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Galaxies, Volume 8, Issue 4 (December 2020) – 19 articles

Cover Story (view full-size image): “Thermodynamic Constraints on the Non-Baryonic Dark Matter Gas Composing Galactic Halos” by Anne M. Hofmeister tests the postulated properties of non-baryonic matter (that NBDM interacts gravitationally with baryonic matter, yet negligibly interacts with photons) against classical physics, which governs gas bodies. Without NBDM–NBDM collisions, the impossible limit of absolute zero can be reached. Collisions with baryons in the intergalactic medium are unavoidable, and would generate light since baryons have finite size and can deform inelastically. The halo must then be detectable. If no thermal energy arises in collisions, NBDM gas would collapse to a tiny, dense volume during any disturbance. NBDM gas should occupy central galactic regions since self-gravitating objects are density stratified. The properties postulated for NBDM in halos would result in a universe unlike that [...] Read more.
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Open AccessArticle
Classification of Planetary Nebulae through Deep Transfer Learning
Galaxies 2020, 8(4), 88; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040088 - 11 Dec 2020
Cited by 1 | Viewed by 597
Abstract
This study investigate the effectiveness of using Deep Learning (DL) for the classification of planetary nebulae (PNe). It focusses on distinguishing PNe from other types of objects, as well as their morphological classification. We adopted the deep transfer learning approach using three ImageNet [...] Read more.
This study investigate the effectiveness of using Deep Learning (DL) for the classification of planetary nebulae (PNe). It focusses on distinguishing PNe from other types of objects, as well as their morphological classification. We adopted the deep transfer learning approach using three ImageNet pre-trained algorithms. This study was conducted using images from the Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory H-alpha Planetary Nebula research platform database (HASH DB) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). We found that the algorithm has high success in distinguishing True PNe from other types of objects even without any parameter tuning. The Matthews correlation coefficient is 0.9. Our analysis shows that DenseNet201 is the most effective DL algorithm. For the morphological classification, we found for three classes, Bipolar, Elliptical and Round, half of objects are correctly classified. Further improvement may require more data and/or training. We discuss the trade-offs and potential avenues for future work and conclude that deep transfer learning can be utilized to classify wide-field astronomical images. Full article
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Open AccessArticle
Development of a Frequency Tunable Green Laser Source for Advanced Virgo+ Gravitational Waves Detector
Galaxies 2020, 8(4), 87; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040087 - 09 Dec 2020
Viewed by 350
Abstract
After one year of data taking, the observing run three (O3), we are preparing for an improved version of the system, named the Advanced Virgo plus. One of the major upgrades will be the installation of the signal recycling mirror to form an [...] Read more.
After one year of data taking, the observing run three (O3), we are preparing for an improved version of the system, named the Advanced Virgo plus. One of the major upgrades will be the installation of the signal recycling mirror to form an additional optical cavity and improve the sensitivity of the interferometer. This also requires a change in the lock acquisition strategy. In particular, the arms will be locked at the beginning with lasers at a different wavelength from the main one. Such a strategy has already been implemented and tested in LIGO and KAGRA, and in this paper we will present how it has been conceived in Virgo. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessArticle
Fine-Tuning the Optical Design of the Advanced Virgo+ Gravitational-Wave Detector Using Binary-Neutron Star Signals
Galaxies 2020, 8(4), 86; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040086 - 08 Dec 2020
Viewed by 321
Abstract
Advanced Virgo+ is a major upgrade of the Advanced Virgo gravitational-wave detector aiming to increase sensitivity in terms of binary neutron star (BNS) range by a factor 3–5 in the next few years. In this work, we present an optimization of the mirror [...] Read more.
Advanced Virgo+ is a major upgrade of the Advanced Virgo gravitational-wave detector aiming to increase sensitivity in terms of binary neutron star (BNS) range by a factor 3–5 in the next few years. In this work, we present an optimization of the mirror transmittances for the second phase of the project (to be implemented for the O5 observation run) using a random walk algorithm implemented with the advGWINC software. In addition to BNS range, a post merger (PM) SNR is also used as a figure of merit to identify configurations that fine-tune the sensitivity curve, as a function of arm-cavity round trip losses. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessArticle
Interferometer Sensing and Control for the Advanced Virgo Experiment in the O3 Scientific Run
Galaxies 2020, 8(4), 85; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040085 - 08 Dec 2020
Viewed by 352
Abstract
Advanced Virgo is a 2nd-generation laser interferometer based in Cascina (Italy) aimed at the detection of gravitational waves (GW) from astrophysical sources. Together with the two USA-based LIGO interferometers they constitute a network which operates in coincidence. The three detectors observed the sky [...] Read more.
Advanced Virgo is a 2nd-generation laser interferometer based in Cascina (Italy) aimed at the detection of gravitational waves (GW) from astrophysical sources. Together with the two USA-based LIGO interferometers they constitute a network which operates in coincidence. The three detectors observed the sky simultaneously during the last part of the second Observing Run (O2) in August 2017, and this led to two paramount discoveries: the first three-detector observation of gravitational waves emitted from the coalescence of a binary black hole system (GW170814), and the first detection ever of gravitational waves emitted from the coalescence of a binary neutron star system (GW170817). Coincident data taking was re-started for the third Observing Run (O3), which started on 1st April 2019 and lasted almost one year. This paper will describe the new techniques implemented for the longitudinal controls with respect to the ones already in use during O2. Then, it will present an extensive description of the full scheme of the angular controls of the interferometer, focusing on the different control strategies that are in place in the different stages of the lock acquisition procedure, which is the complex sequence of operations by which an uncontrolled, “free” laser interferometer is brought to the final working point, which allows the detector to reach the best sensitivity. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessArticle
Advanced LIGO Laser Systems for O3 and Future Observation Runs
Galaxies 2020, 8(4), 84; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040084 - 08 Dec 2020
Viewed by 433
Abstract
The advanced LIGO gravitational wave detectors need high power laser sources with excellent beam quality and low-noise behavior. We present a pre-stabilized laser system with 70 W of output power that was used in the third observing run of the advanced LIGO detectors. [...] Read more.
The advanced LIGO gravitational wave detectors need high power laser sources with excellent beam quality and low-noise behavior. We present a pre-stabilized laser system with 70 W of output power that was used in the third observing run of the advanced LIGO detectors. Furthermore, the prototype of a 140 W pre-stabilized laser system for future use in the LIGO observatories is described and characterized. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessArticle
Anomalous Sun Flyby of 1I/2017 U1 (`Oumuamua)
Galaxies 2020, 8(4), 83; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040083 - 07 Dec 2020
Viewed by 609
Abstract
The findings of Micheli et al. (Nature2018, 559, 223–226) that 1I/2017 U1 (`Oumuamua) showed anomalous orbital accelerations have motivated us to apply an impact model of gravity in search for an explanation. A small deviation from the 1/ [...] Read more.
The findings of Micheli et al. (Nature2018, 559, 223–226) that 1I/2017 U1 (`Oumuamua) showed anomalous orbital accelerations have motivated us to apply an impact model of gravity in search for an explanation. A small deviation from the 1/r potential, where r is the heliocentric distance, is expected for the gravitational interaction of extended bodies as a consequence of this model. This modification of the potential results from an offset of the effective gravitational centre from the geometric centre of a spherically symmetric body. Applied to anomalous Earth flybys, the model accounts for energy gains relative to an exact Kepler orbit and an increased speed of several spacecraft. In addition, the flat rotation profiles of eight disk galaxies could be explained, as well as the anomalous perihelion advances of the inner planets and the asteroid Icarus. The solution in the case of `Oumuamua is also based on the proposal that the offset leads to an approach and flyby trajectory different from a Kepler orbit without postulating cometary activity. As a consequence, an adjustment of the potential and centrifugal orbital energies can be envisaged outside the narrow uncertainty ranges of the published post-perihelion data without a need to re-analyse the original data. The observed anomalous acceleration has been modelled with respect to the orbit solutions JPL 16 and “Pseudo-MPEC” for 1I/`Oumuamua. Full article
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Open AccessArticle
The Hunt for Environmental Noise in Virgo during the Third Observing Run
Galaxies 2020, 8(4), 82; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040082 - 07 Dec 2020
Cited by 2 | Viewed by 495
Abstract
The first twenty years of operation of gravitational-wave interferometers have shown that these detectors are affected by physical disturbances from the surrounding environment. These are seismic, acoustic, or electromagnetic disturbances that are mainly produced by the experiment infrastructure itself. Ambient noise can limit [...] Read more.
The first twenty years of operation of gravitational-wave interferometers have shown that these detectors are affected by physical disturbances from the surrounding environment. These are seismic, acoustic, or electromagnetic disturbances that are mainly produced by the experiment infrastructure itself. Ambient noise can limit the interferometer sensitivity or potentially generate transients of non-astrophysical origin. Between 1 April 2019 and 27 March 2020, the network of second generation interferometers—LIGO, Virgo and GEO—performed the third joined observing run, named O3, searching for gravitational signals from the deep universe. A thorough investigation has been done on each detector before and during data taking in order to optimize its sensitivity and duty cycle. In this paper, we first revisit typical sources of environmental noise and their coupling paths, and we then describe investigation methods and tools. Finally, we illustrate applications of these methods in the hunt for environmental noise at the Virgo interferometer during the O3 run and its preparation phase. In particular, we highlight investigation techniques that might be useful for the next observing runs and the future generation of terrestrial interferometers. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessConcept Paper
Modulated Differential Wavefront Sensing: Alignment Scheme for Beams with Large Higher Order Mode Content
Galaxies 2020, 8(4), 81; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040081 - 03 Dec 2020
Viewed by 396
Abstract
Modulated differential wavefront sensing (MDWS) is an alignment control scheme in the regime of beams with strong higher order transversal modes (HOMs). It is based on the differential wavefront sensing (DWS) technique. MDWS represents a significant upgrade over conventional techniques used in the [...] Read more.
Modulated differential wavefront sensing (MDWS) is an alignment control scheme in the regime of beams with strong higher order transversal modes (HOMs). It is based on the differential wavefront sensing (DWS) technique. MDWS represents a significant upgrade over conventional techniques used in the presence of high HOM content as it allows for higher control bandwidths while eliminating the need of auxiliary alignment modulations, that otherwise cause loss of applied squeezing. The output port of gravitational wave (GW) interferometers (IFO) is one such place where a lot of HOMs are present. These are filtered out by a cavity called the output mode cleaner (OMC), whose alignment gets challenging due to the presence of HOMs. In this paper, we present the first demonstration of the MDWS scheme for aligning the fundamental mode from the IFO to the OMC at the gravitational wave detector-GEO 600. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessArticle
Temperature Control for an Intra-Mirror Etalon in Interferometric Gravitational Wave Detector Fabry–Perot Cavities
Galaxies 2020, 8(4), 80; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040080 - 01 Dec 2020
Viewed by 332
Abstract
The sensitivity of interferometric gravitational wave detectors is optimized, in part, by balanced finesse in the long Fabry–Perot arm cavities. The input test mass mirrors of Advanced Virgo feature parallel faces, which creates an etalon within the substrate, adding variability in the total [...] Read more.
The sensitivity of interferometric gravitational wave detectors is optimized, in part, by balanced finesse in the long Fabry–Perot arm cavities. The input test mass mirrors of Advanced Virgo feature parallel faces, which creates an etalon within the substrate, adding variability in the total mirror reflectivity, in order to correct imbalanced finesse due to manufacturing tolerances. Temperature variations in mirror substrate change the optical path length primarily through varying the index of refraction and are tuned to correct for a finesse imbalance of up to 2.8% by a full etalon fringe of 0.257 K. A negative feedback control system was designed to control the mirror temperature by using an electrical resistive heating belt actuator for a heat transfer process modeled as a two-pole plant. A zero controller filter was designed which achieves temperature control within 2.3% of the etalon fringe and recovers to within 10% of the working point within 32 hours after a step input of one etalon fringe. A preliminary unlock condition control designed to compensate when the interferometer unlocks shows that the control remains stable even after a drastic change in the plant due to the absence of the laser heating. Further improvements to the control must also consider the full heat transfer mechanisms by using modern control state space models. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessFeature PaperArticle
The Squeezed Light Source for the Advanced Virgo Detector in the Observation Run O3
Galaxies 2020, 8(4), 79; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040079 - 26 Nov 2020
Viewed by 512
Abstract
From 1 April 2019 to 27 March 2020, the Advanced Virgo detector, together with the two Advanced LIGO detectors, conducted the third joint scientific observation run O3, aiming for further detections of gravitational wave signals from astrophysical sources. One of the upgrades to [...] Read more.
From 1 April 2019 to 27 March 2020, the Advanced Virgo detector, together with the two Advanced LIGO detectors, conducted the third joint scientific observation run O3, aiming for further detections of gravitational wave signals from astrophysical sources. One of the upgrades to the Virgo detector for O3 was the implementation of the squeezed light technology to improve the detector sensitivity beyond its classical quantum shot noise limit. In this paper, we present a detailed description of the optical setup and performance of the employed squeezed light source. The squeezer was constructed as an independent, stand-alone sub-system operated in air. The generated squeezed states are tailored to exhibit high purity at intermediate squeezing levels in order to significantly reduce the interferometer shot noise level while keeping the correlated enhancement of quantum radiation pressure noise just below the actual remaining technical noise in the Advanced Virgo detector. Full article
(This article belongs to the Special Issue Gravitational Wave Detectors)
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Open AccessReview
Study of Eclipsing Binaries: Light Curves & O-C Diagrams Interpretation
Galaxies 2020, 8(4), 78; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040078 - 13 Nov 2020
Viewed by 481
Abstract
The continuous improvement in observational methods of eclipsing binaries, EBs, yield more accurate data, while the development of their light curves, that is magnitude versus time, analysis yield more precise results. Even so, and in spite the large number of EBs and the [...] Read more.
The continuous improvement in observational methods of eclipsing binaries, EBs, yield more accurate data, while the development of their light curves, that is magnitude versus time, analysis yield more precise results. Even so, and in spite the large number of EBs and the huge amount of observational data obtained mainly by space missions, the ways of getting the appropriate information for their physical parameters etc. is either from their light curves and/or from their period variations via the study of their (O-C) diagrams. The latter express the differences between the observed, O, and the calculated, C, times of minimum light. Thus, old and new light curves analysis methods of EBs to obtain their principal parameters will be considered, with examples mainly from our own observational material, and their subsequent light curves analysis using either old or new methods. Similarly, the orbital period changes of EBs via their (O-C) diagrams are referred to with emphasis on the use of continuous methods for their treatment in absence of sudden or abrupt events. Finally, a general discussion is given concerning these two topics as well as to a few related subjects. Full article
Open AccessArticle
Thermodynamic Constraints on the Non-Baryonic Dark Matter Gas Composing Galactic Halos
Galaxies 2020, 8(4), 77; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040077 - 08 Nov 2020
Viewed by 789
Abstract
To explain rotation curves of spiral galaxies through Newtonian orbital models, massive halos of non-baryonic dark matter (NBDM) are commonly invoked. The postulated properties are that NBDM interacts gravitationally with baryonic matter, yet negligibly interacts with photons. Since halos are large, low-density gaseous [...] Read more.
To explain rotation curves of spiral galaxies through Newtonian orbital models, massive halos of non-baryonic dark matter (NBDM) are commonly invoked. The postulated properties are that NBDM interacts gravitationally with baryonic matter, yet negligibly interacts with photons. Since halos are large, low-density gaseous bodies, their postulated attributes can be tested against classical thermodynamics and the kinetic theory of gas. Macroscopic models are appropriate because these make few assumptions. NBDM–NBDM collisions must be elastic to avoid the generation of light, but this does not permit halo gas temperature to evolve. If no such collisions exist, then the impossible limit of absolute zero would be attainable since the other available energy source, radiation, does not provide energy to NBDM. The alternative possibility, an undefined temperature, is also inconsistent with basic thermodynamic principles. However, a definable temperature could be attained via collisions with baryons in the intergalactic medium since these deliver kinetic energy to NBDM. In this case, light would be produced since some proportion of baryon collisions are inelastic, thereby rendering the halo detectable. Collisions with baryons are unavoidable, even if NBDM particles are essentially point masses. Note that <0.0001 × the size of a proton is needed to avoid scattering with γ-rays, the shortest wavelength used to study halos. If only elastic collisions exist, NBDM gas would collapse to a tiny, dense volume (zero volume for point masses) during a disturbance—e.g., cosmic rays. NBDM gas should occupy central galactic regions, not halos, since self-gravitating objects are density stratified. In summary, properties of NBDM halos as postulated would result in violations of thermodynamic laws and in a universe unlike that observed. Full article
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Open AccessArticle
Magnetized Particle Motion in γ-Spacetime in a Magnetic Field
Galaxies 2020, 8(4), 76; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040076 - 29 Oct 2020
Cited by 3 | Viewed by 544
Abstract
In the present work we explored the dynamics of magnetized particles around the compact object in γ-spacetime in the presence of an external asymptotically-uniform magnetic field. The analysis of the circular orbits of magnetized particles around the compact object in the spacetime [...] Read more.
In the present work we explored the dynamics of magnetized particles around the compact object in γ-spacetime in the presence of an external asymptotically-uniform magnetic field. The analysis of the circular orbits of magnetized particles around the compact object in the spacetime of a γ-object immersed in the external magnetic field has shown that the area of stable circular orbits of magnetized particles increases with the increase of γ-parameter. We have also investigated the acceleration of the magnetized particles near the γ-object and shown that the center-of-mass energy of colliding magnetized particles increases with the increase of γ-parameter. Finally, we have applied the obtained results to the astrophysical scenario and shown that the values of γ-parameter in the range of γ(0.5,1) can mimic the spin of Kerr black hole up to a0.85, while the magnetic interaction can mimic the γ-parameter at γ(0.8,1) and spin of a Kerr black hole up to a0.3. Full article
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Open AccessArticle
Asteroseismic Analysis of δ Scuti Components of Binary Systems: The Case of KIC 8504570
Galaxies 2020, 8(4), 75; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040075 - 28 Oct 2020
Cited by 1 | Viewed by 418
Abstract
The present work concerns the Asteroseismology of the Kepler-detached eclipsing binary KIC 8504570. Particularly, it focuses on the pulsational behaviour of the oscillating component of this system and the estimation of its physical parameters in order to [...] Read more.
The present work concerns the Asteroseismology of the Kepler-detached eclipsing binary KIC 8504570. Particularly, it focuses on the pulsational behaviour of the oscillating component of this system and the estimation of its physical parameters in order to enrich the so far poor sample of systems of this kind. Using spectroscopic observations, the spectral type of the primary component was determined and used to create accurate light curve models and estimate its absolute parameters. The light curve residuals were subsequently analysed using Fourier transformation techniques to obtain the pulsation models. Theoretical models of δ Scuti stars were employed to identify the oscillation modes of the six detected independent frequencies of the pulsator. In addition, more than 385 combination frequencies were also detected. The absolute and the pulsational properties of the δ Scuti star of this system are discussed and compared with all the currently known similar cases. Moreover, using a recent(empirical) luminosity–pulsation period relationship for δ Scuti stars, the distance of the system was estimated. Full article
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Open AccessFeature PaperArticle
Imprint of Pressure on Characteristic Dark Matter Profiles: The Case of ESO0140040
Galaxies 2020, 8(4), 74; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040074 - 22 Oct 2020
Cited by 1 | Viewed by 481
Abstract
We investigate the dark matter distribution in the spiral galaxy ESO0140040, employing the most widely used density profiles: the pseudo-isothermal, exponential sphere, Burkert, Navarro-Frenk-White, Moore and Einasto profiles. We infer the model parameters and estimate the total dark matter [...] Read more.
We investigate the dark matter distribution in the spiral galaxy ESO0140040, employing the most widely used density profiles: the pseudo-isothermal, exponential sphere, Burkert, Navarro-Frenk-White, Moore and Einasto profiles. We infer the model parameters and estimate the total dark matter content from the rotation curve data. For simplicity, we assume that dark matter distribution is spherically symmetric without accounting for the complex structure of the galaxy. Our predictions are compared with previous results and the fitted parameters are statistically confronted for each profile. We thus show that although one does not include the galaxy structure it is possible to account for the same dynamics assuming that dark matter provides a non-zero pressure in the Newtonian approximation. In this respect, we solve the hydrostatic equilibrium equation and construct the dark matter pressure as a function for each profile. Consequently, we discuss the dark matter equation of state and calculate the speed of sound in dark matter. Furthermore, we interpret our results in view of our approach and we discuss the role of the refractive index as an observational signature to discriminate between our approach and the standard one. Full article
(This article belongs to the Special Issue Dark Cosmology: Shedding Light on Our Current Universe)
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Open AccessBrief Report
Equiaffine Braneworld
Galaxies 2020, 8(4), 73; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040073 - 21 Oct 2020
Viewed by 375
Abstract
Higher dimensional theories, wherein our four dimensional universe is immersed into a bulk ambient, have received much attention recently, and the directions of investigation had, as far as we can discern, all followed the ordinary Euclidean hypersurface theory’s isometric immersion recipe, with the [...] Read more.
Higher dimensional theories, wherein our four dimensional universe is immersed into a bulk ambient, have received much attention recently, and the directions of investigation had, as far as we can discern, all followed the ordinary Euclidean hypersurface theory’s isometric immersion recipe, with the spacetime metric being induced by an ambient parent. We note, in this paper, that the indefinite signature of the Lorentzian metric perhaps hints at the lesser known equiaffine hypersurface theory as being a possibly more natural, i.e., less customized beyond minimal mathematical formalism, description of our universe’s extrinsic geometry. In this alternative, the ambient is deprived of a metric, and the spacetime metric becomes conformal to the second fundamental form of the ordinary theory, therefore is automatically indefinite for hyperbolic shapes. Herein, we advocate investigations in this direction by identifying some potential physical benefits to enlisting the help of equiaffine differential geometry. In particular, we show that a geometric origin for dark energy can be proposed within this framework. Full article
(This article belongs to the Special Issue Dark Cosmology: Shedding Light on Our Current Universe)
Open AccessFeature PaperReview
Leptonic and Hadronic Radiative Processes in Supermassive-Black-Hole Jets
Galaxies 2020, 8(4), 72; https://doi.org/10.3390/galaxies8040072 - 01 Oct 2020
Cited by 1 | Viewed by 885
Abstract
Supermassive black holes lying in the center of galaxies can launch relativistic jets of plasma along their polar axis. The physics of black-hole jets is a very active research topic in astrophysics, owing to the fact that many questions remain open on the [...] Read more.
Supermassive black holes lying in the center of galaxies can launch relativistic jets of plasma along their polar axis. The physics of black-hole jets is a very active research topic in astrophysics, owing to the fact that many questions remain open on the physical mechanisms of jet launching, of particle acceleration in the jet, and on the radiative processes. In this work I focus on the last item, and present a review of the current understanding of radiative emission processes in supermassive-black-hole jets. Full article
(This article belongs to the Special Issue Jet Physics of Accreting Super Massive Black Holes)
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Open AccessArticle
Unraveling the Physics of Quasar Jets: Optical Polarimetry and Implications for the X-ray Emission Process
Galaxies 2020, 8(4), 71; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040071 - 27 Sep 2020
Cited by 1 | Viewed by 520
Abstract
Since the launch of Chandra twenty years ago, one of the greatest mysteries surrounding Quasar Jets is the production mechanism for their extremely high X-ray luminosity. Two mechanisms have been proposed. In the first view, the X-ray emission is inverse-Comptonized CMB photons. This [...] Read more.
Since the launch of Chandra twenty years ago, one of the greatest mysteries surrounding Quasar Jets is the production mechanism for their extremely high X-ray luminosity. Two mechanisms have been proposed. In the first view, the X-ray emission is inverse-Comptonized CMB photons. This view requires a jet that is highly relativistic (bulk Lorentz factor >20–40) on scales of hundreds of kiloparsecs, and a jet that is comparably or more powerful than the black hole’s Eddington luminosity. The second possibility is synchrotron emission from a high-energy population of electrons. This requires a much less powerful jet that does not need to be relativistically beamed, but it imposes other extreme requirements, namely the need to accelerate particles to >100 TeV energies at distances of hundreds of kiloparsecs from the active nucleus. We are exploring these questions using a suite of observations from a diverse group of telescopes, including the Hubble Space Telescope (HST), Chandra X-ray Observatory (CXO), Fermi Gamma-ray Space Telescope and various radio telescope arrays. Our results strongly favor the hypothesis that the X-ray emission is synchrotron radiation from a separate, high-energy electron population. We discuss the observations, results and new questions brought up by these surprising results. We investigate the physical processes and magnetic field structure that may help to accelerate particles to such extreme energies. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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Open AccessArticle
Variable Magellanic HMXB Sources versus Variable ULX Sources: Nothing to Brag about the ULX Sources
Galaxies 2020, 8(4), 70; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies8040070 - 24 Sep 2020
Viewed by 578
Abstract
We carry out a meta-analysis of ultraluminous X-ray (ULX) sources that show large variabilities (by factors of >10) between their highest and lowest emission states in the X-ray energy range of 0.3–10 keV. We are guided by a recent stringent compilation [...] Read more.
We carry out a meta-analysis of ultraluminous X-ray (ULX) sources that show large variabilities (by factors of >10) between their highest and lowest emission states in the X-ray energy range of 0.3–10 keV. We are guided by a recent stringent compilation of 25 such X-ray sources by Song et al. We examine the relation of logN versus logSmax, where N is the number of sources radiating above the maximum-flux level Smax. We find a strong deviation from all previously determined slopes in various high-mass X-ray binary (HMXB) samples. In fact, the ULX data clearly show a slope of 0.91. Thus, ULX sources do not appear to be uniform and isotropic in our Universe. We compare the ULX results against the local X-ray luminosity function of HMXBs in the Small Magellanic Cloud (SMC) constructed from our latest library that includes 41 Chandra 0.3–8 keV sources and 56 XMM-Newton 0.2–12 keV sources. The ULX data are not drawn from the same continuous distribution as the SMC data (the ULX data peak at the low tails of the SMC distributions), and none of our data sets is drawn from a normal distribution or from a log-normal distribution (they all show marked excesses at both tails). At a significance level of α=0.05 (2σ), the two-sample p-value of the Kolmogorov–Smirnov (KS) test gives p=4.7×103<α for the ULX versus the small Chandra sample and p=1.1×105<<α for the ULX versus the larger XMM-Newton sample, respectively. This adds to the evidence that ULX sources are not simply the higher end of the known local Be/X-ray pulsar distribution, but they represent a class of X-ray sources different from the young sources found in the SMC and in individual starburst galaxies. On the other hand, our two main SMC data sets are found to be statistically consistent, as they are drawn from the same continuous parent distribution (null hypothesis H0): at the α=0.05 significance level, the two-sample KS test shows an asymptotic p-value of 0.308>α, which tells us to accept H0. Full article
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