Galaxies

750 KiB

Open AccessArticle

by Giorgio Papini

Galaxies 2017, 5(4), 103; https://doi.org/10.3390/galaxies5040103 - 20 Dec 2017

Cited by 7 | Viewed by 2714

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 [...] Read more.

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. Full article

289 KiB

Open AccessFeature PaperArticle

Electromagnetic Casimir Effect in AdS Spacetime

by Anna S. Kotanjyan, Aram A. Saharian and Astghik A. Saharyan

Galaxies 2017, 5(4), 102; https://doi.org/10.3390/galaxies5040102 - 19 Dec 2017

Cited by 6 | Viewed by 3060

Abstract

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. [...] Read more.

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. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

► Show Figures

Figure 1

711 KiB

Open AccessFeature PaperArticle

Explicit Second Partial Derivatives of the Ferrers Potential

by Lucas Antonio Caritá, Irapuan Rodrigues and Ivânio Puerari

Galaxies 2017, 5(4), 101; https://doi.org/10.3390/galaxies5040101 - 15 Dec 2017

Cited by 1 | Viewed by 2327

Abstract

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 [...] Read more.

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. Full article

1279 KiB

Open AccessFeature PaperArticle

Bell Inequality and Its Application to Cosmology

by Sugumi Kanno and Jiro Soda

Galaxies 2017, 5(4), 99; https://doi.org/10.3390/galaxies5040099 - 12 Dec 2017

Cited by 5 | Viewed by 3966

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

► Show Figures

Figure 1

231 KiB

Open AccessFeature PaperArticle

Cosmological Effects of Quantum Vacuum Condensates

by Antonio Capolupo

Galaxies 2017, 5(4), 98; https://doi.org/10.3390/galaxies5040098 - 12 Dec 2017

Cited by 8 | Viewed by 3099

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

2602 KiB

Open AccessFeature PaperArticle

Magnetic Field Studies in BL Lacertae through Faraday Rotation and a Novel Astrometric Technique

by Sol N. Molina, José L. Gómez, Richard Dodson and María J. Rioja

Galaxies 2017, 5(4), 97; https://doi.org/10.3390/galaxies5040097 - 12 Dec 2017

Viewed by 2995

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

978 KiB

Open AccessFeature PaperArticle

Suborbital Fermi/LAT Analysis of the Brightest Gamma-Ray Flare of Blazar 3C 454.3

by Krzysztof Nalewajko

Galaxies 2017, 5(4), 100; https://doi.org/10.3390/galaxies5040100 - 12 Dec 2017

Cited by 4 | Viewed by 3366

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

► Show Figures

Figure 1

299 KiB

Open AccessArticle

Exploring String Axions with Gravitational Waves

by Jiro Soda and Daiske Yoshida

Galaxies 2017, 5(4), 96; https://doi.org/10.3390/galaxies5040096 - 11 Dec 2017

Cited by 13 | Viewed by 3611

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

► Show Figures

Figure 1

1245 KiB

Open AccessFeature PaperArticle

Decoding Galactic Merger Histories

by Eric F. Bell, Antonela Monachesi, Richard D’Souza, Benjamin Harmsen, Roelof S. De Jong, David Radburn-Smith, Jeremy Bailin and Benne W. Holwerda

Galaxies 2017, 5(4), 95; https://doi.org/10.3390/galaxies5040095 - 8 Dec 2017

Cited by 2 | Viewed by 3565

Abstract

Galaxy mergers are expected to influence galaxy properties, yet measurements of individual merger histories are lacking. Models predict that merger histories can be measured using stellar halos and that these halos can be quantified using observations of resolved stars along their minor axis. [...] Read more.

Galaxy mergers are expected to influence galaxy properties, yet measurements of individual merger histories are lacking. Models predict that merger histories can be measured using stellar halos and that these halos can be quantified using observations of resolved stars along their minor axis. Such observations reveal that Milky Way-mass galaxies have a wide range of stellar halo properties and show a correlation between their stellar halo masses and metallicities. This correlation agrees with merger-driven models where stellar halos are formed by satellite galaxy disruption. In these models, the largest accreted satellite dominates the stellar halo properties. Consequently, the observed diversity in the stellar halos of Milky Way-mass galaxies implies a large range in the masses of their largest merger partners. In particular, the Milky Way’s low mass halo implies an unusually quiet merger history. We used these measurements to seek predicted correlations between the bulge and central black hole (BH) mass and the mass of the largest merger partner. We found no significant correlations: while some galaxies with large bulges and BHs have large stellar halos and thus experienced a major or minor merger, half have small stellar halos and never experienced a significant merger event. These results indicate that bulge and BH growth is not solely driven by merger-related processes. Full article

(This article belongs to the Special Issue On the Origin (and Evolution) of Baryonic Galaxy Halos)

► Show Figures

Figure 1

820 KiB

Open AccessFeature PaperArticle

Multi-Band Intra-Night Optical Variability of BL Lacertae

by Haritma Gaur, Alok C. Gupta, Rumen Bachev, Anton Strigachev, Evgeni Semkov, Paul J. Wiita, Minfeng Gu and Sunay Ibryamov

Galaxies 2017, 5(4), 94; https://doi.org/10.3390/galaxies5040094 - 8 Dec 2017

Cited by 5 | Viewed by 3614

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

► Show Figures

Figure 1

9002 KiB

Open AccessFeature PaperArticle

Linear Polarization Properties of Parsec-Scale AGN Jets

by Alexander B. Pushkarev, Yuri Y. Kovalev, Matthew L. Lister, Tuomas Savolainen, Margo F. Aller, Hugh D. Aller and Mary A. Hodge

Galaxies 2017, 5(4), 93; https://doi.org/10.3390/galaxies5040093 - 1 Dec 2017

Cited by 21 | Viewed by 4775

Abstract

We used 15 GHz multi-epoch Very Long Baseline Array (VLBA) polarization sensitive observations of 484 sources within a time interval 1996–2016 from the MOJAVE program, and also from the NRAO data archive. We have analyzed the linear polarization characteristics of the compact core [...] Read more.

We used 15 GHz multi-epoch Very Long Baseline Array (VLBA) polarization sensitive observations of 484 sources within a time interval 1996–2016 from the MOJAVE program, and also from the NRAO data archive. We have analyzed the linear polarization characteristics of the compact core features and regions downstream, and their changes along and across the parsec-scale active galactic nuclei (AGN) jets. We detected a significant increase of fractional polarization with distance from the radio core along the jet as well as towards the jet edges. Compared to quasars, BL Lacs have a higher degree of polarization and exhibit more stable electric vector position angles (EVPAs) in their core features and a better alignment of the EVPAs with the local jet direction. The latter is accompanied by a higher degree of linear polarization, suggesting that compact bright jet features might be strong transverse shocks, which enhance magnetic field regularity by compression. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

825 KiB

Open AccessFeature PaperArticle

Multi-Frequency VLBA Polarimetry and the Twin-Jet Quasar 0850+581

by Evgeniya Kravchenko and Yuri Y. Kovalev

Galaxies 2017, 5(4), 92; https://doi.org/10.3390/galaxies5040092 - 30 Nov 2017

Cited by 1 | Viewed by 2975

Abstract

We present the first multi-frequency VLBA study of the quasar 0850+581 which appears to have a two-sided relativistic jet. Apparent velocity in the approaching jet changes from 3.4c to 7c with the separation from the core. The jet-to-counter-jet ratio of about 5 and [...] Read more.

We present the first multi-frequency VLBA study of the quasar 0850+581 which appears to have a two-sided relativistic jet. Apparent velocity in the approaching jet changes from 3.4c to 7c with the separation from the core. The jet-to-counter-jet ratio of about 5 and apparent superluminal velocities suggest that the observing angle of the inner jet is

\leq 17^{\circ}

. It is likely that this orientation significantly changes downstream due to an interaction of the jet with the surrounding medium; signs of this are seen in polarization. A dense inhomogeneous Faraday screen is detected in the innermost regions of this quasar. We suggest that there is a presence of ionized gas in its nucleus, which might be responsible for the free-free absorption of the synchrotron emission in the jet and counter-jet at frequencies below 8.4 GHz. The experiment makes use of slowly varying instrumental polarisation factors (polarization leakage or D-terms) in time. We report application of the “D-term connection” technique for the calibration of an absolute orientation of electric vector position angle (EVPA) observed by VLBA at 4.6, 5.0, 8.1, 8.4, 15.4, 22.3, and 43.3 GHz bands during the 2007–2011. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

1780 KiB

Open AccessFeature PaperArticle

Behaviour of the Blazar CTA 102 during Two Giant Outbursts

by Valeri M. Larionov, Svetlana G. Jorstad, Alan P. Marscher, Paul S. Smith, Sergey Savchenko, Daria Morozova, Tatiana Grishina, Evgenia Kopatskaya, Liudmila Larionova, Elena Larionova, Anna Mokrushina, Ivan Troitsky, Yulia Troitskaya and George Borman

Galaxies 2017, 5(4), 91; https://doi.org/10.3390/galaxies5040091 - 30 Nov 2017

Cited by 7 | Viewed by 3764

Abstract

Blazar CTA 102 underwent exceptional optical and high-energy outbursts in 2012 and 2016–2017. We analyze its behaviour during these events, focusing on polarimetry as a tool that allows us to trace changes in the physical conditions and geometric configuration of the emission source [...] Read more.

Blazar CTA 102 underwent exceptional optical and high-energy outbursts in 2012 and 2016–2017. We analyze its behaviour during these events, focusing on polarimetry as a tool that allows us to trace changes in the physical conditions and geometric configuration of the emission source close to the central black hole. We also use Fermi

γ

-ray data in conjunction with optical photometry in an effort to localize the origin of the outbursts. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

2463 KiB

Open AccessArticle

Long-Term Multi-Band and Polarimetric View of Mkn 421: Motivations for an Integrated Open-Data Platform for Blazar Optical Polarimetry

by Ulisses Barres de Almeida, Bernardo M. O. Fraga, Paolo Giommi, Narek Sahakyan, Sargis Gasparyan and Carlos H. Brandt

Galaxies 2017, 5(4), 90; https://doi.org/10.3390/galaxies5040090 - 30 Nov 2017

Cited by 3 | Viewed by 3352

Abstract

In this work, by making use of the large software and database resources made available through online facilities such as the ASI Science Data Center (ASDC), we present a novel approach to the modelling of blazar emission whereby the multi-epoch SED for Mkn [...] Read more.

In this work, by making use of the large software and database resources made available through online facilities such as the ASI Science Data Center (ASDC), we present a novel approach to the modelling of blazar emission whereby the multi-epoch SED for Mkn 421 is modelled considering, in a self-consistent way, the temporal lags between bands (both in short and long-timescales). These are obtained via a detailed cross-correlation analysis, spanning data from radio to VHE gamma-rays from 2008 to 2015. In addition to that, long-term optical polarisation data is used to aid and complement our physical interpretation of the state and evolution of the source. Blazar studies constitute a clear example that astrophysics is becoming increasingly dominated by “big data”. Specific questions, such as the interpretation of polarimetric information—namely the evolution of the polarisation degree (PD) and specially the polarisation angle (PA) of a source—are very sensitive to the density of data coverage. Improving data accessibility and integration, in order to respond to these necessities, is thus extremely important and has a potentially large impact for blazar science. For this reason, we present also the project to create an open-access database for optical polarimetry, aiming to circumvent the issues raised above, by integrating long-term optical polarisation information on a number sources from several observatories and data providers in a consistent way. The platform, to be launched by the end of 2017 is built as part of the Brazilian Science Data Center (BSDC), a project hosted at CBPF, in Rio de Janeiro, and developed with the support of the Italian Space Agency (ASI) and ICRANet. The BSDC is Virtual Observatory-compliant and is built in line with “Open Universe”, a global space science open-data initiative to be launched in November under the auspices of the United Nations. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

2050 KiB

Open AccessArticle

Implications of Geometry and the Theorem of Gauss on Newtonian Gravitational Systems and a Caveat Regarding Poisson’s Equation

by Anne M. Hofmeister and Robert E. Criss

Galaxies 2017, 5(4), 89; https://doi.org/10.3390/galaxies5040089 - 29 Nov 2017

Cited by 9 | Viewed by 6397

Abstract

Galactic mass consistent with luminous mass is obtained by fitting rotation curves (RC = tangential velocities vs. equatorial radius r) using Newtonian force models, or can be unambiguously calculated from RC data using a model based on spin. In contrast, mass exceeding [...] Read more.

Galactic mass consistent with luminous mass is obtained by fitting rotation curves (RC = tangential velocities vs. equatorial radius r) using Newtonian force models, or can be unambiguously calculated from RC data using a model based on spin. In contrast, mass exceeding luminous mass is obtained from multi-parameter fits using potentials associated with test particles orbiting in a disk around a central mass. To understand this disparity, we explore the premises of these mainstream disk potential models utilizing the theorem of Gauss, thermodynamic concepts of Gibbs, the findings of Newton and Maclaurin, and well-established techniques and results from analytical mathematics. Mainstream models assume that galactic density in the axial (z) and r directions varies independently: we show that this is untrue for self-gravitating objects. Mathematics and thermodynamic principles each show that modifying Poisson’s equation by summing densities is in error. Neither do mainstream models differentiate between interior and exterior potentials, which is required by potential theory and has been recognized in seminal astronomical literature. The theorem of Gauss shows that: (1) density in Poisson’s equation must be averaged over the interior volume; (2) logarithmic gravitational potentials implicitly assume that mass forms a long, line source along the z axis, unlike any astronomical object; and (3) gravitational stability for three-dimensional shapes is limited to oblate spheroids or extremely tall cylinders, whereas other shapes are prone to collapse. Our findings suggest a mechanism for the formation of the flattened Solar System and of spiral galaxies from gas clouds. The theorem of Gauss offers many advantages over Poisson’s equation in analyzing astronomical problems because mass, not density, is the key parameter. Full article

► Show Figures

Figure 1

962 KiB

Open AccessFeature PaperArticle

Identification of the OGLE Blazars behind the Large and Small Magellanic Clouds

by Natalia Żywucka, Arti Goyal, Marek Jamrozy, Łukasz Stawarz, Michał Ostrowski, Szymon Kozłowski and Andrzej Udalski

Galaxies 2017, 5(4), 88; https://doi.org/10.3390/galaxies5040088 - 28 Nov 2017

Viewed by 3127

Abstract

We report the selection of blazar candidates behind the Large and Small Magellanic Clouds. Both flat spectrum radio quasar and BL Lacreate objects were selected based on the long-term, multi-colour Optical Gravitational Lensing Experiment photometric data. We cross-correlated the Magellanic Quasar Survey catalogue [...] Read more.

We report the selection of blazar candidates behind the Large and Small Magellanic Clouds. Both flat spectrum radio quasar and BL Lacreate objects were selected based on the long-term, multi-colour Optical Gravitational Lensing Experiment photometric data. We cross-correlated the Magellanic Quasar Survey catalogue of spectroscopically confirmed quasars and quasar candidates located behind the Magellanic Clouds with the radio data at six frequencies from 0.8 to 20 GHz. Among the 1654 objects visible in optical range, we identified a sample of 44 newly selected blazar candidates, including 27 flat spectrum radio quasars and 17 BL Lacs. We examined selected objects with respect to their radio, optical, and mid-infrared properties. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

935 KiB

Open AccessFeature PaperConference Report

The Mimetic Born-Infeld Gravity: The Primordial Cosmos and Spherically Symmetric Solutions

by Che-Yu Chen, Mariam Bouhmadi-López and Pisin Chen

Galaxies 2017, 5(4), 87; https://doi.org/10.3390/galaxies5040087 - 27 Nov 2017

Cited by 2 | Viewed by 2955

Abstract

The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the mimetic approach. In the presence of a mimetic field, the model contains non-trivial vacuum solutions. We study a realistic primordial vacuum universe and we prove the existence of regular solutions. Besides, the linear instabilities in [...] Read more.

The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the mimetic approach. In the presence of a mimetic field, the model contains non-trivial vacuum solutions. We study a realistic primordial vacuum universe and we prove the existence of regular solutions. Besides, the linear instabilities in the EiBI model are found to be avoidable for some bouncing solutions. For a vacuum, static and spherically symmetric geometry, a new branch of solutions in which the black hole singularity that is replaced with a lightlike singularity is found. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

► Show Figures

Figure 1

469 KiB

Open AccessFeature PaperArticle

The Giant Flares of the Microquasar Cygnus X-3: X-Rays States and Jets

by Sergei Trushkin, Michael McCollough, Nikolaj Nizhelskij and Peter Tsybulev

Galaxies 2017, 5(4), 86; https://doi.org/10.3390/galaxies5040086 - 27 Nov 2017

Cited by 12 | Viewed by 4237

Abstract

We report on two giant radio flares of the X-ray binary microquasar Cyg X-3, consisting of a Wolf–Rayet star and probably a black hole. The first flare occurred on 13 September 2016, 2000 days after a previous giant flare in February 2011, as [...] Read more.

We report on two giant radio flares of the X-ray binary microquasar Cyg X-3, consisting of a Wolf–Rayet star and probably a black hole. The first flare occurred on 13 September 2016, 2000 days after a previous giant flare in February 2011, as the RATAN-600 radio telescope daily monitoring showed. After 200 days on 1 April 2017, we detected a second giant flare. Both flares are characterized by the increase of the fluxes by almost 2000-times (from 5–10 to 17,000 mJy at 4–11 GHz) during 2–7 days, indicating relativistic bulk motions from the central region of the accretion disk around a black hole. The flaring light curves and spectral evolution of the synchrotron radiation indicate the formation of two relativistic collimated jets from the binaries. Both flares occurred when the source went from hypersoft X-ray states to soft ones, i.e. hard fluxes (Swift/BAT 15–50 keV data) dropped to zero, the soft X-ray fluxes (MAXI 2–10 keV data) staying high, and then later, the binary came back to a hard state. Both similar giant flares indicated the unchanged mechanism of the jets’ formation in Cyg X-3, probably in conditions of strong stellar wind and powerful accretion onto a black hole. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

1723 KiB

Open AccessFeature PaperArticle

Intra-Night Variability of OJ 287 with Long-Term Multiband Optical Monitoring

by Wei Zeng, Qing-Jiang Zhao, Ze-Jun Jiang, Zhi-Hui Kong, Zhen Liu, Dong-Dong Wang, Xiong-Fei Geng, Shen-Bang Yang and Ben-Zhong Dai

Galaxies 2017, 5(4), 85; https://doi.org/10.3390/galaxies5040085 - 22 Nov 2017

Cited by 6 | Viewed by 4141

Abstract

We present long-term optical multi-band photometric monitoring of the blazar OJ 287 from 6 March 2010 to 3 April 2016, with high temporal resolution in the

V R I

-bands. The flux variations and colour-magnitude variations on long and short timescales were investigated [...] Read more.

We present long-term optical multi-band photometric monitoring of the blazar OJ 287 from 6 March 2010 to 3 April 2016, with high temporal resolution in the

V R I

-bands. The flux variations and colour-magnitude variations on long and short timescales were investigated to understand the emission mechanisms. In our observation, the major outbursts occurred in January 2016, as predicted by the binary pair of black holes model for OJ 287, with

F_{v a r}

of 1.3∼2.1%, and variability amplitude (Amp) of 5.8∼9.0%. The intra-night variability (IDV) durations were from 18.5 to 51.3 min, and the minimal variability timescale was about 4.7 min. The colour-magnitude variation showed a weak positive correlation on the long timescale with Pearson’s

r = 0.450

, while a negative correlation was found on intra-night timescales. We briefly discuss the possible physical mechanisms that are most likely to be responsible for the observed flux and colour-magnitude correlation variability. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

► Show Figures

Figure 1

797 KiB

Open AccessFeature PaperArticle

The Jets of Microquasars during Giant Flares and Quiet State

by Sergei Trushkin, Nikolaj Nizhelskij, Peter Tsybulev and Gennadij Zhekanis

Galaxies 2017, 5(4), 84; https://doi.org/10.3390/galaxies5040084 - 21 Nov 2017

Cited by 3 | Viewed by 3113

Abstract

We report on the radio properties of jets of the following microquasars, as determined from daily multi-frequency monitoring observations with the RATAN-600 radio telescope during 2010–2017: V404 Cyg, SS433, Cyg X-1, GRS1915+105 and LSI+61

^{\circ}

303. We have detected many giant flares from [...] Read more.

We report on the radio properties of jets of the following microquasars, as determined from daily multi-frequency monitoring observations with the RATAN-600 radio telescope during 2010–2017: V404 Cyg, SS433, Cyg X-1, GRS1915+105 and LSI+61

^{\circ}

303. We have detected many giant flares from SS433, a powerful flare from V404 Cyg in June 2015, an active state of Cyg X-1 in 2017 and fifty periodic flares from LSI+61

^{\circ}

303. We describe the properties of massive ejections based on multi-band (radio, X-ray and

γ

-ray) studies. The general properties of the light curves are closely connected with the processes of jet formation in microquasars. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

2732 KiB

Open AccessFeature PaperArticle

Polarization and Spectral Energy Distribution in OJ 287 during the 2016/17 Outbursts

by Mauri Valtonen, Stanislaw Zola, Helen Jermak, Stefano Ciprini, Rene Hudec, Lankeswar Dey, Achamveedu Gopakumar, Daniel L. Reichart, Daniel B. Caton, Kosmas Gazeas, Katsura Matsumoto, Waldemar Ogloza, Marek Drozdz, Fahri Alicavus, Oleksandr Baransky, Andrei Berdyugin, Panos Boumis, Yurii Bufan, Bartlomiej Debski, Huseyin Er, Ahmet Erdem, Vira Godunova, Shirin Haque, Vivian L. Hoette, Jan Janik, Mark Kidger, Tomasz Kundera, Sebastian Kurowski, Alexis Liakos, Isa Mohammed, Kari Nilsson, Urszula Pajdosz, Vilppu Piirola, Tapio Pursimo, Brandon Rajkumar, Andrii O. Simon, Michal Siwak, Eda Sonbas, Ian A. Steele, Volodymir V. Vasylenko, Michal Zejmo and Pawel Zielinski Show full author list Hide full author list

Galaxies 2017, 5(4), 83; https://doi.org/10.3390/galaxies5040083 - 20 Nov 2017

Cited by 24 | Viewed by 6108

Abstract

We report optical photometric and polarimetric observations of the blazar OJ 287 gathered during 2016/17. The high level of activity, noticed after the General Relativity Centenary flare, is argued to be part of the follow-up flares that exhibited high levels of polarization and [...] Read more.

We report optical photometric and polarimetric observations of the blazar OJ 287 gathered during 2016/17. The high level of activity, noticed after the General Relativity Centenary flare, is argued to be part of the follow-up flares that exhibited high levels of polarization and originated in the primary black hole jet. We propose that the follow-up flares were induced as a result of accretion disk perturbations, travelling from the site of impact towards the primary SMBH. The timings inferred from our observations allowed us to estimate the propagation speed of these perturbations. Additionally, we make predictions for the future brightness of OJ 287. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

973 KiB

Open AccessFeature PaperArticle

Circular Polarization in Turbulent Blazar Jets

by Nicholas Roy MacDonald

Galaxies 2017, 5(4), 82; https://doi.org/10.3390/galaxies5040082 - 20 Nov 2017

Cited by 1 | Viewed by 3292

Abstract

Circular polarization (CP) provides an invaluable probe into the underlying plasma content of relativistic jets. CP can be generated within the jet through a physical process known as linear birefringence. This is a physical mechanism through which initially linearly polarized emission produced in [...] Read more.

Circular polarization (CP) provides an invaluable probe into the underlying plasma content of relativistic jets. CP can be generated within the jet through a physical process known as linear birefringence. This is a physical mechanism through which initially linearly polarized emission produced in one region of the jet is attenuated by Faraday rotation as it passes through other regions of the jet with distinct magnetic field orientations. Marscher developed the turbulent extreme multi-zone (TEMZ) model of blazar emission which mimics these types of magnetic geometries with collections of thousands of plasma cells passing through a standing conical shock. I have recently developed a radiative transfer algorithm to generate synthetic images of the time-dependent circularly polarized intensity emanating from the TEMZ model at different radio frequencies. In this study, we produce synthetic multi-epoch observations that highlight the temporal variability in the circular polarization produced by the TEMZ model. We also explore the effect that different plasma compositions within the jet have on the resultant levels of CP. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

434 KiB

Open AccessFeature PaperArticle

Full-Stokes, Multi-Frequency Radio Polarimetry of Fermi Blazars; Monitoring and Modelling

by Emmanouil Angelakis, Ioannis Myserlis and J. Anton Zensus

Galaxies 2017, 5(4), 81; https://doi.org/10.3390/galaxies5040081 - 20 Nov 2017

Cited by 3 | Viewed by 3236

Abstract

The polarised emission from active galactic nuclei (AGN) jets carries information about the physical conditions at the emitting plasma elements, while its temporal evolution probes the physical processes that introduce variability and dynamically modify the local conditions. Here we present the analysis of [...] Read more.

The polarised emission from active galactic nuclei (AGN) jets carries information about the physical conditions at the emitting plasma elements, while its temporal evolution probes the physical processes that introduce variability and dynamically modify the local conditions. Here we present the analysis of multi-frequency radio linear and circular polarisation datasets with the aim of exactly quantifying the conditions in blazar jets. Our analysis includes both the careful treatment of observational datasets and numerical modelling for the reproduction of synthetic polarisation curves that can be compared to the observed ones. In our approach, the variability is attributed to traveling shocks. The emission from the cells of our jet model is computed with radiative transfer of all Stokes parameters. The model also accounts for Faraday effects which map the low-energy particle populations. We present two extreme cases in terms of the significance of Faraday conversion in the production of circular polarisation. As we show, in both regimes the model gives a realistic reproduction of the observed emission. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

236 KiB

Open AccessArticle

Polarized Emission from Gamma-Ray Burst Jets

by Shiho Kobayashi

Galaxies 2017, 5(4), 80; https://doi.org/10.3390/galaxies5040080 - 17 Nov 2017

Cited by 2 | Viewed by 3377

Abstract

I review how polarization signals have been discussed in the research field of Gamma-Ray Bursts (GRBs). I mainly discuss two subjects in which polarimetry enables us to study the nature of relativistic jets. (1) Jet breaks: Gamma-ray bursts are produced in ultra-relativistic jets. [...] Read more.

I review how polarization signals have been discussed in the research field of Gamma-Ray Bursts (GRBs). I mainly discuss two subjects in which polarimetry enables us to study the nature of relativistic jets. (1) Jet breaks: Gamma-ray bursts are produced in ultra-relativistic jets. Due to the relativistic beaming effect, the emission can be modeled in a spherical model at early times. However, as the jet gradually slows down, we begin to see the edge of the jet together with polarized signals at some point. (2) Optical flash: later time afterglow is known to be insensitive to the properties of the original ejecta from the GRB central engine. However, a short-lived, reverse shock emission would enable us to study the nature of of GRB jets. I also briefly discuss the recent detection of optical circular polarization in GRB afterglow. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

1424 KiB

Open AccessFeature PaperArticle

Synchrotron Radiation Maps from Relativistic MHD Jet Simulations

by Dimitrios Millas, Oliver Porth and Rony Keppens

Galaxies 2017, 5(4), 79; https://doi.org/10.3390/galaxies5040079 - 15 Nov 2017

Viewed by 2953

Abstract

Relativistic jets from active galactic nuclei (AGN) often display a non-uniform structure and are, under certain conditions, susceptible to a number of instabilities. An interesting example is the development of non-axisymmetric, Rayleigh-Taylor type instabilities in the case of differentially rotating two-component jets, with [...] Read more.

Relativistic jets from active galactic nuclei (AGN) often display a non-uniform structure and are, under certain conditions, susceptible to a number of instabilities. An interesting example is the development of non-axisymmetric, Rayleigh-Taylor type instabilities in the case of differentially rotating two-component jets, with the toroidal component of the magnetic field playing a key role in the development or suppression of these instabilities. We have shown that higher magnetization leads to stability against these non-axisymmetric instabilities. Using ray-casting on data from relativistic MHD simulations of two-component jets, we now investigate the effect of these instabilities on the synchrotron emission pattern from the jets. We recover many well known trends from actual observations, e.g., regarding the polarization fraction and the distribution of the position angle of the electric field, in addition to a different emitting region, depending on the stability of the jet. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

772 KiB

Open AccessFeature PaperArticle

Reheating via Gravitational Particle Production in Simple Models of Quintessence or ΛCDM Inflation

by Jaume De Haro and Llibert Aresté Saló

Galaxies 2017, 5(4), 78; https://doi.org/10.3390/galaxies5040078 - 14 Nov 2017

Cited by 1 | Viewed by 2586

Abstract

We have tested some simple

Λ

CDM (the same test is also valid for quintessence) inflation models, imposing that they match with the recent observational data provided by the BICEP and Planck’s team and leading to a reheating temperature, which is obtained via [...] Read more.

We have tested some simple

Λ

CDM (the same test is also valid for quintessence) inflation models, imposing that they match with the recent observational data provided by the BICEP and Planck’s team and leading to a reheating temperature, which is obtained via gravitational particle production after inflation, supporting the nucleosynthesis success. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

2841 KiB

Open AccessArticle

Coordinated Micro-Variability CIRCE Polarimetry and SARA JKT Multi-Frequency Photometry Observations of the Blazar S5 0716+71

by James R. Webb, Stephen Eikenberry, Yigit Dalliar, Nankun Meng and Douglas H. Laurence

Galaxies 2017, 5(4), 77; https://doi.org/10.3390/galaxies5040077 - 13 Nov 2017

Cited by 4 | Viewed by 3600

Abstract

A critical observation for testing the KRM jet model, in development at FIU, is to observe high time resolution, high accuracy photometry and polarimetry over a wide range of frequencies in the optically thin portion of the synchrotron spectrum. The detection of micro-variability [...] Read more.

A critical observation for testing the KRM jet model, in development at FIU, is to observe high time resolution, high accuracy photometry and polarimetry over a wide range of frequencies in the optically thin portion of the synchrotron spectrum. The detection of micro-variability during these observations would be ideal so the background and flaring components could be separated. Target of opportunity H-band photometry and polarimetry observations on the Gran Telescopio Canarias (GTC) 10.4-m with the Canarias InfraRed Camera Experiment (CIRCE) instrument were made in conjunction with the Southeastern Association for Research in Astronomy (SARA) JKT observing in the optical VRI bands in order to test the model. Here we present simultaneous micro-variability observations of Blazar S5 0716+71 made on 14 April 2017, with the CIRCE instrument on the GTC 10.4-m telescope and optical observations made with the 1.0-m SARA JKT in La Palma. The CIRCE observations consisted of high time resolution polarimetric observations in the H band over a period of 2.4 h on source, measuring both the H-band flux and the polarization degree and angle. Simultaneous observations with the SARA JKT 1.0-m yielded VRI light curves with about three minute time resolution over ∼4 h. 0716+71 showed only small amounts of variability during the observation. We present here the resulting data and a comparison to previous observations. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

► Show Figures

Figure 1

854 KiB

Open AccessArticle

Gravitational Lens Time Delays Using Polarization Monitoring

by Andrew Biggs and Ian Browne

Galaxies 2017, 5(4), 76; https://doi.org/10.3390/galaxies5040076 - 13 Nov 2017

Cited by 1 | Viewed by 3098

Abstract

Gravitational lens time delays provide a means of measuring the expansion of the Universe at high redshift (and therefore in the ‘Hubble flow’) that is independent of local calibrations. It was hoped that many of the radio lenses found in the JVAS/CLASS survey [...] Read more.

Gravitational lens time delays provide a means of measuring the expansion of the Universe at high redshift (and therefore in the ‘Hubble flow’) that is independent of local calibrations. It was hoped that many of the radio lenses found in the JVAS/CLASS survey would yield time delays as these were selected to have flat spectra and are dominated by multiple compact components. However, despite extensive monitoring with the Very Large Array (VLA), time delays have only been measured for three of these systems (out of 22). We have begun a programme to reanalyse the existing VLA monitoring data with the goal of producing light curves in polarized flux and polarization position angle, either to improve delay measurements or to find delays for new sources. Here, we present preliminary results on the lens system B1600+434 which demonstrate the presence of correlated and substantial polarization variability in each image. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

1173 KiB

Open AccessFeature PaperArticle

The University of Michigan Centimeter-Band All Stokes Blazar Monitoring Program: Single-Dish Polarimetry as a Probe of Parsec-Scale Magnetic Fields

by Margo F. Aller, Hugh D. Aller and Philip A. Hughes

Galaxies 2017, 5(4), 75; https://doi.org/10.3390/galaxies5040075 - 13 Nov 2017

Cited by 14 | Viewed by 3362

Abstract

The University of Michigan 26-m paraboloid was dedicated to obtaining linear polarization and total flux density observations of blazars from the mid-1960s until June 2012, providing an unprecedented record tracking centimeter-band variability over decades at 14.5, 8.0, and 4.8 GHz for both targeted [...] Read more.

The University of Michigan 26-m paraboloid was dedicated to obtaining linear polarization and total flux density observations of blazars from the mid-1960s until June 2012, providing an unprecedented record tracking centimeter-band variability over decades at 14.5, 8.0, and 4.8 GHz for both targeted objects and members of flux-limited samples. In the mid-1970s through the mid-1980s, and during the last decade of the program, observations were additionally obtained of circular polarization for a small sample of radio-bright (S > 5 Jy), active sources. Key program results include evidence supporting class-dependent differences in the magnetic field geometry of BL Lac and QSO jets, identification of linear polarization changes temporally associated with flux outbursts supporting a shock-in-jet scenario, and determination of the spectral evolution of the Stokes V amplitude and polarity for testing proposed models. Recent radiative transfer modeling during large flares supports a jet scenario with a kinetically-dominated, relativistic flow at parsec scales with embedded turbulent magnetic fields and dynamically-weak ordered components which may be helical; the circular polarization observations are consistent with linear-to-circular mode conversion within this turbulent jet environment. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

► Show Figures

Figure 1

399 KiB

Open AccessFeature PaperArticle

Modified Newtonian Gravity as an Alternative to the Dark Matter Hypothesis

by Luis Acedo

Galaxies 2017, 5(4), 74; https://doi.org/10.3390/galaxies5040074 - 7 Nov 2017

Cited by 8 | Viewed by 5385

Abstract

[-5]The applications of Newtonian dynamics in galactic scales have shown that the inverse square law is incompatible with the amount of visible mass in the form of stars and molecular clouds. This manifests as the rotational curves of galaxies being asymptotically flat instead [...] Read more.

[-5]The applications of Newtonian dynamics in galactic scales have shown that the inverse square law is incompatible with the amount of visible mass in the form of stars and molecular clouds. This manifests as the rotational curves of galaxies being asymptotically flat instead of decaying with the distance to the center of the galaxy. In the context of Newtonian gravity, the standard explanation requires a huge amount of dark mass in the form of hypothetical particles that still remain undetected. A different theory was provided as a modification of Newtonian dynamics (MOND) at low accelerations . This MOND theory still has many supporters and it can easily explain some features of the rotation curves, such as the Tully–Fisher (TF) phenomenological relation between luminosity and velocity. In this paper, we revisit the third approach of a non-Newtonian force, that has resurfaced from time to time, in order to reconcile it with a finite apparent dark mass and the TF relation. Full article

► Show Figures

Figure 1

Journal Menu

Journal Browser

Galaxies, Volume 5, Issue 4 (December 2017) – 48 articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI