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  • Articles  (102)
  • 2015-2019  (102)
  • 2017  (102)
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  • Articles  (102)
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  • 2015-2019  (102)
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  • 1
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    Galaxies, Vol. 6, Pages 1: Multi-Wavelength Intra-Day Variability and Quasi-Periodic Oscillation in Blazars (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-26
    Description: Galaxies, Vol. 6, Pages 1: Multi-Wavelength Intra-Day Variability and Quasi-Periodic Oscillation in Blazars Galaxies doi: 10.3390/galaxies6010001 Authors: Alok Gupta We reviewed multi-wavelength blazars variability and detection of quasi-periodic oscillations on intra-day timescales. The variability timescale from a few minutes to up to less than a days is commonly known as intra-day variability. These fast variations are extremely useful to constrain the size of the emitting region, black hole mass estimation, etc. It is noticed that in general, blazars show intra-day variability in the complete electromagnetic spectrum. However, some classes of blazars either do not show or show very little intra-day variability in a specific band of the electromagnetic spectrum. Blazars show rarely quasi-periodic oscillations in time series data in optical and X-ray bands. Other properties and emission mechanisms of blazars are also briefly discussed.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 2
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    Galaxies, Vol. 5, Pages 103: Spin and Maximal Acceleration (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-21
    Description: Galaxies, Vol. 5, Pages 103: Spin and Maximal Acceleration Galaxies doi: 10.3390/galaxies5040103 Authors: Giorgio Papini We study the spin current tensor of a Dirac particle at accelerations close to the upper limit introduced by Caianiello. Continual interchange between particle spin and angular momentum is possible only when the acceleration is time-dependent. This represents a stringent limit on the effect that maximal acceleration may have on spin physics in astrophysical applications. We also investigate some dynamical consequences of maximal acceleration.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 3
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    Galaxies, Vol. 5, Pages 102: Electromagnetic Casimir Effect in AdS Spacetime (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-20
    Description: Galaxies, Vol. 5, Pages 102: Electromagnetic Casimir Effect in AdS Spacetime Galaxies doi: 10.3390/galaxies5040102 Authors: Anna Kotanjyan Aram Saharian Astghik Saharyan We investigate the vacuum expectation value (VEV) of the energy-momentum tensor for the electromagnetic field in anti-de Sitter (AdS) spacetime in the presence of a boundary parallel to the AdS horizon. On the boundary, the field obeys the generalized perfect conductor boundary condition. The VEV of the energy-momentum tensor is decomposed into the boundary-free and boundary-induced contributions. In this way, for points away from the boundary, the renormalization is reduced to that for AdS spacetime without the boundary. The boundary-induced energy density is negative everywhere, and the normal stress is positive in the region between the boundary and the AdS boundary and is negative in the region between the boundary and the AdS horizon. Near both the AdS boundary and horizon, the boundary-induced VEV decays exponentially as a function of the corresponding proper distance. Applications are given for even and odd vector fields in Randall–Sundrum model with a single brane.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 4
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    Galaxies, Vol. 5, Pages 101: Explicit Second Partial Derivatives of the Ferrers Potential (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-16
    Description: Galaxies, Vol. 5, Pages 101: Explicit Second Partial Derivatives of the Ferrers Potential Galaxies doi: 10.3390/galaxies5040101 Authors: Lucas Caritá Irapuan Rodrigues Ivânio Puerari One of the algebraic potentials most commonly used to represent a galactic bar in the stellar orbits integration is the Ferrers potential. Some researchers may be inclined to implement a numerical differentiation for it in the motion or variational equations, since it can be very laborious to calculate such derivatives algebraically, despite a possible polynomial form, and there are no publications showing the second partial explicit derivatives. The purpose of this work is to present the explicit algebraic form of the partial derivatives of the Ferrers potential using the simplifications suggested by Pfenniger.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 5
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    Galaxies, Vol. 5, Pages 99: Bell Inequality and Its Application to Cosmology (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-13
    Description: Galaxies, Vol. 5, Pages 99: Bell Inequality and Its Application to Cosmology Galaxies doi: 10.3390/galaxies5040099 Authors: Sugumi Kanno Jiro Soda One of the cornerstones of inflationary cosmology is that primordial density fluctuations have a quantum mechanical origin. However, most physicists consider that such quantum mechanical effects disappear in CMB data due to decoherence. In this conference report, we show that the violation of Bell inequalities in an initial state of our universe increases exponentially with the number of modes to measure in inflation. This indicates that some evidence that our universe has a quantum mechanical origin may survive in CMB data, even if quantum entanglement decays exponentially afterward due to decoherence.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 6
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    Galaxies, Vol. 5, Pages 98: Cosmological Effects of Quantum Vacuum Condensates (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-13
    Description: Galaxies, Vol. 5, Pages 98: Cosmological Effects of Quantum Vacuum Condensates Galaxies doi: 10.3390/galaxies5040098 Authors: Antonio Capolupo In quantum field theory, many phenomena are characterized by a condensed structure of their vacua. Such a structure is responsible of a non trivial vacuum energy. Here we analyze disparate systems and we show that the thermal vacuum state for hot plasmas, the vacuum for boson field in curved space and the vacuum for mixed neutrinos have the state equation of dark matter, w = 0 , and values of the energy density which are in agreement with the one estimated for dark matter. Moreover, we show that the vacuum of axions mixed with photons has the state equation of the cosmological constant w = − 1 and a value of the energy density compatible with the one of dark energy.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 7
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    Galaxies, Vol. 5, Pages 97: Magnetic Field Studies in BL Lacertae through Faraday Rotation and a Novel Astrometric Technique (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-13
    Description: Galaxies, Vol. 5, Pages 97: Magnetic Field Studies in BL Lacertae through Faraday Rotation and a Novel Astrometric Technique Galaxies doi: 10.3390/galaxies5040097 Authors: Sol Molina José Gómez Richard Dodson María Rioja It is thought that dynamically important helical magnetic fields twisted by the differential rotation of the black hole’s accretion disk or ergosphere play an important role in the launching, acceleration, and collimation of active galactic nuclei (AGN) jets. We present multi-frequency astrometric and polarimetric Very Long Baseline Array (VLBA) images at 15, 22, and 43 GHz, as well as Faraday rotation analyses of the jet in BL Lacertae as part of a sample of AGN jets aimed to probe the magnetic field structure at the innermost scales to test jet formation models. The novel astrometric technique applied allows us to obtain the absolute position at mm wavelengths without any external calibrator.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 8
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    Galaxies, Vol. 5, Pages 100: Suborbital Fermi/LAT Analysis of the Brightest Gamma-Ray Flare of Blazar 3C 454.3 (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-13
    Description: Galaxies, Vol. 5, Pages 100: Suborbital Fermi/LAT Analysis of the Brightest Gamma-Ray Flare of Blazar 3C 454.3 Galaxies doi: 10.3390/galaxies5040100 Authors: Krzysztof Nalewajko Recent detection of suborbital gamma-ray variability of Flat Spectrum Radio Quasar (FSRQ) 3C 279 by Fermi Large Area Telescope (LAT) is in severe conflict with established models of blazar emission. This paper presents the results of suborbital analysis of the Fermi/LAT data for the brightest gamma-ray flare of another FSRQ blazar 3C 454.3 in November 2010 (Modified Julian Date; MJD 55516-22). Gamma-ray light curves are calculated for characteristic time bin lengths as short as 3 min. The measured variations of the 0.1–10 GeV photon flux are tested against the hypothesis of steady intraorbit flux. In addition, the structure function is calculated for absolute photon flux differences and for their significances. Significant gamma-ray flux variations are measured only over time scales longer than ∼5 h, which is consistent with the standard blazar models.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 9
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    Galaxies, Vol. 5, Pages 96: Exploring String Axions with Gravitational Waves (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-12
    Description: Galaxies, Vol. 5, Pages 96: Exploring String Axions with Gravitational Waves Galaxies doi: 10.3390/galaxies5040096 Authors: Jiro Soda Daiske Yoshida We explore the string axion dark matter with gravitational waves in Chern–Simons gravity. We show that the parametric resonance of gravitational waves occurs due to the axion coherent oscillation. Remarkably, the circular polarization of gravitational waves is induced by the parity violating Chern–Simons coupling. In fact, the gravitational waves should be enhanced ten times every 10 − 8 pc propagation in the presence of the axion dark matter with mass 10 − 10 eV provided the coupling constant ℓ = 10 8 km . Hence, after 10 kpc propagation, the amplitude of gravitational waves is enhanced significantly and the polarization of gravitational waves becomes circular. However, we have never observed these signatures. This implies that the Chern–Simons coupling constant and/or the abundance of string axions should be constrained much stronger than the current limits.
    Electronic ISSN: 2075-4434
    Topics: Physics
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  • 10
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    Galaxies, Vol. 5, Pages 94: Multi-Band Intra-Night Optical Variability of BL Lacertae (2017)
    MDPI
    In: Galaxies
    Details
    Publication Date: 2017-12-09
    Description: Galaxies, Vol. 5, Pages 94: Multi-Band Intra-Night Optical Variability of BL Lacertae Galaxies doi: 10.3390/galaxies5040094 Authors: Haritma Gaur Alok Gupta Rumen Bachev Anton Strigachev Evgeni Semkov Paul Wiita Minfeng Gu Sunay Ibryamov We monitored BL Lacertae frequently during 2014–2016 when it was generally in a high state. We searched for intra-day variability for 43 nights using quasi-simultaneous measurements in the B, V, R, and I bands (totaling 143 light curves); the typical sampling interval was about eight minutes. On hour-like timescales, BL Lac exhibited significant variations during 13 nights in various optical bands. Significant spectral variations are seen during most of these nights such that the optical spectrum becomes bluer when brighter. The amplitude of variability is usually greater for longer observations but is lower when BL Lac is brighter. No evidence for periodicities or characteristic variability time-scales in the light curves was found. The color variations are mildly chromatic on long timescales.
    Electronic ISSN: 2075-4434
    Topics: Physics
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