Time filter

Source Type

Sokolov N.A.,Central Astronomical Observatory at Pulkovo
Astrophysics and Space Science | Year: 2010

The spectrophotometric variability of the magnetic CP star 56 Arietis (56 Ari) in the ultraviolet spectral region from 1950 to 3200 Å is investigated. This study is based on the archival International Ultraviolet Explorer data obtained at different phases of the rotational cycle. The brightness of 56 Ari is not constant in the investigated wavelengths over the whole rotational period. The monochromatic light curves continuously change their shape with wavelength. This indicates that we do not observe a truly 'null wavelength region' where the monochromatic light curve has a zero amplitude. Probably, an uneven surface distribution of silicon and iron mainly influences the flux redistribution from the far-UV to near-UV spectral regions, although additional sources of opacity may be involved. The redistribution of the flux at phase 0.25 is connected with the nonuniform distribution of silicon on the stellar surface of 56 Ari. On the other hand, the redistribution of the flux at phase 0.65 is quite complex, because there are additional blocking and redistribution of the flux by iron lines in the near-UV spectral region. © 2010 Springer Science+Business Media B.V.

Krasnikov S.,Central Astronomical Observatory at Pulkovo
Classical and Quantum Gravity | Year: 2015

General relativity lacks the notion of the speed of gravity. This is inconvenient, and the current paper is aimed at filling this gap. To that end I introduce the concept of the 'alternative' and argue that its variation, called the 'superluminal alternative', describes exactly what one understands by the 'superluminal gravitational signal'. Another, closely related, object called the 'semi-superluminal alternative' corresponds to the situation in which a massive (and therefore gravitating) body reaches its destination sooner than a photon would if the latter were sent instead of the body. I prove that in general relativity constrained by the condition that only globally hyperbolic spacetimes are allowed, (1) semi-superluminal alternatives are absent, and (2) under some natural conditions and conventions, admissible superluminal alternatives are absent too. © 2015 IOP Publishing Ltd.

Tokovinin A.,Cerro Tololo Inter American Observatory | Kiyaeva O.,Central Astronomical Observatory at Pulkovo
Monthly Notices of the Royal Astronomical Society | Year: 2015

A sample of 477 solar-type binaries within 67 pc with projected separations larger than 50 au is studied by a new statistical method. Speed and direction of the relative motion are determined from the short observed arcs or known orbits, and their joint distribution is compared to the numerical simulations. By inverting the observed distribution with the help of simulations, we find that average eccentricity of wide binaries is 0.59 ± 0.02 and the eccentricity distribution can be modelled as f(e) ≈ 1.2e + 0.4. However, wide binaries containing inner subsystems, i.e. triple or higher order multiples, have significantly smaller eccentricities with the average e = 0.52±0.05 and the peak at e~0.5.We find that the catalogue of visual orbits is strongly biased against large eccentricities. A marginal evidence of eccentricity increasing with separation (or period) is found for this sample. Comparison with spectroscopic binaries proves the reality of the controversial period-eccentricity relation. The average eccentricity does increase with binary period, being 0.39 for periods from 102 to 103 d and 0.59 for the binaries studied here (105-106 d). © 2015 The Authors. Published by Oxford University Press.

Krasnikov S.V.,Central Astronomical Observatory at Pulkovo
Gravitation and Cosmology | Year: 2013

I show that there is a significant lacuna in the proof of the theorem known as "Topological Censorship" (a theorem forbidding a solution of Einstein's equations to have some topological features, such as traversable wormholes, without violating the averaged null energy condition). To fill the lacuna one would probably have to revise the class of spacetimes for which the theorem is formulated. © 2013 Pleiades Publishing, Ltd.

Krasnikov S.,Central Astronomical Observatory at Pulkovo
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

The building of a time machine, if possible at all, requires the relevant regions of spacetime to be compact (that is, physically speaking, free from sources of unpredictability such as infinities and singularities). Motivated by this argument we consider the spacetimes with the compactly determined Cauchy horizons (CDCHs), the defining property of which is the compactness of J-(U)̄ J+(S0), where U is an open subset of the Cauchy horizon and S0 is a Cauchy surface of the initial globally hyperbolic region Min. The following two facts are established: (1) Min has no globally hyperbolic maximal extension. This means that, by shaping appropriately a precompact portion of a globally hyperbolic region, one can force the Universe to produce either a closed causal curve, or a quasiregular singularity, whichever it abhors less. (2) Before a CDCH is formed a null geodesic appears which infinitely approaches the horizon returning again and again in the same - arbitrarily small - region. The energy of the photon moving on such a geodesic increases with each passage, or at least falls insufficiently fast. As a result, an observer located in the mentioned region would see a bunch of photons passing through his laboratory with the arbitrarily large total energy. We speculate that this phenomenon may have observable consequences. © 2014 American Physical Society.

Discover hidden collaborations