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Leonid I. Gurvits

Bio: Leonid I. Gurvits is an academic researcher from Joint Institute for VLBI in Europe. The author has contributed to research in topics: Very-long-baseline interferometry & Quasar. The author has an hindex of 34, co-authored 140 publications receiving 4180 citations. Previous affiliations of Leonid I. Gurvits include Delft University of Technology & Japan Aerospace Exploration Agency.


Papers
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Journal ArticleDOI
TL;DR: In this article, the authors examined the compact structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array (VLBA) at 15 GHz.
Abstract: We have examined the compact structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array (VLBA) at 15 GHz. With projected baselines out to 440 Mλ, we are able to investigate source structure on typical angular scales as small as 0.05 mas. This scale is similar to the resolution of the VLBI Space Observatory Programme data obtained on longer baselines at a lower frequency and with somewhat poorer accuracy. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines; these will be useful as fringe finders for short-wavelength VLBA observations. The total flux densities recovered in the VLBA images at 15 GHz are generally close to the values measured around the same epoch at the same frequency with the RATAN-600 and University of Michigan Radio Astronomy Observatory telescopes. We have modeled the core of each source with an elliptical Gaussian component. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lac objects are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. Intraday variability (IDV) sources typically have a more compact, more core-dominated structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intraday variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray-loud appear to have a more compact VLBA structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron–emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 1011 and 1013 K, but they extend up to 5 × 1013 K, apparently in excess of the equipartition brightness temperature or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming. Longer baselines, which can be obtained by space VLBI observations, will be needed to resolve the most compact high brightness temperature regions in these sources.

338 citations

Journal ArticleDOI
N. S. Kardashev1, V. V. Khartov, V. V. Abramov2, V. Yu. Avdeev1, A. V. Alakoz1, Yu. A. Aleksandrov1, S. Ananthakrishnan3, V. V. Andreyanov1, A. S. Andrianov1, N. M. Antonov1, M. I. Artyukhov, M. Yu. Arkhipov1, W. Baan4, N.G. Babakin1, V. E. Babyshkin, N. Bartel5, K. G. Belousov1, A. A. Belyaev, J. J. Berulis1, Bernard F. Burke6, A. V. Biryukov1, A. E. Bubnov2, M. S. Burgin1, G. Busca, A. A. Bykadorov, V. S. Bychkova1, V. I. Vasil’kov1, K. J. Wellington7, I. S. Vinogradov1, R. Wietfeldt8, P. A. Voitsik1, A. S. Gvamichava1, I. A. Girin1, Leonid I. Gurvits9, Leonid I. Gurvits10, R. D. Dagkesamanskii1, L. D’Addario8, Gabriele Giovannini11, Gabriele Giovannini12, D. L. Jauncey7, Peter E. Dewdney, A. A. D’yakov2, Vladimir Zharov13, V. I. Zhuravlev1, G. S. Zaslavskii2, M. V. Zakhvatkin2, A. N. Zinov’ev1, Yu. Ilinen, A. V. Ipatov2, B. Z. Kanevskii1, I. A. Knorin1, J. L. Casse10, K. I. Kellermann14, Yu. A. Kovalev1, Yu. Yu. Kovalev15, Yu. Yu. Kovalev1, A. V. Kovalenko1, B. L. Kogan16, R. V. Komaev, A. A. Konovalenko17, G. D. Kopelyanskii1, Yu. A. Korneev1, V. I. Kostenko1, A. N. Kotik1, B. B. Kreisman1, A. Yu. Kukushkin2, V. F. Kulishenko17, D. N. Cooper7, A. M. Kutkin1, Wayne Cannon5, M. G. Larionov1, Mikhail M. Lisakov1, L. N. Litvinenko17, S. F. Likhachev1, L. N. Likhacheva1, A. P. Lobanov15, S. V. Logvinenko1, Glen Langston14, K. McCracken7, S. Yu. Medvedev, M. V. Melekhin, A. V. Menderov, David W. Murphy8, T. A. Mizyakina1, Yu. V. Mozgovoi, N. Ya. Nikolaev1, B. S. Novikov1, B. S. Novikov2, I. D. Novikov1, V. V. Oreshko1, Yu. K. Pavlenko, I. N. Pashchenko1, Yu. N. Ponomarev1, M. V. Popov1, A. Pravin-Kumar3, Robert A. Preston8, V. N. Pyshnov1, I. A. Rakhimov2, V. M. Rozhkov, Jonathan D. Romney14, P. Rocha, V. A. Rudakov1, Antti V. Räisänen18, S. V. Sazankov1, Boris A. Sakharov, S. K. Semenov, V. A. Serebrennikov, R. T. Schilizzi, D. P. Skulachev2, V. I. Slysh1, A. I. Smirnov1, Joel Smith8, V. A. Soglasnov1, K. V. Sokolovskii1, K. V. Sokolovskii13, L. H. Sondaar4, V. A. Stepan’yants2, M. S. Turygin2, S. Yu. Turygin2, A. G. Tuchin2, S. Urpo18, S.D. Fedorchuk1, A. M. Finkel’shtein2, Ed Fomalont14, I. Fejes, A. N. Fomina, Yu. B. Khapin2, G. S. Tsarevskii1, J. A. Zensus15, A. A. Chuprikov1, M. V. Shatskaya1, N. Ya. Shapirovskaya1, A. I. Sheikhet, A. E. Shirshakov, A. Schmidt15, L. A. Shnyreva1, V. V. Shpilevskii2, R. D. Ekers7, V. E. Yakimov1 
TL;DR: The RadioAstron project as mentioned in this paper is targeted at systematic studies of compact radio-emitting sources and their dynamics, including supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei.
Abstract: The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun.

259 citations

Journal ArticleDOI
08 Dec 2005-Nature
TL;DR: An overview of the Huygens mission is reported, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape.
Abstract: Titan, Saturn's largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titan's atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. Here we report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity--probably due to methane-and a darker area that looked like a river- or lake-bed. Post-landing images showed centimetre-sized surface details.

231 citations

Journal ArticleDOI
01 Mar 2019-Science
TL;DR: In this paper, the authors present very-long-baseline interferometry observations, performed 207.4 days after the binary neutron star merger event GW170817, using a global network of 32 radio telescopes.
Abstract: The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.

227 citations

Journal ArticleDOI
TL;DR: In this article, the authors used VLBA fringe visibility data obtained at 15 GHz to examine the compact structure in 250 extragalactic radio sources and found that most of the sources have unresolved core components in one direction, usually transverse to the direction into which the jet extends.
Abstract: We have used VLBA fringe visibility data obtained at 15 GHz to examine the compact structure in 250 extragalactic radio sources. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lacs are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. IDV sources typically have a more compact, more core-dominated structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intra-day variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray loud appear to have a more compact VLBA structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 10^11 and 10^13 K, but they extend up to 5x10^13 K, apparently in excess of the equipartition brightness temperature, or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming (abridged).

204 citations


Cited by
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TL;DR: In this article, a combination of seven-year data from WMAP and improved astrophysical data rigorously tests the standard cosmological model and places new constraints on its basic parameters and extensions.
Abstract: The combination of seven-year data from WMAP and improved astrophysical data rigorously tests the standard cosmological model and places new constraints on its basic parameters and extensions. By combining the WMAP data with the latest distance measurements from the baryon acoustic oscillations (BAO) in the distribution of galaxies and the Hubble constant (H0) measurement, we determine the parameters of the simplest six-parameter ΛCDM model. The power-law index of the primordial power spectrum is ns = 0.968 ± 0.012 (68% CL) for this data combination, a measurement that excludes the Harrison–Zel’dovich–Peebles spectrum by 99.5% CL. The other parameters, including those beyond the minimal set, are also consistent with, and improved from, the five-year results. We find no convincing deviations from the minimal model. The seven-year temperature power spectrum gives a better determination of the third acoustic peak, which results in a better determination of the redshift of the matter-radiation equality epoch. Notable examples of improved parameters are the total mass of neutrinos, � mν < 0.58 eV (95% CL), and the effective number of neutrino species, Neff = 4.34 +0.86 −0.88 (68% CL), which benefit from better determinations of the third peak and H0. The limit on a constant dark energy equation of state parameter from WMAP+BAO+H0, without high-redshift Type Ia supernovae, is w =− 1.10 ± 0.14 (68% CL). We detect the effect of primordial helium on the temperature power spectrum and provide a new test of big bang nucleosynthesis by measuring Yp = 0.326 ± 0.075 (68% CL). We detect, and show on the map for the first time, the tangential and radial polarization patterns around hot and cold spots of temperature fluctuations, an important test of physical processes at z = 1090 and the dominance of adiabatic scalar fluctuations. The seven-year polarization data have significantly improved: we now detect the temperature–E-mode polarization cross power spectrum at 21σ , compared with 13σ from the five-year data. With the seven-year temperature–B-mode cross power spectrum, the limit on a rotation of the polarization plane due to potential parity-violating effects has improved by 38% to Δα =− 1. 1 ± 1. 4(statistical) ± 1. 5(systematic) (68% CL). We report significant detections of the Sunyaev–Zel’dovich (SZ) effect at the locations of known clusters of galaxies. The measured SZ signal agrees well with the expected signal from the X-ray data on a cluster-by-cluster basis. However, it is a factor of 0.5–0.7 times the predictions from “universal profile” of Arnaud et al., analytical models, and hydrodynamical simulations. We find, for the first time in the SZ effect, a significant difference between the cooling-flow and non-cooling-flow clusters (or relaxed and non-relaxed clusters), which can explain some of the discrepancy. This lower amplitude is consistent with the lower-than-theoretically expected SZ power spectrum recently measured by the South Pole Telescope Collaboration.

11,309 citations

Journal ArticleDOI
TL;DR: A review of dark energy can be found in this paper, where the authors present the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.
Abstract: Physics welcomes the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, \ensuremath{\Lambda}; today the concept is termed dark energy or quintessence. Physics also suggests that dark energy could be dynamical, allowing for the arguably appealing picture of an evolving dark-energy density approaching its natural value, zero, and small now because the expanding universe is old. This would alleviate the classical problem of the curious energy scale of a millielectron volt associated with a constant \ensuremath{\Lambda}. Dark energy may have been detected by recent cosmological tests. These tests make a good scientific case for the context, in the relativistic Friedmann-Lema\^{\i}tre model, in which the gravitational inverse-square law is applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one-quarter of the critical Einstein--de Sitter value. The case for detection of dark energy is not yet as convincing but still serious; we await more data, which may be derived from work in progress. Planned observations may detect the evolution of the dark-energy density; a positive result would be a considerable stimulus for attempts at understanding the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.

4,783 citations

Journal ArticleDOI
TL;DR: A comprehensive survey of recent work on modified theories of gravity and their cosmological consequences can be found in this article, where the authors provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a selfcontained, comprehensive and up-to-date introduction to the subject as a whole.

3,674 citations

01 Jan 2016
TL;DR: In this paper, the authors present the principles of optics electromagnetic theory of propagation interference and diffraction of light, which can be used to find a good book with a cup of coffee in the afternoon, instead of facing with some infectious bugs inside their computer.
Abstract: Thank you for reading principles of optics electromagnetic theory of propagation interference and diffraction of light. As you may know, people have search hundreds times for their favorite novels like this principles of optics electromagnetic theory of propagation interference and diffraction of light, but end up in harmful downloads. Rather than enjoying a good book with a cup of coffee in the afternoon, instead they are facing with some infectious bugs inside their computer.

2,213 citations

01 Jan 2005
TL;DR: The Monthly Notices as mentioned in this paper is one of the three largest general primary astronomical research publications in the world, published by the Royal Astronomical Society (RAE), and it is the most widely cited journal in astronomy.
Abstract: Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

2,091 citations