A gravitational energy-momentum and the thermodynamic description of gravity

Giovanni Acquaviva

05.11.2018 14:00

A proposal for the gravitational energy-momentum tensor, known in the literature as the square root of Bel-Robinson tensor (SQBR), is analyzed in detail. Being constructed exclusively from the Weyl part of the Riemann tensor, such tensor encapsulates the geometric properties of free gravitational fields in terms of optical scalars of null congruences: making use of the general decomposition of any energy–momentum tensor, we explore the thermodynamic interpretation of such geometric quantities. While the matter energy-momentum is identically conserved due to Einstein’s field equations, the SQBR is not necessarily conserved and dissipative terms could arise in its vacuum continuity equation. We discuss the possible physical interpretations of such mathematical properties.

Manifold Dynamics in the Solar System and in Galaxies

Christos Efthymiopoulos

19.11.2018 14:00

Manifold dynamics started as a concept allowing to unveil how the dynamical behavior of the trajectories close to unstable equilibrium points or unstable periodic orbits can be exploited in trajectory design in astrodynamics (e.g. the design of spacecraft orbits in the framework of the restricted three body problem). Nowadays, manifold dynamics serves also to interpret a variety of natural phenomena in gravitational systems, from planetesimal accretion, and planet or asteroid migration within our Solar System, to the spiral arms and the tidal streams and tails in galaxies. The seminar will present the basics of manifold dynamics from the viewpoint of dynamical astronomy. Then, we will refer to results obtained so far from manifold computations (analytical and numerical) in conjunction with N-body simulations.

TBA

Vyacheslav Lukin

19.11.2018 15:30

Prospecting the wind accretion in the sgHMXB IGR J16320-4751

Federico Garcia

26.11.2018 14:00

IGR J16320-4751 is a High Mass X-ray Binary (HMXB) formed by a neutron star (NS) spinning with a period of ~1300 s which orbits around a supergiant companion in about ~9 days accreting material from its powerful wind. It is a highly-obscured system characterized by a typical intrinsic absorption column density of 10^{23} cm^{-2}, an order of magnitude higher than the Galactic column density along the line of sight. In this talk, I will present our recent results arising from an orbital monitoring of this source performed with XMM-Newton and Swift/BAT X-ray satellites. By means of the hard X-ray data from Swift/BAT we re-calculated the ephemeris of the source, updating its reported orbital period. Based on the soft X-ray light curves of nine XMM-Newton observations performed at different orbital phases we generated time-resolved spectra that we fitted using an intrinsically-absorbed comptonization model for the continuum emission and three narrow gaussians Fe lines. We found that the spectral evolution is mainly governed by variations in the absorption column which correlate with the intensity of the Fe lines. Moreover, we also found that the absorption column peaks before the maximum of the Swift/BAT light curve is reached. Using the information gathered from this X-ray monitoring we propose a simple model assuming a typical wind profile for the supergiant companion, which enables us are to simultaneously fit the evolution of the hard X-ray light curve and the orbital evolution of the absorption column and to constrain the eccentricity and inclination of the binary system.

Mathematical aspects of the black holes Meissner effect

Martin Scholtz

03.12.2018 14:00

Meissner effect is a well-known property describing the expulsion of external electromagnetic fields from the horizon of extremal axially symmetric black holes. This phenomenon has been "observed theoretically" first for test fields in the Kerr spacetime, but today many examples of exact solutions exhibiting the Meissner effect exist, e.g. black hole immersed in Melvin magnetic universe. Nevertheless, whether the Meissner effect is a generic property of black holes remained unclear. We provide a general proof showing that Meissner effect holds for any axially symmetric horizon representing a black hole in equilibrium. In this talk we present the mathematical background of our proof, namely the Ashtekar's formalism of isolated horizons and show the intimate relation between the presence of the symmetries and the Meissner effect. Then we show that the presence of the Meissner effect is independent of the deformations of the horizon induced by an external matter. We demonstrate the transition from under-extremal to extremal case on a distorted Kerr black hole for which a new coordinate system and tetrad had to be developed. We briefly discuss possible implications of the Meissner effect on the efficiency of the Blandford-Znajek process. We extend our original proof also to the case of electrically charged black holes which requires a discussion on the actual meaning of the Meissner effect in such a case, since the non-linearity of general relativity makes it difficult to disentangle contributions of the external fields and the field induced by the black hole itself. Finally, we relate the Meissner effect to the uniqueness of extremal horizons and generalize previous uniqueness theorem by Lewandowski and Pawlowski to the case of black holes admitting conical singularities, showing that the Meissner effect holds also for C-metric black holes with possibly non-vanishing NUT parameter.

Slowly rotating Schwarzschild star and the gravastar limit

Camilo Posada

10.12.2018 14:00

The Schwarzschild interior solution, or Schwarzschild star, which describes a spherically symmetric homogeneous mass with a constant energy density, shows a divergence in pressure when the radius of the star R = (9/4)M. Recently, Mazur and Mottola showed that this divergence is integrable, inducing non-isotropic transverse stresses on a surface of some radius R_0. When this radius approaches the Schwarzschild radius, the interior solution becomes one of negative pressure evoking a de Sitter spacetime. This gravitational condensate star, or gravastar, is an alternative solution to the idea of a black hole as the final state of gravitational collapse. Using Hartles model to calculate equilibrium configurations of slowly rotating masses, we report results of surface and integral properties for a slowly rotating Schwarzschild star in the long-ignored region R_S < R < (9/4)M. We found that in the gravastar limit, the angular velocity of the fluid relative to the local inertial frame tends to zero, indicating rigid rotation. Remarkably, the moment of inertia and the mass quadrupole moment approach the corresponding values for the Kerr metric to second order in the angular velocity. These results provide a solution to the problem of the source of a slowly rotating Kerr black hole.