is not in accordance with a site-specific improvisational practice with changing spatial circumstances impacting the listening experience, nor does it take into account the agency of the instrument and objects used or the performer’s movements and gestures. In my practice, I have found a concept as part of the creating process in improvised music which has compelling potential: Timbre orchestration. My research takes the many and complex aspects of a performance environment into account and offers an extended understanding of timbre, which embraces spatial, material and bodily aspects of sound in improvised music performance. The investigative projects described in this exposition offer a methodology to explore timbral improvisational processes integrated into my practice, which is further extended through collaborations with sound engineers, an instrument builder and a choreographer: -experiments in amplification and recording, resulting in Memory piece, a series of works for amplified piano and multichannel playback - Piano mapping, a performance approach, with a custom-built device for live spatialization as means to expand and deepen spatio-timbral relationships; - Accretion, a project with choreographer Toby Kassell for three grand pianos and a pianist, where gestural approaches are used to activate and compose timbre in space. Together, the projects explore memory as a structural, reflective and performative tool and the creation of performing and listening modes as integrated parts of timbre orchestration. Orchestration and choreography of timbre turn into an open and hybrid compositional approach, which can be applied to various contexts, engaging with dynamic relationships and re-configuring them.