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Galaxies, Volume 5, Issue 4 (December 2017)

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Open AccessArticle Exoplanet Predictions Based on Harmonic Orbit Resonances
Galaxies 2017, 5(4), 56; doi:10.3390/galaxies5040056
Received: 2 June 2017 / Revised: 13 September 2017 / Accepted: 13 September 2017 / Published: 25 September 2017
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Abstract
The current exoplanet database includes 5454 confirmed and candidate planets observed with the Kepler mission. We find 932 planet pairs from which we extract distance and orbital period ratios. While earlier studies used a logarithmic spacing, which lacks a physical model, we employ
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The current exoplanet database includes 5454 confirmed and candidate planets observed with the Kepler mission. We find 932 planet pairs from which we extract distance and orbital period ratios. While earlier studies used a logarithmic spacing, which lacks a physical model, we employ here the theory of harmonic orbit resonances, which contains quantized ratios instead, to explain the observed planet distance ratios and to predict undetected exoplanets. We find that the most prevailing harmonic ratios are (2:1), (3:2), and (5:3) in 73% of the cases, while alternative harmonic ratios of (5:4), (4:3), (5:2), and (3:1) occur in the other 27% of the cases. Our orbital predictions include 171 exoplanets, 2 Jupiter moons, 1 Saturn moon, 3 Uranus moons, and 4 Neptune moons. The accuracy of the predicted planet distances amounts to a few percent, which fits the data significantly better than the logarithmic spacing. This information may be useful for targeted exoplanet searches with Kepler data and to estimate the number of live-carrying planets in habitable zones. Full article
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Open AccessFeature PaperArticle On the Time Variable Rotation Measure in the Core Region of Markarian 421
Galaxies 2017, 5(4), 57; doi:10.3390/galaxies5040057
Received: 6 September 2017 / Revised: 18 September 2017 / Accepted: 20 September 2017 / Published: 26 September 2017
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Abstract
In this conference contribution, we discuss and interpret the time variable rotation measure (RM) detected in the core region of the TeV blazar Markarian 421 (Mrk 421). We monitored Mrk 421 during 2011 with one observing run per month at 15, 24, and
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In this conference contribution, we discuss and interpret the time variable rotation measure (RM) detected in the core region of the TeV blazar Markarian 421 (Mrk 421). We monitored Mrk 421 during 2011 with one observing run per month at 15, 24, and 43 GHz with the American Very Long Baseline Array. We explore the possible connection between the RM and the accretion rate, and we investigate the Faraday screen properties and its location with respect to the jet emitting region. Among the various scenarios, the jet sheath is the most promising candidate for being the main source of Faraday rotation. We interpret the RM sign reversals observed during the one-year monitoring within the context of the magnetic tower models by invoking the presence of two nested helical magnetic fields in the relativistic jet with opposite helicities, originating through the Poynting–Robertson cosmic battery effect. The net observed RM values result from the relative contribution of both inner and outer helical fields. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle Microscopic Processes in Global Relativistic Jets Containing Helical Magnetic Fields: Dependence on Jet Radius
Galaxies 2017, 5(4), 58; doi:10.3390/galaxies5040058
Received: 23 August 2017 / Revised: 21 September 2017 / Accepted: 22 September 2017 / Published: 26 September 2017
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Abstract
In this study, we investigate the interaction of jets with their environment at a microscopic level, which is a key open question in the study of relativistic jets. Using small simulation systems during past research, we initially studied the evolution of both electron–proton
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In this study, we investigate the interaction of jets with their environment at a microscopic level, which is a key open question in the study of relativistic jets. Using small simulation systems during past research, we initially studied the evolution of both electron–proton and electron–positron relativistic jets containing helical magnetic fields, by focusing on their interactions with an ambient plasma. Here, using larger jet radii, we have performed simulations of global jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities, such as the Weibel instability, the kinetic Kelvin–Helmholtz instability (kKHI) and the mushroom instability (MI). We found that the evolution of global jets strongly depends on the size of the jet radius. For example, phase bunching of jet electrons, in particular in the electron–proton jet, is mixed with a larger jet radius as a result of the more complicated structures of magnetic fields with excited kinetic instabilities. In our simulation, these kinetic instabilities led to new types of instabilities in global jets. In the electron–proton jet simulation, a modified recollimation occurred, and jet electrons were strongly perturbed. In the electron–positron jet simulation, mixed kinetic instabilities occurred early, followed by a turbulence-like structure. Simulations using much larger (and longer) systems are required in order to further thoroughly investigate the evolution of global jets containing helical magnetic fields. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Hot Gaseous Halos in Early Type Galaxies
Galaxies 2017, 5(4), 60; doi:10.3390/galaxies5040060
Received: 31 July 2017 / Revised: 26 September 2017 / Accepted: 27 September 2017 / Published: 4 October 2017
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Abstract
The hot gas in early type galaxies (ETGs) plays a crucial role in their formation and evolution. As the hot gas is often extended to the outskirts beyond the optical size, the large scale structural features identified by Chandra (including cavities, cold fronts,
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The hot gas in early type galaxies (ETGs) plays a crucial role in their formation and evolution. As the hot gas is often extended to the outskirts beyond the optical size, the large scale structural features identified by Chandra (including cavities, cold fronts, filaments, and tails) point to key evolutionary mechanisms, e.g., AGN feedback, merging history, accretion/stripping, as well as star formation and quenching. We systematically analyze the archival Chandra data of ETGs to study the hot ISM. Using uniformly derived data products with spatially resolved spectral information, we revisit the X-ray scaling relations of ETGs and address their implications by comparing them with those of groups/clusters and simulations. Full article
(This article belongs to the Special Issue On the Origin (and Evolution) of Baryonic Galaxy Halos)
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Open AccessArticle Evidence for Toroidal B-Field Components in AGN Jets on Kiloparsec Scales
Galaxies 2017, 5(4), 61; doi:10.3390/galaxies5040061
Received: 23 August 2017 / Revised: 26 September 2017 / Accepted: 28 September 2017 / Published: 4 October 2017
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Abstract
Though helical magnetic fields are generally believed to arise when the jets of Active Galactic Nuclei (AGN) are launched, it is still unclear what role they play (and if they survive) to the largest jet scales. A helical or toroidal B-field may contribute
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Though helical magnetic fields are generally believed to arise when the jets of Active Galactic Nuclei (AGN) are launched, it is still unclear what role they play (and if they survive) to the largest jet scales. A helical or toroidal B-field may contribute substantially to the collimation of the jet. This B-field structure can be detected in images of the Faraday rotation measure (RM)—a measure of the change in polarisation angle of an electromagnetic wave as it passes through a magneto-ionic medium. The Faraday rotation measure is directly proportional to the line-of-sight magnetic field; therefore a monotonic gradient in the RM transverse to the jet indicates similar behaviour of the line-of-sight B-field component. This type of analysis has mostly been done on parsec scales using VLBI observations at centimetre wavelengths, while relatively few studies have probed decaparsec to kiloparsec scales. The detection of RM gradients with significances of 3 σ or more on such large scales can demonstrate the presence of a toroidal field component, which may be associated with a helical field that has persisted to these distances from the centre of the AGN. We present the results of new Faraday rotation analyses for 2 AGN on kiloparsec scales based on multiwavelength VLA observations, with robust transverse RM gradients detected in both. Furthermore, the direction of the inferred toroidal B-fields on the sky supports previous results indicating a predominance of outward currents in the jets on kiloparsec scales. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Searching for Jet Emission in LMXBs: A Polarimetric View
Galaxies 2017, 5(4), 62; doi:10.3390/galaxies5040062
Received: 25 August 2017 / Revised: 23 September 2017 / Accepted: 25 September 2017 / Published: 7 October 2017
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Abstract
We present results taken from a study aiming at detecting the emission from relativistic particles jets in neutron star-low mass X-ray binaries using optical polarimetric observations. First, we focus on a polarimetric study performed on the persistent LMXB 4U 0614+091. Once corrected for
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We present results taken from a study aiming at detecting the emission from relativistic particles jets in neutron star-low mass X-ray binaries using optical polarimetric observations. First, we focus on a polarimetric study performed on the persistent LMXB 4U 0614+091. Once corrected for interstellar effects, we measured an intrinsic linear polarization in the r-band of ~3% at a 3σ confidence level. This is in-line with the observation of an infrared excess in the spectral energy distribution (SED) of the source, reported in a previous work, which the authors linked to the optically thin synchrotron emission of a jet. We then present a study performed on the transitional millisecond pulsar PSR J1023+0038 during quiescence. We measured a linear polarization of 1.09 ± 0.27% and 0.90 ± 0.17% in the V and R bands, respectively. The phase-resolved polarimetric curve of the source in the R-band reveals a hint of a sinusoidal modulation at the source orbital period. The NIR -optical SED of the system did not suggest the presence of a jet. We conclude that the optical linear polarization observed for PSR J1023+0038 is possibly due to Thomson scattering with electrons in the disc, as also suggested by the hint of the modulation of the R-band linear polarization at the system orbital period. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle Modeling the Time-Dependent Polarization of Blazars
Galaxies 2017, 5(4), 63; doi:10.3390/galaxies5040063
Received: 21 September 2017 / Revised: 6 October 2017 / Accepted: 9 October 2017 / Published: 13 October 2017
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Abstract
Linear polarization is an extremely valuable observational tool for probing the dynamic physical conditions of blazar jets. Some patterns are seen in the data, suggestive of order that can be explained by shock waves and helical magnetic field components. However, much disorder is
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Linear polarization is an extremely valuable observational tool for probing the dynamic physical conditions of blazar jets. Some patterns are seen in the data, suggestive of order that can be explained by shock waves and helical magnetic field components. However, much disorder is apparent, which implies that turbulence plays a major role as well, especially in the fluctuations of flux and polarization, and perhaps particle acceleration. Here, we present some actual flux and polarization versus time data, plus simulations of model jets. We analyze the output of the simulations in a manner that can be compared with observational data. The results suggest that the ratio of turbulent to ordered magnetic fields varies with time. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle A Model of Polarisation Rotations in Blazars from Kink Instabilities in Relativistic Jets
Galaxies 2017, 5(4), 64; doi:10.3390/galaxies5040064
Received: 15 September 2017 / Revised: 28 September 2017 / Accepted: 7 October 2017 / Published: 12 October 2017
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Abstract
This paper presents a simple model of polarisation rotation in optically thin relativistic jets of blazars. The model is based on the development of helical (kink) mode of current-driven instability. A possible explanation is suggested for the observational connection between polarisation rotations and
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This paper presents a simple model of polarisation rotation in optically thin relativistic jets of blazars. The model is based on the development of helical (kink) mode of current-driven instability. A possible explanation is suggested for the observational connection between polarisation rotations and optical/gamma-ray flares in blazars, if the current-driven modes are triggered by secular increases of the total jet power. The importance of intrinsic depolarisation in limiting the amplitude of coherent polarisation rotations is demonstrated. The polarisation rotation amplitude is thus very sensitive to the viewing angle, which appears to be inconsistent with the observational estimates of viewing angles in blazars showing polarisation rotations. Overall, there are serious obstacles to explaining large-amplitude polarisation rotations in blazars in terms of current-driven kink modes. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle High-Sensitivity AGN Polarimetry at Sub-Millimeter Wavelengths
Galaxies 2017, 5(4), 65; doi:10.3390/galaxies5040065
Received: 31 August 2017 / Revised: 17 September 2017 / Accepted: 25 September 2017 / Published: 15 October 2017
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Abstract
The innermost regions of radio loud Active Galactic Nuclei (AGN) jets are heavily affected by synchrotron self-absorption, due to the strong magnetic fields and high particle densities in these extreme zones. The only way to overcome this absorption is to observe at sub-millimeter
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The innermost regions of radio loud Active Galactic Nuclei (AGN) jets are heavily affected by synchrotron self-absorption, due to the strong magnetic fields and high particle densities in these extreme zones. The only way to overcome this absorption is to observe at sub-millimeter wavelengths, although polarimetric observations at such frequencies have so far been limited by sensitivity and calibration accuracy. However, new generation instruments such as the Atacama Large mm/sub-mm Array (ALMA) overcome these limitations and are starting to deliver revolutionary results in the observational studies of AGN polarimetry. Here we present an overview of our state-of-the-art interferometric mm/sub-mm polarization observations of AGN jets with ALMA (in particular, the gravitationally-lensed sources PKS 1830−211 and B0218+359), which allow us to probe the magneto-ionic conditions at the regions closest to the central black holes. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle Radio Polarisation Study of High Rotation Measure AGNs
Galaxies 2017, 5(4), 66; doi:10.3390/galaxies5040066
Received: 14 September 2017 / Revised: 3 October 2017 / Accepted: 6 October 2017 / Published: 16 October 2017
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Abstract
As radio polarised emission from astrophysical objects traverse through foreground magnetised plasma, the physical conditions along the lines of sight are encrypted in the form of rotation measure (RM). We performed broadband spectro-polarimetric observations of high rotation measure (|RM|
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As radio polarised emission from astrophysical objects traverse through foreground magnetised plasma, the physical conditions along the lines of sight are encrypted in the form of rotation measure (RM). We performed broadband spectro-polarimetric observations of high rotation measure ( | RM | 300 rad m 2 ) sources away from the Galactic plane ( | b | > 10 ) selected from the NVSS RM catalogue. The main goals are to verify the NVSS RM values, which could be susceptible to n π -ambiguity, as well as to identify the origin of the extreme RM values. We show that 40 % of our sample suffer from n π -ambiguity in the NVSS RM catalogue. There are also hints of RM variabilities over ∼20 years epoch for most of our sources, as revealed by comparing the RM values of the two studies in the same frequency ranges after correcting for n π -ambiguity. At last, we demonstrate the possibility of applying QU-fitting to study the ambient media of AGNs. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle 3 mm GMVA Observations of Total and Polarized Emission from Blazar and Radio Galaxy Core Regions
Galaxies 2017, 5(4), 67; doi:10.3390/galaxies5040067
Received: 13 September 2017 / Revised: 12 October 2017 / Accepted: 13 October 2017 / Published: 18 October 2017
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Abstract
We present total and linearly polarized 3 mm Global mm-VLBI Array (GMVA; mm-VLBI: Very Long Baseline Interferometry observations at millimetre wavelengths) images of a sample of blazars and radio galaxies from the VLBA-BU-BLAZAR 7 mm monitoring program designed to probe the innermost regions
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We present total and linearly polarized 3 mm Global mm-VLBI Array (GMVA; mm-VLBI: Very Long Baseline Interferometry observations at millimetre wavelengths) images of a sample of blazars and radio galaxies from the VLBA-BU-BLAZAR 7 mm monitoring program designed to probe the innermost regions of active galactic nuclei (AGN) jets and locate the sites of gamma-ray emission observed by the Fermi-LAT. The lower opacity at 3 mm and improved angular resolution—on the order of 50 microarcseconds—allow us to distinguish features in the jet not visible in the 7 mm VLBA data. We also compare two different methods used for the calibration of instrumental polarisation and we analyze the resulting images for some of the sources in the sample. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle An Old Fogey’s History of Radio Jets
Galaxies 2017, 5(4), 68; doi:10.3390/galaxies5040068
Received: 13 September 2017 / Revised: 13 October 2017 / Accepted: 14 October 2017 / Published: 18 October 2017
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Abstract
This paper describes a personal view of the discovery of radio jets in celestial radio sources. The existence of narrow, collimated optical features in distant objects has been known about since the early 20th century; however, the advent of radio astronomy in the
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This paper describes a personal view of the discovery of radio jets in celestial radio sources. The existence of narrow, collimated optical features in distant objects has been known about since the early 20th century; however, the advent of radio astronomy in the 1940s and 1950s revealed the existence of a large number of discrete radio sources. The realization that many of these objects were not primarily stellar or local to our own galaxy, but rather extragalactic, followed the determination of accurate radio positions, enabling identifications with optical objects. High-resolution radio interferometers found that they were often compact, and with a double lobed structure, implying outflow from a central object. Shortly afterwards, accurate techniques for the measurement of polarization were developed. However it was not until the advent of synthesis instruments in the 1970s that radio images of the sources were produced, and the existence of radio jets firmly established and their polarization characteristics found. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle Polarization Monitoring of the Lens System JVAS B0218+357
Galaxies 2017, 5(4), 69; doi:10.3390/galaxies5040069
Received: 15 September 2017 / Revised: 2 October 2017 / Accepted: 6 October 2017 / Published: 18 October 2017
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Abstract
Monitoring of the lens system JVAS B0218+357 with the Fermi Gamma-ray Space Telescope measured a different time delay to that derived from radio observations. We have re-analysed three months of archival Very Large Array data to produce variability curves with an average sampling
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Monitoring of the lens system JVAS B0218+357 with the Fermi Gamma-ray Space Telescope measured a different time delay to that derived from radio observations. We have re-analysed three months of archival Very Large Array data to produce variability curves with an average sampling of one epoch per day in total flux, polarized flux and polarization position angle (PPA) at 15, 8.4 and 5 GHz. The variability is particularly strong in polarized flux. Dense sampling and improved subtraction of the Einstein ring has allowed us to produce superior variability curves and a preliminary analysis has resulted in a time delay (11.5 days) which agrees well with the γ -ray value. Both images of 0218+357 are subject to strong Faraday rotation and depolarization as a result of the radio waves passing through the interstellar medium of the spiral lens galaxy. Our data reveal frequency-dependent variations in the PPA that are different in each image and which must therefore result from variable Faraday rotation in the lens galaxy on timescales of a few days. Our analysis has revealed systematic errors in the polarization position angle measurements that strongly correlate with hour angle. Although we have been able to correct for these, we caution that all VLA polarization observations are potentially affected. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Automated Polarimetry with Smaller Aperture Telescopes: The ROVOR Observatory
Galaxies 2017, 5(4), 70; doi:10.3390/galaxies5040070
Received: 13 September 2017 / Revised: 9 October 2017 / Accepted: 11 October 2017 / Published: 23 October 2017
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Abstract
To better understand possible blazar jet mechanisms and morphologies, brighter prototypical objects are regularly monitored for variability in optical broad-band light. If the monitoring filters are polarized, the position angles and polarization percentages can be measured and their evolution monitored over time. However,
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To better understand possible blazar jet mechanisms and morphologies, brighter prototypical objects are regularly monitored for variability in optical broad-band light. If the monitoring filters are polarized, the position angles and polarization percentages can be measured and their evolution monitored over time. However, building up a statistically significant time base of polarization parameters requires the arduous task of monitoring sources for months or years to catch and follow interesting events such as flares. Fortunately, monitoring an object is easily done using remotely operated or robotic telescopes. The Remote Observatory for Variable Object Research (ROVOR) is a small-aperture telescope that has monitored blazars in broad-band Johnson filters since 2009. Calibration data using a set of four plane-polarized filters suggest that it is suitable for polarimetric monitoring as well. We have successfully collected data on CTA 102 and are encouraged at the prospects of monitoring it and other similar objects. Long-term monitoring campaigns are a scientifically and educationally-effective use of underutilized smaller-aperture telescopes. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
Open AccessArticle Electric Currents along Astrophysical Jets
Galaxies 2017, 5(4), 71; doi:10.3390/galaxies5040071
Received: 14 July 2017 / Revised: 23 October 2017 / Accepted: 23 October 2017 / Published: 25 October 2017
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Abstract
Astrophysical black holes and their surrounding accretion disks are believed to be threaded by grand design helical magnetic fields. There is strong theoretical evidence that the main driver of their winds and jets is the Lorentz force generated by these fields and their
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Astrophysical black holes and their surrounding accretion disks are believed to be threaded by grand design helical magnetic fields. There is strong theoretical evidence that the main driver of their winds and jets is the Lorentz force generated by these fields and their associated electric currents. Several researchers have reported direct evidence for large scale electric currents along astrophysical jets. Quite unexpectedly, their directions are not random as would have been the case if the magnetic field were generated by a magnetohydrodynamic dynamo. Instead, in all kpc-scale detections, the inferred electric currents are found to flow away from the galactic nucleus. This unexpected break of symmetry suggests that a battery mechanism is operating around the central black hole. In the present article, we summarize observational evidence for the existence of large scale electric currents and their associated grand design helical magnetic fields in kpc-scale astrophysical jets. We also present recent results of general relativistic radiation magnetohydrodynamic simulations which show the action of the Cosmic Battery in the vicinity of astrophysical black holes. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Linear Polarimetry with γe+e Conversions
Galaxies 2017, 5(4), 72; doi:10.3390/galaxies5040072
Received: 25 September 2017 / Revised: 22 October 2017 / Accepted: 25 October 2017 / Published: 1 November 2017
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Abstract
γ-rays are emitted by cosmic sources by non-thermal processes that yield either non-polarized photons, such as those from π0 decay in hadronic interactions, or linearly polarized photons from synchrotron radiation and the inverse-Compton up-shifting of these on high-energy charged particles. Polarimetry
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γ -rays are emitted by cosmic sources by non-thermal processes that yield either non-polarized photons, such as those from π 0 decay in hadronic interactions, or linearly polarized photons from synchrotron radiation and the inverse-Compton up-shifting of these on high-energy charged particles. Polarimetry in the MeV energy range would provide a powerful tool to discriminate among “leptonic” and “hadronic” emission models of blazars, for example, but no polarimeter sensitive above 1 MeV has ever been flown into space. Low-Z converter telescopes such as silicon detectors are developed to improve the angular resolution and the point-like sensitivity below 100 MeV. We have shown that in the case of a homogeneous, low-density active target such as a gas time-projection chamber (TPC), the single-track angular resolution is even better and is so good that in addition the linear polarimetry of the incoming radiation can be performed. We actually characterized the performance of a prototype of such a telescope on beam. Track momentum measurement in the tracker would enable calorimeter-free, large effective area telescopes on low-mass space missions. An optimal unbiased momentum estimate can be obtained in the tracker alone based on the momentum dependence of multiple scattering, from a Bayesian analysis of the innovations of Kalman filters applied to the tracks. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Radiative Signatures of Parsec-Scale Magnetised Jets
Galaxies 2017, 5(4), 73; doi:10.3390/galaxies5040073
Received: 18 September 2017 / Revised: 1 November 2017 / Accepted: 2 November 2017 / Published: 6 November 2017
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Abstract
Relativistic jets are launched from the immediate vicinity of black holes and can reach kilo-parsec scales. During their evolution from the smallest to the largest scales, they encounter different physical conditions (e.g., ambient configurations and magnetic fields) which can modify their morphology and
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Relativistic jets are launched from the immediate vicinity of black holes and can reach kilo-parsec scales. During their evolution from the smallest to the largest scales, they encounter different physical conditions (e.g., ambient configurations and magnetic fields) which can modify their morphology and dynamics. Using state-of-the-art relativistic magneto-hydrodynamical simulations and ray-tracing algorithms, we model the dynamics of jets along with their radiation microphysics, investigating the impact of magnetisation on the jet dynamics and the observed emission. During the post-processing procedure we account for the properties of the observing array (sparse uv-plane) and the imaging algorithm, enabling a more direct comparison between simulations and ground-or space-based very-long-baseline interferometry (VLBI) observations. The different jet models can be distinguished from the reconstructed radio images. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Modified Newtonian Gravity as an Alternative to the Dark Matter Hypothesis
Galaxies 2017, 5(4), 74; doi:10.3390/galaxies5040074
Received: 19 September 2017 / Revised: 30 October 2017 / Accepted: 1 November 2017 / Published: 7 November 2017
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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
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[-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
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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
Galaxies 2017, 5(4), 75; doi:10.3390/galaxies5040075
Received: 7 September 2017 / Revised: 31 October 2017 / Accepted: 8 November 2017 / Published: 13 November 2017
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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
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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)
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Open AccessArticle Gravitational Lens Time Delays Using Polarization Monitoring
Galaxies 2017, 5(4), 76; doi:10.3390/galaxies5040076
Received: 15 September 2017 / Revised: 25 October 2017 / Accepted: 8 November 2017 / Published: 13 November 2017
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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
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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)
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Open AccessArticle Coordinated Micro-Variability CIRCE Polarimetry and SARA JKT Multi-Frequency Photometry Observations of the Blazar S5 0716+71
Galaxies 2017, 5(4), 77; doi:10.3390/galaxies5040077
Received: 6 October 2017 / Revised: 25 October 2017 / Accepted: 26 October 2017 / Published: 13 November 2017
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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
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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)
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Open AccessFeature PaperArticle Reheating via Gravitational Particle Production in Simple Models of Quintessence or ΛCDM Inflation
Galaxies 2017, 5(4), 78; doi:10.3390/galaxies5040078
Received: 11 October 2017 / Revised: 9 November 2017 / Accepted: 10 November 2017 / Published: 14 November 2017
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Abstract
We have tested some simple ΛCDM (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
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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)
Open AccessFeature PaperArticle Synchrotron Radiation Maps from Relativistic MHD Jet Simulations
Galaxies 2017, 5(4), 79; doi:10.3390/galaxies5040079
Received: 15 September 2017 / Revised: 8 November 2017 / Accepted: 10 November 2017 / Published: 15 November 2017
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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
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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)
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Open AccessArticle Polarized Emission from Gamma-Ray Burst Jets
Galaxies 2017, 5(4), 80; doi:10.3390/galaxies5040080
Received: 18 September 2017 / Revised: 4 November 2017 / Accepted: 14 November 2017 / Published: 17 November 2017
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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.
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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)
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Open AccessFeature PaperArticle Full-Stokes, Multi-Frequency Radio Polarimetry of Fermi Blazars; Monitoring and Modelling
Galaxies 2017, 5(4), 81; doi:10.3390/galaxies5040081
Received: 31 August 2017 / Revised: 10 November 2017 / Accepted: 15 November 2017 / Published: 20 November 2017
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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
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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)
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Open AccessFeature PaperArticle Circular Polarization in Turbulent Blazar Jets
Galaxies 2017, 5(4), 82; doi:10.3390/galaxies5040082
Received: 15 September 2017 / Revised: 14 November 2017 / Accepted: 15 November 2017 / Published: 20 November 2017
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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
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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)
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Open AccessFeature PaperArticle Polarization and Spectral Energy Distribution in OJ 287 during the 2016/17 Outbursts
Galaxies 2017, 5(4), 83; doi:10.3390/galaxies5040083
Received: 29 August 2017 / Revised: 8 November 2017 / Accepted: 9 November 2017 / Published: 20 November 2017
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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
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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)
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Open AccessFeature PaperArticle The Jets of Microquasars during Giant Flares and Quiet State
Galaxies 2017, 5(4), 84; doi:10.3390/galaxies5040084
Received: 2 October 2017 / Revised: 9 November 2017 / Accepted: 13 November 2017 / Published: 21 November 2017
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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+61303. We have detected many giant flares from
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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+61 303. 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+61 303. 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)
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Open AccessFeature PaperArticle Intra-Night Variability of OJ 287 with Long-Term Multiband Optical Monitoring
Galaxies 2017, 5(4), 85; doi:10.3390/galaxies5040085
Received: 28 October 2017 / Revised: 17 November 2017 / Accepted: 20 November 2017 / Published: 22 November 2017
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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 VRI-bands. The flux variations and colour-magnitude variations on long and short timescales were investigated
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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)
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Open AccessFeature PaperArticle The Giant Flares of the Microquasar Cygnus X-3: X-Rays States and Jets
Galaxies 2017, 5(4), 86; doi:10.3390/galaxies5040086
Received: 18 September 2017 / Revised: 20 November 2017 / Accepted: 21 November 2017 / Published: 27 November 2017
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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
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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)
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Open AccessFeature PaperArticle Identification of the OGLE Blazars behind the Large and Small Magellanic Clouds
Galaxies 2017, 5(4), 88; doi:10.3390/galaxies5040088
Received: 30 October 2017 / Revised: 11 November 2017 / Accepted: 22 November 2017 / Published: 28 November 2017
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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
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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)
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Open AccessArticle Implications of Geometry and the Theorem of Gauss on Newtonian Gravitational Systems and a Caveat Regarding Poisson’s Equation
Galaxies 2017, 5(4), 89; doi:10.3390/galaxies5040089
Received: 30 August 2017 / Revised: 29 September 2017 / Accepted: 21 November 2017 / Published: 29 November 2017
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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
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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
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Open AccessArticle Long-Term Multi-Band and Polarimetric View of Mkn 421: Motivations for an Integrated Open-Data Platform for Blazar Optical Polarimetry
Galaxies 2017, 5(4), 90; doi:10.3390/galaxies5040090
Received: 18 September 2017 / Revised: 7 November 2017 / Accepted: 27 November 2017 / Published: 30 November 2017
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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
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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)
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Open AccessFeature PaperArticle Behaviour of the Blazar CTA 102 during Two Giant Outbursts
Galaxies 2017, 5(4), 91; doi:10.3390/galaxies5040091
Received: 23 September 2017 / Revised: 22 November 2017 / Accepted: 27 November 2017 / Published: 30 November 2017
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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
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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)
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Open AccessFeature PaperArticle Multi-Frequency VLBA Polarimetry and the Twin-Jet Quasar 0850+581
Galaxies 2017, 5(4), 92; doi:10.3390/galaxies5040092
Received: 9 October 2017 / Revised: 21 November 2017 / Accepted: 28 November 2017 / Published: 30 November 2017
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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
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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 17 . 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)
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Open AccessFeature PaperArticle Linear Polarization Properties of Parsec-Scale AGN Jets
Galaxies 2017, 5(4), 93; doi:10.3390/galaxies5040093
Received: 6 October 2017 / Revised: 21 November 2017 / Accepted: 27 November 2017 / Published: 1 December 2017
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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
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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)
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Open AccessFeature PaperArticle Multi-Band Intra-Night Optical Variability of BL Lacertae
Galaxies 2017, 5(4), 94; doi:10.3390/galaxies5040094
Received: 30 October 2017 / Revised: 1 December 2017 / Accepted: 5 December 2017 / Published: 8 December 2017
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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
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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)
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Open AccessFeature PaperArticle Decoding Galactic Merger Histories
Galaxies 2017, 5(4), 95; doi:10.3390/galaxies5040095
Received: 30 June 2017 / Revised: 30 November 2017 / Accepted: 1 December 2017 / Published: 8 December 2017
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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.
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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)
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Open AccessArticle Exploring String Axions with Gravitational Waves
Galaxies 2017, 5(4), 96; doi:10.3390/galaxies5040096
Received: 9 November 2017 / Revised: 4 December 2017 / Accepted: 7 December 2017 / Published: 11 December 2017
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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
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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 = 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)
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Open AccessFeature PaperArticle Magnetic Field Studies in BL Lacertae through Faraday Rotation and a Novel Astrometric Technique
Galaxies 2017, 5(4), 97; doi:10.3390/galaxies5040097
Received: 25 September 2017 / Revised: 5 December 2017 / Accepted: 5 December 2017 / Published: 12 December 2017
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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)
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Open AccessFeature PaperArticle Cosmological Effects of Quantum Vacuum Condensates
Galaxies 2017, 5(4), 98; doi:10.3390/galaxies5040098
Received: 22 October 2017 / Revised: 5 December 2017 / Accepted: 6 December 2017 / Published: 12 December 2017
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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
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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)
Open AccessFeature PaperArticle Bell Inequality and Its Application to Cosmology
Galaxies 2017, 5(4), 99; doi:10.3390/galaxies5040099
Received: 9 November 2017 / Revised: 9 December 2017 / Accepted: 11 December 2017 / Published: 12 December 2017
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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
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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)
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Open AccessFeature PaperArticle Suborbital Fermi/LAT Analysis of the Brightest Gamma-Ray Flare of Blazar 3C 454.3
Galaxies 2017, 5(4), 100; doi:10.3390/galaxies5040100
Received: 31 October 2017 / Revised: 21 November 2017 / Accepted: 1 December 2017 / Published: 12 December 2017
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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
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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)
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Open AccessFeature PaperArticle Explicit Second Partial Derivatives of the Ferrers Potential
Galaxies 2017, 5(4), 101; doi:10.3390/galaxies5040101 (registering DOI)
Received: 3 November 2017 / Revised: 4 December 2017 / Accepted: 5 December 2017 / Published: 15 December 2017
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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
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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

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Open AccessFeature PaperConference Report Constraints on the Formation of M31’s Stellar Halo from the SPLASH Survey
Galaxies 2017, 5(4), 59; doi:10.3390/galaxies5040059
Received: 16 August 2017 / Revised: 12 September 2017 / Accepted: 13 September 2017 / Published: 30 September 2017
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Abstract
The SPLASH (Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo) Survey has observed fields throughout M31’s stellar halo, dwarf satellites, and stellar disk. The observations and derived measurements have either been compared to predictions from simulations of stellar halo formation or modeled directly
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The SPLASH (Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo) Survey has observed fields throughout M31’s stellar halo, dwarf satellites, and stellar disk. The observations and derived measurements have either been compared to predictions from simulations of stellar halo formation or modeled directly in order to derive inferences about the formation and evolution of M31’s stellar halo. We summarize some of the major results from the SPLASH survey and the resulting implications for our understanding of the build-up of M31’s stellar halo. Full article
(This article belongs to the Special Issue On the Origin (and Evolution) of Baryonic Galaxy Halos)
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Open AccessFeature PaperConference Report The Mimetic Born-Infeld Gravity: The Primordial Cosmos and Spherically Symmetric Solutions
Galaxies 2017, 5(4), 87; doi:10.3390/galaxies5040087
Received: 31 October 2017 / Revised: 14 November 2017 / Accepted: 15 November 2017 / Published: 27 November 2017
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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
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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)
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