13,352 research outputs found
A magnetohydrodynamic model for multi-wavelength flares from Sagittarius~A (I): model and the near-infrared and X-ray flares
Flares from the supermassive black hole in our Galaxy, Sagittarius~A
(Sgr A), are routinely observed over the last decade or so. Despite
numerous observational and theoretical efforts, the nature of such flares still
remains poorly understood, although a few phenomenological scenarios have been
proposed. In this work, we develop the Yuan et al. (2009) scenario into a
magnetohydrodynamic (MHD) model for Sgr A flares. This model is
analogous with the theory of solar flares and coronal mass ejection in solar
physics. In the model, magnetic field loops emerge from the accretion flow onto
Sgr A and are twisted to form flux ropes because of shear and
turbulence. The magnetic energy is also accumulated in this process until a
threshold is reached. This then results in a catastrophic evolution of a flux
rope with the help of magnetic reconnection in the current sheet. In this
catastrophic process, the magnetic energy is partially converted into the
energy of non-thermal electrons. We have quantitatively calculated the
dynamical evolution of the height, size, and velocity of the flux rope, as well
as the magnetic field in the flare regions, and the energy distribution of
relativistic electrons in this process. We further calculate the synchrotron
radiation from these electrons and compare the obtained light curves with the
observed ones. We find that the model can reasonably explain the main
observations of near-infrared (NIR) and X-ray flares including their light
curves and spectra. It can also potentially explain the frequency-dependent
time delay seen in radio flare light curves.Comment: 17 pages, 13 figures, accepted by MNRA
Interactions between gaussian processes and bayesian estimation
Lâapprentissage (machine) de modĂšle et lâestimation dâĂ©tat sont cruciaux pour interprĂ©ter les phĂ©nomĂšnes sous-jacents Ă de nombreuses applications du monde rĂ©el. Toutefois, il est souvent difficile dâapprendre le modĂšle dâun systĂšme et de capturer les Ă©tats latents, efficacement et avec prĂ©cision, en raison du fait que la connaissance du monde est gĂ©nĂ©ralement incertaine. Au cours des derniĂšres annĂ©es, les approches dâestimation et de modĂ©lisation bayĂ©siennes ont Ă©tĂ© extensivement Ă©tudiĂ©es afin que lâincertain soit rĂ©duit Ă©lĂ©gamment et de maniĂšre flexible. Dans la pratique cependant, diffĂ©rentes limitations au niveau de la modĂ©lisation et de lâestimation bayĂ©siennes peuvent dĂ©tĂ©riorer le pouvoir dâinterprĂ©tation bayĂ©sienne. Ainsi, la performance de lâestimation est souvent limitĂ©e lorsque le modĂšle de systĂšme manque de souplesse ou/et est partiellement inconnu. De mĂȘme, la performance de la modĂ©lisation est souvent restreinte lorsque lâestimateur BayĂ©sien est inefficace. InspirĂ© par ces faits, nous proposons dâĂ©tudier dans cette thĂšse, les connections possibles entre modĂ©lisation bayĂ©sienne (via le processus gaussien) et lâestimation bayĂ©sienne (via le filtre de Kalman et les mĂ©thodes de Monte Carlo) et comment on pourrait amĂ©liorer lâune en utilisant lâautre. Ă cet effet, nous avons dâabord vu de plus prĂšs comment utiliser les processus gaussiens pour lâestimation bayĂ©sienne. Dans ce contexte, nous avons utilisĂ© le processus gaussien comme un prior non-paramĂ©trique des modĂšles et nous avons montrĂ© comment cela permettait dâamĂ©liorer lâefficacitĂ© et la prĂ©cision de lâestimation bayĂ©sienne. Ensuite, nous nous somme intĂ©ressĂ© au fait de savoir comment utiliser lâestimation bayĂ©sienne pour le processus gaussien. Dans ce cadre, nous avons utilisĂ© diffĂ©rentes estimations bayĂ©siennes comme le filtre de Kalman et les filtres particulaires en vue dâamĂ©liorer lâinfĂ©rence au niveau du processus gaussien. Ceci nous a aussi permis de capturer diffĂ©rentes propriĂ©tĂ©s au niveau des donnĂ©es dâentrĂ©e. Finalement, on sâest intĂ©ressĂ© aux interactions dynamiques entre estimation bayĂ©sienne et processus gaussien. On sâest en particulier penchĂ© sur comment lâestimation bayĂ©sienne et le processus gaussien peuvent âtravaillerâ de maniĂšre interactive et complĂ©mentaire de façon Ă amĂ©liorer Ă la fois le modĂšle et lâestimation. LâefficacitĂ© de nos approches, qui contribuent Ă la fois au processus gaussien et Ă lâestimation bayĂ©sienne, est montrĂ©e au travers dâune analyse mathĂ©matique rigoureuse et validĂ©e au moyen de diffĂ©rentes expĂ©rimentations reflĂ©tant des applications rĂ©elles.Model learning and state estimation are crucial to interpret the underlying phenomena in many real-world applications. However, it is often challenging to learn the system model and capture the latent states accurately and efficiently due to the fact that the knowledge of the world is highly uncertain. During the past years, Bayesian modeling and estimation approaches have been significantly investigated so that the uncertainty can be elegantly reduced in a flexible probabilistic manner. In practice, however, several drawbacks in both Bayesian modeling and estimation approaches deteriorate the power of Bayesian interpretation. On one hand, the estimation performance is often limited when the system model lacks in flexibility and/or is partially unknown. On the other hand, the modeling performance is often restricted when a Bayesian estimator is not efficient and/or accurate. Inspired by these facts, we propose Interactions Between Gaussian Processes and Bayesian Estimation where we investigate the novel connections between Bayesian model (Gaussian processes) and Bayesian estimator (Kalman filter and Monte Carlo methods) in different directions to address a number of potential difficulties in modeling and estimation tasks. Concretely, we first pay our attention to Gaussian Processes for Bayesian Estimation where a Gaussian process (GP) is used as an expressive nonparametric prior for system models to improve the accuracy and efficiency of Bayesian estimation. Then, we work on Bayesian Estimation for Gaussian Processes where a number of Bayesian estimation approaches, especially Kalman filter and particle filters, are used to speed up the inference efficiency of GP and also capture the distinct input-dependent data properties. Finally, we investigate Dynamical Interaction Between Gaussian Processes and Bayesian Estimation where GP modeling and Bayesian estimation work in a dynamically interactive manner so that GP learner and Bayesian estimator are positively complementary to improve the performance of both modeling and estimation. Through a number of mathematical analysis and experimental demonstrations, we show the effectiveness of our approaches which contribute to both GP and Bayesian estimation
(De-)activating the growth machine for redevelopment: the case of Liede urban village in Guangzhou
This research investigates the mechanism of urban village redevelopment in south China. Through a revised typology of place entrepreneurs based on the growth machine thesis and a case study of Liede village in central Guangzhou, it illustrates how land-based interests embedded in an imbalanced power relationship can (de-)activate urban village redevelopment. The study reveals that while urban villagers, as represented by the village collective, have entrenched interests in the redevelopment process, the city government â as monopolistic land manager and place entrepreneur â plays the deciding role in forging and halting a growth machine geared towards urban village redevelopment. Although developers are also part of the process, the (de-)activation of redevelopment growth machine/coalition in Guangzhou has largely been dominated by the city government. With a comparative view on the original growth machine model, it is hoped that this study would furnish both theoretical and practical thoughts for future research
Residue cross sections of Ti-induced fusion reactions based on the two-step model
Ti-induced fusion reactions to synthesize superheavy elements are
studied systematically with the two-step model developed recently, where fusion
process is divided into approaching phase and formation phase. Furthermore, the
residue cross sections for different neutron evaporation channels are evaluated
with the statistical evaporation model. In general, the calculated cross
sections are much smaller than that of Ca-induced fusion reactions, but
the results are within the detection capability of experimental facilities
nowadays. The maximum calculated residue cross section for producing superheavy
element is in the reaction Ti+Bk in channels with
pb at = 37.0 MeV.Comment: 6 pages, 7 figure
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