1,721,062 research outputs found
Probing the presence of a single or binary black hole in the globular cluster NGC 6752 with pulsar dynamics
No full textGRMHD simulations of accretion flows onto massive binary black hole mergers embedded in a thin slab of gas
Full text linkWe present general relativistic magnetohydrodynamic simulations of merging
equal-mass, spinning black holes embedded in an equatorial thin slab of
magnetized gas. We evolve black holes either non-spinning, with spins aligned
to the orbital angular momentum, and with misaligned spins. The rest-mass
density of the gas slab follows a Gaussian profile symmetric relative to the
equatorial plane and it is initially either stationary or with Keplerian
rotational support. As part of our diagnostics, we follow the accretion of
matter onto the black hole horizons and the Poynting luminosity. Throughout the
inspiral phase, the configurations with non-zero spins display modulations in
the mass accretion rate that are proportional to the orbital frequency and its
multiples. Our frequency analysis suggests that these modulations are
influenced by the initial geometry and angular momentum of the gas
distribution. In contrast to binary models evolved in the gas cloud scenario,
we do not observe a significant increase in the mass accretion rate after the
merger in any of our simulations. This observation brings attention to a
potential link between the electromagnetic signatures of massive binary black
hole mergers and the geometrical distribution of the surrounding gas. It also
suggests the possibility of not detecting a peak luminosity at the time of
merger in future electromagnetic observations.Comment: 16 pages, 8 figures, published in Phys. Rev.
Overlapping inflow events as catalysts for supermassive black hole growth
Full text linkOne of the greatest issues in modelling black hole fuelling is our lack of understanding of the processes by which gas loses angular momentum and falls from galactic scales down to the nuclear region where an accretion disc forms, subsequently guiding the inflow of gas down to the black hole horizon. It is feared that gas at larger scales might still retain enough angular momentum and settle into a larger scale disc with very low or no inflow to form or replenish the inner accretion disc (on similar to 0.01 pc scales). In this paper we report on hydrodynamical simulations of rotating infalling gas shells impacting at different angles on to a pre-existing, primitive large-scale (similar to 10 pc) disc around a supermassive black hole. The aim is to explore how the interaction between the shell and the disc redistributes the angular momentum on scales close to the black hole's sphere of influence. Angular momentum redistribution via hydrodynamical shocks leads to inflows of gas across the inner boundary, enhancing the inflow rate by more than 2-3 orders of magnitude. In all cases, the gas inflow rate across the inner parsec is higher than in the absence of the interaction, and the orientation of the angular momentum of the flow in the region changes with time due to gas mixing. Warped discs or nested misaligned rings form depending on the angular momentum content of the infalling shell relative to the disc. In the cases in which the shell falls in near counter-rotation, part of the resulting flows settle into an inner dense disc which becomes more susceptible to mass transfer
Overlapping inflows as catalysts of AGN activity - II. Relative importance of turbulence and inflow-disc interaction
Full text linkThe main challenge for understanding the fuelling of supermassive black holes in active galactic nuclei is not to account for the source of fuel, but rather to explain its delivery from the boundaries of the black hole sphere of influence (10-100 pc) down to sub-parsec scales. In this work, we report on a series of numerical experiments aimed at exploring in further depth our model of 'overlapping inflow events' as catalysts for rapid accretion, seeding a turbulent field in the infalling gas. We initially set a gaseous shell in non-equilibrium rotation around a supermassive black hole. After infall, the shell stalls in a disc-like structure. A second shell is then set in either corotation or counterrotation with respect to the first and is let to impinge on the previously formed disc. We find that combined turbulence and overlap significantly enhance accretion in counterrotating inflows, while turbulence dominates for corotating inflows. The leftovers of overlapping inflows are warped nuclear discs, whose morphology depend on the relative orientation and angular momentum of the disc and the shell. Overlapping inflows leave observational signatures in the gas rotation curves
Gravity: where do we stand ?
No full textThis book presents an overview of the current understanding of gravitation, with a focus on the current efforts to test its theories, especially general relativity. It shows how the quest for a deeper understanding, which would possibly incorporate gravity in the quantum realm, is more than ever an open field. The majority of the contributions deals with the manifold facets of “experimental gravitation”, but the book goes beyond this and covers a broad range of subjects from the foundations of gravitational theories to astrophysics and cosmology. The book is divided into three parts. The first part deals with foundations and Solar System tests. An introductory pedagogical chapter reviews first Newtonian gravitational theory, special relativity, the equivalence principle and the basics of general relativity. Then it focuses on approximation methods, mainly the post-Newtonian formalism and the relaxed Einstein equations, with a discussion on how they are used in treating experimental tests and in the problem of generation and detection of gravitational waves. Following this is a set of chapters describing the most recent experiments, techniques and observations to test the theories of gravitation in the laboratory, around the Earth and in the Solar System. The second part is dedicated to astrophysical topics deeply linked with the study of gravitation, namely binary pulsars and the perspective of direct detection of gravitational waves. These cases are paradigmatic in that the gravitational signals act at the same time as messengers helping us to understand the properties of important and wide classes of astrophysical objects. The third part explores the many open issues in the current knowledge of gravitation machinery, especially related to astrophysical and cosmological problems, and the way possible solutions to them impact the quest for a quantum theory of gravitation and unified theory. Included is a selection of the many possible paths, giving a hint to the subtleties one is called upon. Whenever possible, a close link to observational constraints and possible experimental tests is provided. In selecting the topics of the various contributions, particular care has been devoted to ensure their fit in a coherent representation of our understanding of gravitational phenomena. The book is aimed at graduate level students and will form a valuable reference for those working in the field
Massive black hole and gas dynamics in mergers of galaxy nuclei – II. Black hole sinking in star-forming nuclear discs
No full textMergers of gas-rich galaxies are key events in the hierarchical built-up of cosmic structures, and can lead to the formation of massive black hole binaries. By means of high-resolution hydrodynamical simulations we consider the late stages of a gas-rich major merger, detailing the dynamics of two circumnuclear discs, and of the hosted massive black holes during their pairing phase. During the merger gas clumps with masses of a fraction of the black hole mass form because of fragmentation. Such high-density gas is very effective in forming stars, and the most massive clumps can substantially perturb the black hole orbits. After similar to 10 Myr from the start of the merger a gravitationally bound black hole binary forms at a separation of a few parsecs, and soon after, the separation falls below our resolution limit of 0.39 pc. At the time of binary formation the original discs are almost completely disrupted because of SNa feedback, while on pc scales the residual gas settles in a circumbinary disc with mass similar to 10(5) M-circle dot. We also test that binary dynamics is robust against the details of the SNa feedback employed in the simulations, while gas dynamics is not. We finally highlight the importance of the SNa time-scale on our result
Constraining the high-redshift formation of black hole seeds in nuclear star clusters with gas inflows
No full textIn this paper, we explore a possible route of black hole seed formation that appeals to a model by Davies, Miller & Bellovary who considered the case of the dynamical collapse of a dense cluster of stellar black holes subjected to an inflow of gas. Here, we explore this case in a broad cosmological context. The working hypotheses are that (i) nuclear star clusters form at high redshifts in pre-galactic discs hosted in dark matter haloes, providing a suitable environment for the formation of stellar black holes in their cores, (ii) major central inflows of gas occur on to these clusters due to instabilities seeded in the growing discs and/or to mergers with other gas-rich haloes and (iii) following the inflow, stellar black holes in the core avoid ejection due to the steepening to the potential well, leading to core collapse and the formation of a massive seed of ≲1000M⊙. We simulate a cosmological box tracing the build-up of the dark matter haloes and their embedded baryons, and explore cluster evolution with a semi-analytical model.We showthat this route is feasible, peaks at redshifts z ≲10 and occurs in concomitance with the formation of seeds from other channels. The channel is competitive relative to others, and is independent of the metal content of the parent cluster. This mechanism of gas-driven core collapse requires inflows with masses at least 10 times larger than the mass of the parent star cluster, occurring on time-scales shorter than the evaporation/ejection time of the stellar black holes from the core. In this respect, the results provide upper limit to the frequency of this process.© 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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