1,721,227 research outputs found
Complete extension of the symmetry axis of the Tomimatsu–Sato solution of the Einstein equations
Full text linkThe symmetry axis of the simplest Tomimatsu-Sato field is considered. Since this manifold is not geodesically complete for every value of the parameters occurring in the metric, a complete extension is given, and it is shown that its causal structure is very similar to that of the symmetry axis of the Kerr field
Some properties and problems of accretion disks about Kerr naked singularities
Full text linkDisk accretion onto a Kerr naked singularity is investigated by examining the stable circular equatorial orbits of a test particle in a Kerr background. It is assumed that there is a mechanism (e.g., viscous stress) at large radii that disperses energy and transfers angular momentum outward and mass inward, while maintaining the accretion flow in quasi-geodesic circular orbits. The results obtained show that: (1) the radius and energy of the last stable orbit increase as the ratio of the angular-momentum density and mass (a/M) of the naked singularity increases; (2) for a/M less than the square root of (32/27) there is a region where the energy becomes negative, suggesting that more energy than the mass-energy of a test particle can be extracted; (3) pathologies related to the definition of positive and negative energy states exist for a/M between unity and the square root of (32/27); and (4) the entire mass-energy of a test particle can be extracted for a/M equal to the square root of (32/27)
Epigenetic Modulation of Chromatin States and Gene Expression by G-Quadruplex Structures
Full text linkG-quadruplexes are four-stranded helical nucleic acid structures formed by guanine-rich sequences. A considerable number of studies have revealed that these noncanonical structural motifs are widespread throughout the genome and transcriptome of numerous organisms, including humans. In particular, G-quadruplexes occupy strategic locations in genomic DNA and both coding and noncoding RNA molecules, being involved in many essential cellular and organismal functions. In this review, we first outline the fundamental structural features of G-quadruplexes and then focus on the concept that these DNA and RNA structures convey a distinctive layer of epigenetic information that is critical for the complex regulation, either positive or negative, of biological activities in different contexts. In this framework, we summarize and discuss the proposed mechanisms underlying the functions of G-quadruplexes and their interacting factors. Furthermore, we give special emphasis to the interplay between G-quadruplex formation/disruption and other epigenetic marks, including biochemical modifications of DNA bases and histones, nucleosome positioning, and three-dimensional organization of chromatin. Finally, epigenetic roles of RNA G-quadruplexes in post-transcriptional regulation of gene expression are also discussed. Undoubtedly, the issues addressed in this review take on particular importance in the field of comparative epigenetics, as well as in translational research
Optical Candidates for the X-Ray Source GX 5-1
No full textThe large distance of GX5-1, suggested by the low-energy X-ray spectrum, restricts the number of possible optical candidates. A near infrared excess from the counterpart is expected
Dimensional reduction of the Kaler-Dirac equation and light fermions in Kaluza-Klein theories
No full textGyromagnetic ratio of Einstein-Maxwell fields
No full textIt is shown that the gyromagnetic factor is g=2 for electrovac solutions of Einstein's equations obtained by the prescription introduced by Ernst. This generalizes Carter's result for the Kerr-Newman metric
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