1,720,983 research outputs found
Bertotti-Robinson type solutions to Dilaton-Axion Gravity
No full text21 pagesInternational audienceWe present a new solution to dilaton-axion gravity which looks like a rotating Bertotti-Robinson (BR) Universe. It is supported by an homogeneous Maxwell field and a linear axion and can be obtained as a near-horizon limit of extremal rotating dilaton-axion black holes. It has the isometry where U(1) is the remnant of the SO(3) symmetry of BR broken by rotation, while corresponds to the sector which no longer factors out of the full spacetime. Alternatively our solution can be obtained from the D=5 vacuum counterpart to the dyonic BR with equal electric and magnetic field strengths. The derivation amounts to smearing it in D=6 and then reducing to D=4 with dualization of one Kaluza-Klein two-form in D=5 to produce an axion. Using a similar dualization procedure, the rotating BR solution is uplifted to D=11 supergravity. We show that it breaks all supersymmetries of N=4 supergravity in D=4, and that its higher dimensional embeddings are not supersymmetric either. But, hopefully it may provide a new arena for corformal mechanics and holography. Applying a complex coordinate transformation we also derive a BR solution endowed with a NUT parameter
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Quantized rotating Taub-bolt instantons
No full text11 pages revtex4We argue that previously suggested metrics for rotating Taub-bolt instantons do not satisfy all the necessary regularity conditions, and we present a family of new regular rotating Taub-bolts labelled by an odd integer . There are two types of rotating bolt solutions. The first infinite sequence starts with non-rotating Taub-NUT with positive mass for and goes to for (or for ), where is the rotation parameter. For the second sequence of rotating Page bolts, the masses go through the value for and asymptote to for
On the Smarr formulas for electrovac spacetimes with line singularities
No full textInternational audienceUsing the revised Komar-Tomimatsu approach, we derive Smarr mass formulas for stationary axisymmetric solutions of the Einstein-Maxwell equations containing line singularities (defects) on the polar axis. In terms of the rod structure associated with Weyl representation of the metric, the horizons and the defects are formally similar up to differences due to their timelike/spacelike character. We derive (previously unknown or incorrect) horizon and global Smarr formulas in presence of a Newman-Unti-Tamburino (NUT) parameter. To avoid the divergence of the Komar angular momentum of semi-infinite Dirac and Misner strings, it is necessary to use a symmetric tuning. We also note that the horizon mass Smarr formula does not include either magnetic charge, or NUT parameter, correcting some statements in the literature. The contribution of each Misner string to the total mass consists in an angular momentum term, an electric charge term, and a length term, which can also be presented as the product of the spacelike analogue of surface gravity and the area of the string
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Stationary double black hole without naked ring singularity
No full textInternational audienceRecently double black hole vacuum and electrovacuum metrics attracted attention as exact solutions suitable for visualization of ultra-compact objects beyond the Kerr paradigm. However, many of the proposed systems are plagued with ring curvature singularities. Here we present a new simple solution of this type which is asymptotically Kerr, has zero electric and magnetic charges, but is endowed with magnetic dipole moment and electric quadrupole moment. It is manifestly free of ring singularities, and contains only a mild string-like singularity on the axis corresponding to a distributional energy–momentum tensor. Its main constituents are two extreme co-rotating black holes carrying equal electric and opposite magnetic and NUT charges
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
F0 fluxbranes, F-walls and new brane worlds
Get PDFWe complete the list of fluxbrane solutions in classical supergravities by
introducing Melvin type space-times supported by antisymmetric forms of rank
and their pseudoscalar duals. In IIB theory these solutions belong to the
same family as the seven-brane and D-instanton. In current notation, a
fluxbrane supported by the D-1 form is an F0 brane, its euclidean continuation
is a cylindrical background which ``interacts'' with a pointlike instanton. The
general F0 brane can have a transverse space S_k times R^{(D-k-2)} with 1 <= k
<= D-2. For k=1 we find the complete solution containing four parameters, three
of them associated with pointlike singularities on the Melvin-type background.
The S-dual to the F0 brane in ten dimensions is the F8 fluxbrane of codimension
one, or F-wall, similar fluxbranes exist in any dimensions if an appropriate
form field is present. F-walls contain naked singularities unless one
introduces source branes. In such a way one obtains new brane-world type
solutions with two bulk scalar fields. Their relation to the supersymmetric
brane-worlds is discussed.Comment: 24 pages, latex, 2 references added, to be published in Class. Quant.
Gra
Solitons and black holes in Einstein-Born-Infeld-dilaton theory
No full textLatex, 16 pages, 6 .ps figuresInternational audienceStatic spherically symmetric asymptotically flat solutions to the U(1) Born-Infeld theory coupled to gravity and to a dilaton are investigated. The dilaton enters in such a way that the theory is - symmetric for a unit value of the dilaton coupling constant. We find globally regular solutions for any value of the effective coupling which is the ratio of the gravitational and dilaton couplings; they form a continuous sequence labeled by the sole free parameter of the local solution near the origin. The allowed values of this parameter are bounded from below, and, as the limiting value is approached, the mass and the dilaton charge rise indefinitely and tend to a strict equality (in suitable units). Together with the electric charge they saturate the BPS bound of the corresponding linear U(1) theory. Beyond this boundary the solutions become compact (singular), while the limiting solution at the exact boundary value is non-compact and non-asymptotically flat. The black holes in this theory exist for any value of the event horizon radius and also form a sequence labelled by a continuously varying parameter restricted to a finite interval. The singularity inside the black hole exhibits a power-law mass inflation
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