Search results for " black"
showing 10 items of 317 documents
SEM–EDS ANALYSIS AS A RAPID TOOL FOR DISTINGUISHING CAMPANIAN A WARE AND SICILIAN IMITATIONS
2013
The aim of this work is to examine whether it is possible to find chemical markers that allow a distinction to be made between the imported black glossed ‘Campanian A’ and the Sicilian imitation (end of fourth to first century BC) of these productions by carrying out quantitative chemical microanalysis of the slip using the SEM–EDS technique. The efficiency of the proposed analytical method has been tested on a set of ceramic samples corresponding to Sicilian black gloss imitations whose ceramic body has already been characterized petrographically by thin-section microscopy and chemically by XRF. The analytical data point to Na2O as a suitable chemical marker to distinguish between original…
Magnetic braking and damping of differential rotation in massive stars
2018
Fragmentation of highly differentially rotating massive stars that undergo collapse has been suggested as a possible channel for binary black hole formation. Such a scenario could explain the formation of the new population of massive black holes detected by the LIGO/VIRGO gravitational wave laser interferometers. We probe that scenario by performing general relativistic magnetohydrodynamic simulations of differentially rotating massive stars supported by thermal radiation pressure plus a gas pressure perturbation. The stars are initially threaded by a dynamically weak, poloidal magnetic field confined to the stellar interior. We find that magnetic braking and turbulent viscous damping via …
On the dynamics of tilted black hole-torus systems
2016
We present results from three-dimensional, numerical relativity simulations of a {\it tilted} black hole-thick accretion disc system. The simulations are analysed using tracer particles in the disc which are advected with the flow. Such tracers, which we employ in these new simulations for the first time, provide a powerful means to analyse in detail the complex dynamics of tilted black hole-torus systems. We show how its use helps to gain insight in the overall dynamics of the system, discussing the origin of the observed black hole precession and the development of a global non-axisymmetric $m=1$ mode in the disc. Our three-dimensional simulations show the presence of quasi-periodic oscil…
Non-linear evolutions of magnetized thick discs around black holes: dependence on the initial data
2020
We build equilibrium solutions of magnetised thick discs around a highly spinning Kerr black hole and evolve these initial data up to a final time of about 100 orbital periods. The numerical simulations reported in this paper solve the general relativistic magnetohydrodynamics equations using the BHAC code and are performed in axisymmetry. Our study assumes non-self-gravitating, polytropic, constant angular momentum discs endowed with a purely toroidal magnetic field. In order to build the initial data we consider three approaches, two of which incorporate the magnetic field in a self-consistent way and a third approach in which the magnetic field is included as a perturbation on to an othe…
Radio observations of active galactic nuclei with mm-VLBI
2017
Over the past few decades, our knowledge of jets produced by active galactic nuclei (AGN) has greatly progressed thanks to the development of very-long-baseline interferometry (VLBI). Nevertheless, the crucial mechanisms involved in the formation of the plasma flow, as well as those driving its exceptional radiative output up to TeV energies, remain to be clarified. Most likely, these physical processes take place at short separations from the supermassive black hole, on scales which are inaccessible to VLBI observations at centimeter wavelengths. Due to their high synchrotron opacity, the dense and highly magnetized regions in the vicinity of the central engine can only be penetrated when …
Propagation and stability of relativistic jets
2020
A simple look at the steady high-energy Universe reveals a clear correlation with outflows generated around compact objects (winds and jets). In the case of relativistic jets, they are thought to be produced as a consequence of the extraction of rotational energy from a Kerr black hole (Blandford-Znajek), or from the disc (Blandford-Payne). A fraction of the large energy budget provided by accretion and/or black hole rotational energy is invested into jet formation. After formation, the acceleration and collimation of these outflows allow them to propagate to large distances away from the compact object. The synchrotron cooling times demand that re-acceleration of particles takes place alon…
Gravitational Waves from the Papaloizou-Pringle Instability in Black-Hole-Torus Systems
2011
Black hole (BH)--torus systems are promising candidates for the central engine of gamma-ray bursts (GRBs), and also possible outcomes of the collapse of supermassive stars to supermassive black holes (SMBHs). By three-dimensional general relativistic numerical simulations, we show that an $m=1$ nonaxisymmetric instability grows for a wide range of self-gravitating tori orbiting BHs. The resulting nonaxisymmetric structure persists for a timescale much longer than the dynamical one, becoming a strong emitter of large amplitude, quasiperiodic gravitational waves. Our results indicate that both, the central engine of GRBs and newly formed SMBHs, can be strong gravitational wave sources observa…
Striped Blandford/Znajek jets from advection of small-scale magnetic field
2020
Black hole - accretion disc systems are the central engines of relativistic jets from stellar to galactic scales. We numerically quantify the unsteady outgoing Poynting flux through the horizon of a rapidly spinning black hole endowed with a rotating accretion disc. The disc supports small-scale, concentric, flux tubes with zero net magnetic flux. Our General Relativistic force-free electrodynamics simulations follow the accretion onto the black hole over several hundred dynamical timescales in 3D. For the case of counter-rotating accretion discs, the average process efficiency reaches up to $\left\langle\epsilon\right\rangle\approx 0.43$, compared to a stationary energy extraction by the B…
Protomagnetar and black hole formation in high-mass stars
2017
Using axisymmetric simulations coupling special relativistic MHD, an approximate post-Newtonian gravitational potential and two-moment neutrino transport, we show different paths for the formation of either protomagnetars or stellar mass black holes. The fraction of prototypical stellar cores which should result in collapsars depends on a combination of several factors, among which the structure of the progenitor star and the profile of specific angular momentum are probably the foremost. Along with the implosion of the stellar core, we also obtain supernova-like explosions driven by neutrino heating and hydrodynamic instabilities or by magneto-rotational effects in cores of high-mass stars…
Influence of pions and hyperons on stellar black hole formation
2013
We present numerical simulations of stellar core-collapse with spherically symmetric, general relativistic hydrodynamics up to black hole formation. Using the CoCoNuT code, with a newly developed grey leakage scheme for the neutrino treatment, we investigate the effects of including pions and \Lambda-hyperons into the equation of state at high densities and temperatures on the black hole formation process. Results show non-negligible differences between the models with reference equation of state without any additional particles and models with the extended ones. For the latter, the maximum masses supported by the proto-neutron star are smaller and the collapse to a black hole occurs earlie…