1,631 research outputs found
Self-similar accretion in thin disks around near-extremal black holes
Near-maximally spinning black holes display conformal symmetry in their
near-horizon region, which is therefore the locus of critical phenomena. In
this paper, we revisit the Novikov-Thorne accretion thin disk model and find a
new self-similar radiation-dominated solution in the extremely high spin
regime. Motivated by the self-consistency of the model, we require that matter
flows at the sound speed at the innermost stable circular orbit (ISCO). We
observe that, when the disk pressure is dominated by radiation at the ISCO,
which occurs for the best-fitting Novikov-Thorne model of GRS 1915+105, the
Shakura-Sunyaev viscosity parameter can be expressed in terms of the spin, mass
accretion rate and radiative efficiency. We quantitatively describe how the
exact thin disk solution approaches the self-similar solution in the vicinity
of the ISCO and for increasing spins.Comment: 13 pages, 6 figures; v2 matches published version in MNRAS; v3: typos
fixed, results unchange
Gravitational multipole moments from Noether charges
We define the mass and current multipole moments for an arbitrary theory of
gravity in terms of canonical Noether charges associated with specific residual
transformations in canonical harmonic gauge, which we call multipole
symmetries. We show that our definition exactly matches Thorne's mass and
current multipole moments in Einstein gravity, which are defined in terms of
metric components. For radiative configurations, the total multipole charges --
including the contributions from the source and the radiation -- are given by
surface charges at spatial infinity, while the source multipole moments are
naturally identified by surface integrals in the near-zone or, alternatively,
from a regularization of the Noether charges at null infinity. The conservation
of total multipole charges is used to derive the variation of source multipole
moments in the near-zone in terms of the flux of multipole charges at null
infinity.Comment: v1: 22 pages + 13 pages of appendices, 1 figure; v2: published
version in JHE
Productive Ambiguity in Mathematics
According to E. Grosholz, there is a phenomenon called `productive ambiguity' which
plays a very important role in mathematics, and the sciences, because it is instrumental to the resolution of many open questions.
The main task of this paper is that of assessing Grosholz's claim with regard to mathematics
Mass of Kerr-Newman Black Holes in an external magnetic field
The explicit solution for a Kerr-Newman black hole immersed in an external
magnetic field, sometimes called the Melvin-Kerr-Newman black hole, has been
derived by Ernst and Wild in 1976. In this paper, we clarify the first law and
Smarr formula for black holes in a magnetic field. We then define the unique
mass which is integrable and reduces to the Kerr-Newman mass in the absence of
magnetic field. This defines the thermodynamic potentials of the black hole.
Quite strikingly, the mass coincides with the standard Christodoulou-Ruffini
mass of a black hole as a function of the entropy, angular momentum and
electric charge.Comment: 21 pages; v2 matches published versio
A triple-GEM telescope for the TOTEM experiment
The TOTEM experiment at LHC has chosen the triple Gas Electron Multiplier
(GEM) technology for its T2 telescope which will provide charged track
reconstruction in the rapidity range 5.3<|eta|<6.5 and a fully inclusive
trigger for diffractive events. GEMs are gas-filled detectors that have the
advantageous decoupling of the charge amplification structure from the charge
collection and readout structure. Furthermore, they combine good spatial
resolution with very high rate capability and a good resistance to radiation.
Results from a detailed T2 GEM simulation and from laboratory tests on a final
design detector performed at CERN are presented.Comment: To appear in the proceedings of 10th Topical Seminar on Innovative
Particle and Radiation Detectors (IPRD06), Siena, Italy, October 1-5 200
Empowering Communication in Emergency Contexts: Reflections From the Italian Coronavirus Outbreak
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On Representing Concepts in High-dimensional Linear Spaces
Producing a mathematical model of concepts is a very important issue
in artificial intelligence, because if such a model were found this, besides being
a very interesting result in its own right, would also contribute to the emergence
of what we could call the \u2018mathematics of thought.\u2019 One of the most interesting
attempts made in this direction is P. Gardenfors\u2019 theory of conceptual spaces, a \ua8
theory which is mostly presented by its author in an informal way. The main aim
of the present article is contributing to Gardenfors\u2019 theory of conceptual spaces \ua8
by discussing some of the advantages which derive from the possibility of representing
concepts in high-dimensional linear spaces
Changes in behavioural response of Mediterranean Seabass (Dicenthratus labrax L.) under different feeding distributions
Captive-induced behavioural deviations may
involve many aspects of fish behaviour such as
swimming activity and enhancement of individual
aggressiveness. We studied seabass
(Dicentrarchus labrax) behaviour as a function
of manual and automatic feeding distribution
modes. Under manual mode, the food is distributed
over an extended area for a longer period,
and its precise location is not always predictable,
while with pneumatic automatic feeders,
fish receive the same amount of resource,
which is concentrated in the same surface area
over a shorter period. We compared seabass
behaviour under automatic and manual conditions
collecting video image recordings before,
during, and after feeding distribution, in the
morning and in afternoon, on two different
days, and analysing data within independent
sessions of measurements. Feeding modes significantly
affected swimming behaviour: automatically-
fed fish were characterised by vertical
movements through the water column (towards
the surface and bottom) and by horizontal
swimming. Manually-fed fish were instead
characterised by sharp direction changes during
their swimming, mostly towards the surface.
Feeding distribution induced changes in
collision frequency and elicited aggressive
behaviour. In particular, agonistic behaviour
(i.e. a fish attacks another fish) was almost
exclusively recorded during the feeding under
automatic distribution, whereas it was constantly
expressed during all the distribution
phases under manual mod
Mathematical Patterns and Cognitive Architectures
Mathematical patterns are an important subclass of the class of patterns. The main task of this paper is examining a particular proposal concerning the nature of mathematical patterns and some elements of the cognitive architecture an agent should have to recognize them
Creativity embedding: A vector to characterise and classify plausible triples in deep learning NLP models
In this paper we define the creativity embedding of a text based on four self-assessment creativity metrics, namely diversity, novelty, serendipity and magnitude, knowledge graphs, and neural networks. We use as basic unit the notion of triple (head, relation, tail). We investigate if additional information about creativity improves natural language processing tasks. In this work, we focus on triple plausibility task, exploiting BERT model and a WordNet11 dataset sample. Contrary to our hypothesis, we do not detect increase in the performance
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