CosmoStat Calendar

Nicolas SCEPI (CU Boulder)

Titre : Formation, Evolution and Radiation of Magnetized Accretion Disks around Black Holes

Résumé : The impact of magnetic fields on the evolution and on the observational signatures of accretion disks is very uncertain. This uncertainty is mainly due to a lack of observational constraints on the magnetic field geometry or strength in accretion disks. However, even from a theoretical point of view our understanding of magnetized disks remains relatively poor. Indeed, analytic models of magnetized disks often need inputs from numerical simulations and numerical simulations of magnetized disks are difficult to perform and/or interpret. Because of this lack of magnetized disk models, standard disk models often reduce the magnetic field to a source of turbulence; turbulence through which the accretion can happen. While this simplification may hold for weakly magnetized disks, a large number of numerical simulations have shown that the role of a strong magnetic field goes far beyond producing turbulence. In particular, a strong magnetic field can produce powerful outflows, induce accretion through vertically elevated layers or non-axisymmetric structures, modify the time scales of accretion, enhance dissipation of gravitational energy in the disk and accelerate particles to very high energies. All of these effects dramatically affect the evolution and observational signature of accretion disks and open up new and exciting avenues to resolve outstanding problems of the standard accretion disk theory. In this talk, I will present an overview of my recent results on how strongly magnetized disks form, evolve and radiate. I will show in particular how strongly magnetized disks could explain events of very strong variability in AGNs, the flaring behavior of the Galactic center and the hardest emission in X-ray binaries.

Joint deconvolution and semi & unsupervised component separation, with application to radio-interferometry

Abstract:
With the development of continent-wide radio telescopes, the analysis of multi-frequency radio-interferometric data has become a significant challenge in signal processing and astrophysics. In this respect, component separation methods are an adequate tool as they allow the separation of multi-frequency data into elementary physical components.
However, standard component separation algorithms are not adapted to radio-interferometric data for several reasons: (i) they are constituted of incomplete measurements in the Fourier domain, also known as the visibility domain, and are potentially further deteriorated by instrumental or non-coplanar effects, and (ii) the sought-after signals can be severely drowned in noise or other emissions.
Consequently, accounting for the telescope instrumental response requires developing dedicated algorithms to solve a joint deconvolution and separation problem. Furthermore, the recovery of weak signals calls for the design of accurate approaches that make use of physical models known a priori; this motivates the development of semi-supervised separation methods that integrate machine learning techniques.

Preparing for postdoc applications

Elisabeth KOHLER (CNRS)

Agir pour l’égalité professionnelle entre les femmes et les hommes

Résumé : De la scolarité aux postes à responsabilité de nombreux stéréotypes persistent qui souvent entravent la carrière scientifique des femmes. L’objectif de ce séminaire est d’identifier ces biais et de présenter des exemples de mesures concrètes pour y remédier et mettre en œuvre des politiques d’égalité professionnelle entre les femmes et les hommes. Différents sujets seront abordés, tels que les critères d’évaluation, les actions de communication, le congé maternité et la parentalité, la lutte et la prévention contre les violences sexistes et sexuelles.