Open AccessConference Report
Interstellar Reddening Effect on the Age Dating of Population II Stars
Galaxies 2017, 5(3), 28; doi:10.3390/galaxies5030028 -
Abstract
The age measurement of the stellar halo component of the Galaxy is based mainly on the comparison of the main sequence turn-off luminosity of the globular cluster (GC) stars with theoretical isochrones. The standard procedure includes a vertical shift, in order to account
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The age measurement of the stellar halo component of the Galaxy is based mainly on the comparison of the main sequence turn-off luminosity of the globular cluster (GC) stars with theoretical isochrones. The standard procedure includes a vertical shift, in order to account for the distance and extinction to the cluster, and a horizontal one, to compensate the reddening. However, the photometry is typically performed with broad-band filters where the shape of the stellar spectra introduces a shift of the effective wavelength response of the system, dependent on the effective temperature (or color index) of the star. The result is an increasing distortion—actually a rotation and a progressive compression with the temperature—of the color-magnitude diagrams relatively to the standard unreddened isochrones, with increasing reddening. This effect is usually negligible for reddening E(B-V) on the order of or smaller than 0.15, but it can be quite relevant at larger extinction values. While the ratio of the absorption to the reddening is widely discussed in the literature, the importance of the latter effect is often overlooked. In this contribution, we present isochron simulations and discuss the expected effects on age dating of high-reddening globular clusters. Full article
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Open AccessArticle
Assembly Pathways and the Growth of Massive Early-Type Galaxies
Galaxies 2017, 5(2), 27; doi:10.3390/galaxies5020027 -
Abstract
Based on data from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, I present results on the assembly pathways, dark matter content and halo growth of massive early-type galaxies. Using galaxy starlight information we find that such galaxies had an early dissipative
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Based on data from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, I present results on the assembly pathways, dark matter content and halo growth of massive early-type galaxies. Using galaxy starlight information we find that such galaxies had an early dissipative phase followed by a second phase of halo growth from largely minor mergers (and in rare cases major mergers). Thus our result fits in well with the two-phase scenario of galaxy formation. We also used globular cluster radial velocities to measure the enclosed mass within 5 effective radii. The resulting dark matter fractions reveal a few galaxies with very low dark matter fractions that are not captured in the latest cosmological models. Multiple solutions are possible, but none yet is convincing. Translating dark matter fractions into epochs of halo assembly, we show that low mass galaxies tend to grow via gas-rich accretion, while high mass galaxies grow via gas-poor mergers. Full article
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Open AccessConference Report
The SLUGGS Survey: Understanding Lenticular Galaxy Formation via Extended Stellar Kinematics
Galaxies 2017, 5(2), 26; doi:10.3390/galaxies5020026 -
Abstract
We present the latest published and preliminary results from the SLUGGS Survey discussing the formation of lenticular galaxies through analysis of their kinematics. These include a comparison of the measured stellar spin of low-mass lenticular galaxies to the spin of remnant galaxies formed
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We present the latest published and preliminary results from the SLUGGS Survey discussing the formation of lenticular galaxies through analysis of their kinematics. These include a comparison of the measured stellar spin of low-mass lenticular galaxies to the spin of remnant galaxies formed by binary merger simulations to assess whether a merger is a likely formation mechanism for these galaxies. We determine that while a portion of lenticular galaxies have properties consistent with these remnants, others are not, indicating that they are likely “faded spirals”. We also present a modified version of the spin–ellipticity diagram, which utilises radial tracks to be able to identify galaxies with intermediate-scale discs. Such galaxies often have conflicting morphological classifications, depending on whether photometric or kinematic measurements are used. Finally, we present preliminary results on the total mass density profile slopes of lenticular galaxies to assess trends as lower stellar masses are probed. Full article
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Open AccessArticle
A Left and Right Truncated Schechter Luminosity Function for Quasars
Galaxies 2017, 5(2), 25; doi:10.3390/galaxies5020025 -
Abstract
The luminosity function for quasars (QSOs) is usually fitted by a Schechter function. The dependence of the number of quasars on the redshift, both in the low and high luminosity regions, requires the inclusion of a lower and upper boundary in the Schechter
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The luminosity function for quasars (QSOs) is usually fitted by a Schechter function. The dependence of the number of quasars on the redshift, both in the low and high luminosity regions, requires the inclusion of a lower and upper boundary in the Schechter function. The normalization of the truncated Schechter function is forced to be the same as that for the Schechter function, and an analytical form for the average value is derived. Three astrophysical applications for QSOs are provided: deduction of the parameters at low redshifts, behavior of the average absolute magnitude at high redshifts, and the location (in redshift) of the photometric maximum as a function of the selected apparent magnitude. The truncated Schechter function with the double power law and an improved Schechter function are compared as luminosity functions for QSOs. The chosen cosmological framework is that of the flat cosmology, for which we provided the luminosity distance, the inverse relation for the luminosity distance, and the distance modulus. Full article
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Open AccessConference Report
Dissecting Halo Components in IFU Data
Galaxies 2017, 5(2), 24; doi:10.3390/galaxies5020024 -
Abstract
While most astronomers are now familiar with tools to decompose images into multiple components such as disks, bulges, and halos, the equivalent techniques for spectral data cubes are still in their infancy. This is unfortunate, as integral field unit (IFU) spectral surveys are
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While most astronomers are now familiar with tools to decompose images into multiple components such as disks, bulges, and halos, the equivalent techniques for spectral data cubes are still in their infancy. This is unfortunate, as integral field unit (IFU) spectral surveys are now producing a mass of data in this format, which we are ill-prepared to analyze effectively. We have therefore been developing new tools to separate out components using this full spectral data. The results of such analyses will prove invaluable in determining not only whether such decompositions have an astrophysical significance, but, where they do, also in determining the relationship between the various elements of a galaxy. Application to a pilot study of IFU data from the cD galaxy NGC 3311 confirms that the technique can separate the stellar halo from the underlying galaxy in such systems, and indicates that, in this case, the halo is older and more metal poor than the galaxy, consistent with it forming from the cannibalism of smaller satellite galaxies. The success of the method bodes well for its application to studying the larger samples of cD galaxies that IFU surveys are currently producing. Full article
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Open AccessEditorial
A Conference on the Origin (and Evolution) of Baryonic Galaxy Halos
Galaxies 2017, 5(2), 23; doi:10.3390/galaxies5020023 -
Abstract
A conference was held in March 2017 in the Galapagos Islands on the topic of The Origin (and Evolution) of Baryonic Galaxy Halos. It attracted some 120 researchers from around the world. They presented 68 talks (nine of which were invited) and 30
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A conference was held in March 2017 in the Galapagos Islands on the topic of The Origin (and Evolution) of Baryonic Galaxy Halos. It attracted some 120 researchers from around the world. They presented 68 talks (nine of which were invited) and 30 posters over five days. A novel element of the talk schedule was that participants were asked which talks they wanted to hear and the schedule was made up based on their votes and those of the Scientific Organizing Committee SOC . The final talk schedule had 34% of the talks given by women. An emphasis was given to discussion time directly after each talk. Combined with limited/no access to the internet, this resulted in high level of engagement and lively discussions. A prize was given to the poster voted the best by participants. A free afternoon included organized excursions to see the local scenery and wildlife of the Galapagos (e.g., the giant tortoises). Four public talks were given, in Spanish, for the local residents of the town. A post-conference survey was conducted, with most participants agreeing that the conference met their scientific needs and helped to initiate new research directions. Although it was challenging to organize such a large international meeting in such an isolated location as the Galapagos Islands (and much credit goes to the Local Organizing Committee LOC and staff of Quito Astronomical Observatory for their logistical efforts, organizing the meeting for over a year), it was very much a successful conference. We hope it will play a small part in further developing astronomy in Ecuador. Full article
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Open AccessArticle
The Disk-Driven Jet of Cygnus A
Galaxies 2017, 5(2), 22; doi:10.3390/galaxies5020022 -
Abstract
Recently published results from VLBI observations at 3 and 7 millimeters of the radio galaxy Cygnus A are reviewed in this article, and discussed within the model of a prominently stratified jet outflow. At the source redshift (z= 0.056), mm-VLBI allows
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Recently published results from VLBI observations at 3 and 7 millimeters of the radio galaxy Cygnus A are reviewed in this article, and discussed within the model of a prominently stratified jet outflow. At the source redshift (z= 0.056), mm-VLBI allows a spatial resolution down to 200 Schwarzschild radii to be achieved, providing an extremely detailed view of the two-sided jet base. Through a study of the kinematic properties of the flow and of its transverse structure, it is shown that the radio emission is produced by an accelerating, mildly relativistic, parabolically expanding disk-wind. The observed transverse stratification, both of the flux density and of the bulk speed, supports the presence of an invisible faster spine close to the jet axis, powered either by the inner regions of the accretion disk or by the spinning black hole. Full article
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Open AccessArticle
The Signature of the Blandford-Znajek Mechanism in GRB Light Curves
Galaxies 2017, 5(2), 21; doi:10.3390/galaxies5020021 -
Abstract
In 1977, Blandford and Znajek showed how the spin energy of a rotating black hole may be extracted electromagnetically through a magnetic field that threads the black hole horizon. A characteristic feature of this mechanism is that, under certain fairly general conditions, the
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In 1977, Blandford and Znajek showed how the spin energy of a rotating black hole may be extracted electromagnetically through a magnetic field that threads the black hole horizon. A characteristic feature of this mechanism is that, under certain fairly general conditions, the energy loss rate decays exponentially. We looked precisely for such behavior in the X-ray light curves of Long and Ultra Long duration Gamma-Ray Bursts (GRBs) observed with the XRT instrument on board the Swift satellite, and found that almost 30% of XRT light curves show an exponential decay before they reach the afterglow plateau. A similar behavior (Fast Rise Exponential Decay-FRED) was observed in γ-rays with the BATSE instrument aboard the CGRO satellite. We consider both of these findings as the signature of the Blandford-Znajek mechanism in action in the central engine of GRBs. Full article
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Open AccessReview
A Search for Blazar-Like Radio-Loud Narrow-Line Seyfert 1 Galaxies
Galaxies 2017, 5(1), 20; doi:10.3390/galaxies5010020 -
Abstract
We report the results of an observational program to investigate the gamma-ray and optical variability properties of the vRL NLSY1 galaxies listed in the Yuan et al. sample. We have identified 17 members of the Yuan et al. sample possibly associated with gamma-ray
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We report the results of an observational program to investigate the gamma-ray and optical variability properties of the vRL NLSY1 galaxies listed in the Yuan et al. sample. We have identified 17 members of the Yuan et al. sample possibly associated with gamma-ray sources based on a combination of their optical polarization and optical variability and their gamma-ray properties. Eight have previously been associated with gamma-ray sources. We find nine additional members that we predict are excellent candidates to be associated with gamma-ray sources in the future. All 17 sources have many properties in common with flat spectrum radio quasars (FSRQs), suggesting that they may, in fact, constitute a new subclass of FSRQs. Full article
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Open AccessArticle
Gaussian Processes for Blazar Variability Studies
Galaxies 2017, 5(1), 19; doi:10.3390/galaxies5010019 -
Abstract
This article briefly introduces Gaussian processes as a new approach for modelling time series in the field of blazar physics. In the second part of the paper, recent results from an application of GP modelling to the multi-wavelength light curves of the blazar
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This article briefly introduces Gaussian processes as a new approach for modelling time series in the field of blazar physics. In the second part of the paper, recent results from an application of GP modelling to the multi-wavelength light curves of the blazar PKS 1502+106 are discussed. Full article
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Open AccessArticle
Small Scale Problems of the ΛCDM Model: A Short Review
Galaxies 2017, 5(1), 17; doi:10.3390/galaxies5010017 -
Abstract
The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems
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The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. ΛCDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the ΛDCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the ΛCDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe. Full article
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Open AccessArticle
Long-Term Monitoring of Bright Blazars in the Multi-GeV to TeV Range with FACT
Galaxies 2017, 5(1), 18; doi:10.3390/galaxies5010018 -
Abstract
Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable,
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Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable, with changing activity levels on timescales between minutes, months, and even years. Several questions are part of the current research, such as the question of the emission regions or the engine of the AGN and the particle acceleration. A dedicated longterm monitoring program is necessary to investigate the properties of blazars in detail. A densely sampled and unbiased light curve allows for observation of both high and low states of the sources, and the combination with multi-wavelength observation could contribute to the answer of several questions mentioned above. FACT (First G-APD Cherenkov Telescope) is the first operational telescope using silicon photomultiplier (SiPM, also known as Geigermode—Avalanche Photo Diode, G-APD) as photon detectors. SiPM have a very homogenous and stable longterm performance, and allow operation even during full moon without any filter, leading to a maximal duty cycle for an Imaging Air Cherenkov Telescope (IACT). Hence, FACT is an ideal device for such a longterm monitoring of bright blazars. A small set of sources (e.g., Markarian 421, Markarian 501, 1ES 1959+650, and 1ES 2344+51.4) is currently being monitored. In this contribution, the FACT telescope and the concept of longterm monitoring of bright blazars will be introduced. The results of the monitoring program will be shown, and the advantages of densely sampled and unbiased light curves will be discussed. Full article
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Open AccessArticle
Extended Radio Emission in the Perhipheral Regions of the Shapley Concentration Core
Galaxies 2017, 5(1), 16; doi:10.3390/galaxies5010016 -
Abstract
The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the
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The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the very early stages of merger, fairly massive clusters with ongoing accretion activity, and smaller groups located in filaments. These features make the SC an ideal place to observe the signatures of the formation of large-scale structures in the Universe. As a matter of fact, the SC has been observed over a broad range of frequencies with the most important observatories, allowing for a unique multiband study. In this paper, we will present new results from an ongoing study of the Shapley Concentration Core, which is being carried out with the Giant Metrewave Radio Telescope (GMRT). Our work confirms the role played by radio observations in disentangling the details of the accretion and merging processes, and delivers a wealth of information in regions well outside the cluster cores. In particular, we will report on the discovery of a relic in the region between the two clusters A 3558 and A 3562, and of the radio properties of the brightest galaxy in the peripheral cluster A 3556. Full article
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Open AccessArticle
Black Hole Accretion in Gamma Ray Bursts
Galaxies 2017, 5(1), 15; doi:10.3390/galaxies5010015 -
Abstract
We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of
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We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on 14 September 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 s and appeared about 0.4 s after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnant’s matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole. Full article
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Open AccessArticle
The Statistical Study of the Daily Fermi Light Curves of 130 Sources
Galaxies 2017, 5(1), 14; doi:10.3390/galaxies5010014 -
Abstract
Blazars show rapid and high amplitude variability. It is interesting to question what kind of process the variability corresponds to. Maybe it is a result of the instability of the accretion flows. In this work, Fermi daily light curves of 130 sources are
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Blazars show rapid and high amplitude variability. It is interesting to question what kind of process the variability corresponds to. Maybe it is a result of the instability of the accretion flows. In this work, Fermi daily light curves of 130 sources are analyzed, and the distributions of daily variability are compared by using a Kolmogorov–Smirnov (K-S) test. The results can be summarized as follows:(1) in most cases, the distributions are not Gaussian; (2) some pairs of the distributions are similar. Full article
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Open AccessArticle
OJ 287 as a Rotating Helix
Galaxies 2017, 5(1), 12; doi:10.3390/galaxies5010012 -
Abstract
We present preliminary data from high-cadence 15-GHz VLBA images of OJ 287 from 1995 to 2015. The ridgelines suggest that the jet is rotating, perhaps with a period of∼30 years. The EVPA of the core rotated by 240° in 2001–2002 and decreased slowly
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We present preliminary data from high-cadence 15-GHz VLBA images of OJ 287 from 1995 to 2015. The ridgelines suggest that the jet is rotating, perhaps with a period of∼30 years. The EVPA of the core rotated by 240° in 2001–2002 and decreased slowly after that. The inner jet apparently moved to a new direction after the rotation,as shown by the emergence of a new component at a new PA at 43 GHz, in 2004. This was presaged by a strong rise in the flux density of the core, and then its sudden fall as the new component was identified. The equivalent sequence of events took place about 5 years later at 15 GHz, but in addition the core EVPA had a step in 2006 and moved to be aligned with the new 43-GHz component. The 15-GHz core became optically thin in 2006, but the angular resolution was insufficient to separate the new component from the core until 2010 Full article
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Open AccessArticle
Study of the Time-Series of Microvariability in Kepler Blazar W2R 1926+42
Galaxies 2017, 5(1), 13; doi:10.3390/galaxies5010013 -
Abstract
One of the remarkable features of blazars is violent variability over a wide wavelength range. The variation mechanism causing the observed complex behavior is still under debate. The variability timescales range from less than a day to decades. Variation on timescales less than
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One of the remarkable features of blazars is violent variability over a wide wavelength range. The variation mechanism causing the observed complex behavior is still under debate. The variability timescales range from less than a day to decades. Variation on timescales less than a day is known as “microvariability.” Such short-term variations can provide insights regarding the origin of the variability after they are distinguished from longer-term variational components. We select about 195 microvariability events from the continuous light curve of blazar W2R 1926+42 with 1-min time resolution obtained by the Kepler spacecraft, and estimate the timescale and amplitude of each event. The rise and decay timescales of the events reveal random variations over short timescales less than a day, but they indicate systematic variations on timescales longer than several days. This result implies that the events are not independent, but rather mutually correlated. Full article
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Open AccessArticle
Area Products for H± in AdS Space
Galaxies 2017, 5(1), 10; doi:10.3390/galaxies5010010 -
Abstract
We derive the thermodynamic products, in particular the area (or entropy) products of H± for a wide variety of black holes (BHs) in anti-de Sitter (AdS) space. We show by explicit and exact calculations that, for this class of BHs, more complicated
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We derive the thermodynamic products, in particular the area (or entropy) products of H± for a wide variety of black holes (BHs) in anti-de Sitter (AdS) space. We show by explicit and exact calculations that, for this class of BHs, more complicated functions of the event horizon area and Cauchy horizon area are indeed mass-independent. This mass-independent results indicate that they could turn out to be a “universal” quantity provided that they depend only on the quantized angular momentum, quantized charges, and cosmological constant, etc. Furthermore, these area (or entropy) product relations for several classes of BHs in AdS space gives us strong indication to understanding the nature of non-extremal BH entropy (both inner and outer) at the microscopic level. Moreover, we compute the famous Cosmic Censorship Inequality (which requires Cosmic-Censorship hypothesis) for these classes of BHs in AdS space. Local thermodynamic stability has been discussed for these BHs and under certain conditions, these classes of BHs displayed second order phase transition. The super-entropic BH does not provide any kind of second order phase transition. Full article
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Open AccessArticle
The Origin and Structure of the Magnetic Fields and Currents of AGN Jets
Galaxies 2017, 5(1), 11; doi:10.3390/galaxies5010011 -
Abstract
This paper reviews observational evidence obtained to date about the overall structure of the magnetic fields in the jets of Active Galactic Nuclei (AGN). Because they are sensitive to the line-of-sight magnetic-field component, Faraday rotation observations of AGN jets provide an effective tool
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This paper reviews observational evidence obtained to date about the overall structure of the magnetic fields in the jets of Active Galactic Nuclei (AGN). Because they are sensitive to the line-of-sight magnetic-field component, Faraday rotation observations of AGN jets provide an effective tool for searching for toroidal jet magnetic fields, whose line-of-sight component changes systematically across the jet. Transverse Faraday rotation measure (RM) gradients providing direct evidence for helical/toroidal magnetic fields have been reliably detected in nearly 40 AGN on parsec scales. Helical magnetic fields are believed to form due to the combined action of the rotation of the central black hole and accretion disk, and these observations demonstrate that at least some of this helical field survives to distances well beyond the Very Long Baseline Interferometry (VLBI) core. Observations of reversals in the direction of the transverse RM gradients in a number of AGN provide evidence for a“return”magnetic field forming a nested helical-field structure with oppositely directed azimuthal components in the inner and outer regions of the helical magnetic field. The collected data now provide firm evidence for a predominance of inward jet currents on parsec scales and outward currents on scales greater than a few tens of parsecs. This suggests a global pattern of magnetic fields and currents with an inward current near the jet axis and an outward current farther from the jet axis, with these currents closing in the accretion disk and far out in the radio lobes, forming a self-consistent set of fields and currents together with the implied nested helical-field structure. Full article
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Open AccessArticle
The Power of (Near) Simultaneous Multi-Frequency Observations for mm-VLBI and Astrometry
Galaxies 2017, 5(1), 9; doi:10.3390/galaxies5010009 -
Abstract
Simultaneous or near-simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the
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Simultaneous or near-simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the signals can be coherently combined, and preventing the use of phase referencing and astrometric measurements. We present two demonstrations of the power of (near) simultaneous multi-frequency observations with the KVN and VLBA, and our recently developed analysis strategies to enable new measurements at mm-VLBI. The first case comprises simultaneous observations at 22, 43, 87 and 130 GHz of a group of five AGNs, the weakest of which is ∼200 mJy at 130 GHz, with angular separations ranging from 3.6 to 11 degrees, using the KVN. We analysed this data using the Frequency Phase Transfer (FPT) and the Source Frequency Phase Referencing (SFPR) techniques, which use the observations at a lower frequency to correct those at a higher frequency. The results of the analysis provide an empirical demonstration of the increase in the coherence times at 130 GHz from a few tens of seconds to about twenty minutes, with FPT, and up to many hours with SFPR. Moreover the astrometric analysis provides high precision relative position measurements between two frequencies, including, for the first time, astrometry at 130 GHz. The second case is a variation of the above, whereby adding dedicated wide-band cm-wavelength observations to measure the ionosphere eliminates the need for a second, calibrator, source. This addresses the scarcity of calibrators at mm-VLBI. We dubbed this technique Multi Frequency Phase Referencing (MFPR). We present bona fide astrometrically aligned VLBA images of BL Lacertae at 22 and 43 GHz using MFPR, which, combined with results from conventional phase referencing at cm-wavelengths, suggests the VLBI core has a recollimation shock that is revealed at mm-wavelengths. These shocks could be responsible for the γ-ray emission in blazar jets. Full article
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