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Galaxies, Volume 10, Issue 5 (October 2022) – 12 articles

Cover Story (view full-size image): Neutron stars are known to withstand very fast rotation. The current record holders, for example, rotate around their axis of rotation more than 700 times per second. At such high rotational frequencies, neutron stars are no longer spherically symmetric objects but may build up mass multipole moments, causing them to begin emitting gravitational waves via the onset of gravitational–radiation reaction (GRR)-driven instabilities. We have calculated the rotational periods at which this happens. They depend on the temperature and mass of a neutron star and are found to be reached before mass shedding from the equator makes the star unstable. This makes neutron stars interesting candidates in the search for sources of gravitational waves. View this paper
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11 pages, 2709 KiB  
Article
Diverse Polarimetric Features of AGN Jets from Various Viewing Angles: Towards a Unified View
by Yuh Tsunetoe, Shin Mineshige, Tomohisa Kawashima, Ken Ohsuga, Kazunori Akiyama and Hiroyuki R. Takahashi
Galaxies 2022, 10(5), 103; https://doi.org/10.3390/galaxies10050103 - 21 Oct 2022
Cited by 1 | Viewed by 1521
Abstract
Here, we demonstrate that polarization properties show a wide diversity depending on viewing angles. To simulate images of a supermassive black hole and surrounding plasma, we performed a full-polarimetric general relativistic radiative transfer based on three-dimensional general relativistic magnetohydrodynamics models with moderate magnetic [...] Read more.
Here, we demonstrate that polarization properties show a wide diversity depending on viewing angles. To simulate images of a supermassive black hole and surrounding plasma, we performed a full-polarimetric general relativistic radiative transfer based on three-dimensional general relativistic magnetohydrodynamics models with moderate magnetic strengths. Under an assumption of a hot-jet and cold-disk in the electron temperature prescription, we confirmed a typical scenario where polarized synchrotron emissions from the funnel jet experience Faraday rotation and conversion in the equatorial disk. Further, we found that linear polarization vectors are inevitably depolarized for edge-on-like observers, whereas a portion of vectors survive and reach the observers in face-on-like cases. We also found that circular polarization components have persistent signs in the face-on cases, and changing signs in the edge-on cases. It is confirmed that these features are smoothly connected via intermediate viewing-angle cases. These results are due to Faraday rotation/conversion for different viewing angles, and suggest that a combination of linear and circular polarimetry can give a constraint on the inclination between the observer and black hole’s (and/or disk’s) rotating-axis and plasma properties in the jet–disk structure. These can also lead to a more statistical and unified interpretation for a diversity of emissions from active galactic nuclei. Full article
(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)
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23 pages, 6794 KiB  
Review
Polarization Observations of AGN Jets: Past and Future
by Jongho Park and Juan Carlos Algaba
Galaxies 2022, 10(5), 102; https://doi.org/10.3390/galaxies10050102 - 20 Oct 2022
Cited by 6 | Viewed by 2417
Abstract
The magnetic field is believed to play a critical role in the bulk acceleration and propagation of jets produced in active galactic nuclei (AGN). Polarization observations of AGN jets provide valuable information about their magnetic fields. As a result of radiative transfer, jet [...] Read more.
The magnetic field is believed to play a critical role in the bulk acceleration and propagation of jets produced in active galactic nuclei (AGN). Polarization observations of AGN jets provide valuable information about their magnetic fields. As a result of radiative transfer, jet structure, and stratification, among other factors, it is not always straightforward to determine the magnetic field structures from observed polarization. We review these effects and their impact on polarization emission at a variety of wavelengths, including radio, optical, and ultraviolet wavelengths in this paper. It is also possible to study the magnetic field in the launching and acceleration regions of AGN jets by using very long baseline interferometry (VLBI), which occurs on a small physical scale. Due to the weak polarization of the jets in these regions, probing the magnetic field is generally difficult. However, recent VLBI observations have detected significant polarization and Faraday rotation in some nearby sources. We present the results of these observations as well as prospects for future observations. Additionally, we briefly discuss recently developed polarization calibration and imaging techniques for VLBI data, which enable more in-depth analysis of the magnetic field structure around supermassive black holes and in AGN jets. Full article
(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)
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27 pages, 7436 KiB  
Article
Assessing the Potential for Liquid Solvents from X-ray Sources: Considerations on Bodies Orbiting Active Galactic Nuclei
by Daniel Rodener, Myriam Schäfer, Michael Hausmann and Georg Hildenbrand
Galaxies 2022, 10(5), 101; https://doi.org/10.3390/galaxies10050101 - 17 Oct 2022
Viewed by 1425
Abstract
We aim to establish a rough first prospect on the potential of certain biorelevant solvents (water, ammonia, and methane) being present in liquid form inside the uppermost few meters of several modeled rocky and icy surfaces of hypothetical bodies orbiting active galactic nuclei [...] Read more.
We aim to establish a rough first prospect on the potential of certain biorelevant solvents (water, ammonia, and methane) being present in liquid form inside the uppermost few meters of several modeled rocky and icy surfaces of hypothetical bodies orbiting active galactic nuclei (AGNs) and investigate under which constraints this might occur. For this, we adjust and average X-ray spectra from a sample of 20 Type-1 Seyfert galaxies to calculate the mean snowline of the sample used. We then vary the hypothetical body’s orbit between 10% and 100% of the snowline radius and calculate a sub-surface attenuation within four different model surface compositions for each. We then use this as a continuous source term for a thermal model. Example bodies are systematically investigated with sizes between 1/30 and 20 earth radii, with further variations also considered (such as possible bound rotation), to end up with a perspective of solvent phases under a wide slew of different conditions. We find that liquid solvents are possible under a multitude of parameters, with temperature being the main constraint to liquid water whereas body size and pressure are the main constraint to liquid methane and ammonia. Full article
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27 pages, 1873 KiB  
Article
Spirality: A Novel Way to Measure Spiral Arm Pitch Angle
by Deanna Shields, Benjamin Boe, Casey Pfountz, Benjamin L. Davis, Matthew Hartley, Ryan Miller, Zac Slade, M. Shameer Abdeen, Daniel Kennefick and Julia Kennefick
Galaxies 2022, 10(5), 100; https://doi.org/10.3390/galaxies10050100 - 17 Oct 2022
Cited by 17 | Viewed by 1807
Abstract
We present the MATLAB code Spirality, a novel method for measuring spiral arm pitch angles by fitting galaxy images to spiral templates of known pitch. Computation time is typically on the order of 2 min per galaxy, assuming 8 GB of working memory. [...] Read more.
We present the MATLAB code Spirality, a novel method for measuring spiral arm pitch angles by fitting galaxy images to spiral templates of known pitch. Computation time is typically on the order of 2 min per galaxy, assuming 8 GB of working memory. We tested the code using 117 synthetic spiral images with known pitches, varying both the spiral properties and the input parameters. The code yielded correct results for all synthetic spirals with galaxy-like properties. We also compared the code’s results to two-dimensional Fast Fourier Transform (2DFFT) measurements for the sample of nearby galaxies defined by DMS PPak. Spirality’s error bars overlapped 2DFFT’s error bars for 26 of the 30 galaxies. The two methods’ agreement correlates strongly with galaxy radius in pixels and also with i-band magnitude, but not with redshift, a result that is consistent with at least some galaxies’ spiral structure being fully formed by z=1.2, beyond which there are few galaxies in our sample. The Spirality code package also includes GenSpiral, which produces FITS images of synthetic spirals, and SpiralArmCount, which uses a one-dimensional Fast Fourier Transform to count the spiral arms of a galaxy after its pitch is determined. All code is freely available. Full article
(This article belongs to the Special Issue Galactic Structure and Dynamics)
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20 pages, 1785 KiB  
Article
Neutron Star Radii, Deformabilities, and Moments of Inertia from Experimental and Ab Initio Theory Constraints of the 208Pb Neutron Skin Thickness
by Yeunhwan Lim and Jeremy W. Holt
Galaxies 2022, 10(5), 99; https://doi.org/10.3390/galaxies10050099 - 20 Sep 2022
Cited by 12 | Viewed by 1609
Abstract
Recent experimental and ab initio theory investigations of the 208Pb neutron skin thickness have the potential to inform the neutron star equation of state. In particular, the strong correlation between the 208Pb neutron skin thickness and the pressure of neutron matter [...] Read more.
Recent experimental and ab initio theory investigations of the 208Pb neutron skin thickness have the potential to inform the neutron star equation of state. In particular, the strong correlation between the 208Pb neutron skin thickness and the pressure of neutron matter at normal nuclear densities leads to modified predictions for the radii, tidal deformabilities, and moments of inertia of typical 1.4M neutron stars. In the present work, we study the relative impact of these recent analyses of the 208Pb neutron skin thickness on bulk properties of neutron stars within a Bayesian statistical analysis. Two models for the equation of state prior are employed in order to highlight the role of the highly uncertain high-density equation of state. From our combined Bayesian analysis of nuclear theory, nuclear experiment, and observational constraints on the dense matter equation of state, we find at the 90% credibility level R1.4=12.360.73+0.38 km for the radius of a 1.4M neutron star, R2.0=11.960.71+0.94 km for the radius of a 2.0M neutron star, Λ1.4=440144+103 for the tidal deformability of a 1.4M neutron star, and I1.338=1.4250.146+0.074×1045gcm2 for the moment of inertia of PSR J0737-3039A whose mass is 1.338M. Full article
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14 pages, 549 KiB  
Article
Fundamental Properties of Late-Type Stars in Eclipsing Binaries
by Juan Carlos Morales, Ignasi Ribas, Álvaro Giménez and David Baroch
Galaxies 2022, 10(5), 98; https://doi.org/10.3390/galaxies10050098 - 17 Sep 2022
Cited by 6 | Viewed by 1482
Abstract
Evidence from the analysis of eclipsing binary systems revealed that late-type stars are larger and cooler than predicted by models, and that this is probably caused by stellar magnetic activity. In this work, we revisit this problem taking into account the advancements in [...] Read more.
Evidence from the analysis of eclipsing binary systems revealed that late-type stars are larger and cooler than predicted by models, and that this is probably caused by stellar magnetic activity. In this work, we revisit this problem taking into account the advancements in the last decade. We provide and updated a list of 32 eclipsing binary or multiple systems, including at least one star with a mass 0.7 M and with mass and radius measured to an accuracy better than 3%. The comparison with stellar structure and evolution theoretical models reveals an overall discrepancy of about 7% and −4% for the radius and effective temperature, respectively, and that it may be larger than previously found below the full convection boundary. Furthermore, the hypothesis of stellar activity is reinforced by the comparison of different systems with similar components. Further eclipsing binaries with accurately determined masses and radii, and with estimated activity levels, as well as the implementation of magnetic activity in theoretical models will help to improve our knowledge of low-mass stars, which are prime targets for exoplanet surveys. Full article
(This article belongs to the Special Issue What’s New under the Binary Suns)
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15 pages, 1374 KiB  
Article
Mass-Accreting Pulsating Components of Algols
by David Mkrtichian, Khemsinan Gunsriviwat, Holger Lehmann, Chris Engelbrecht, Andrew Tkachenko and Victor Nazarenko
Galaxies 2022, 10(5), 97; https://doi.org/10.3390/galaxies10050097 - 16 Sep 2022
Cited by 2 | Viewed by 1837
Abstract
We present a review of the latest results of studies of the class of mass-accreting pulsating components of semi-detached eclipsing binaries known as oEA stars. The application of the techniques of asteroseismology to this class of stars unlocks new pathways for gaining a [...] Read more.
We present a review of the latest results of studies of the class of mass-accreting pulsating components of semi-detached eclipsing binaries known as oEA stars. The application of the techniques of asteroseismology to this class of stars unlocks new pathways for gaining a deeper understanding of the short-term evolution and magnetic activity of binary stars. We report the discovery of 49 new pulsating components of eclipsing binaries, based on data from NASA’s TESS space telescope. Recent observational results on the pulsation characteristics of these stars are summarized. The effects of the interaction of the magnetic and spot activity of the Roche-lobe-filling component of a system with the pulsations of the mass-accreting component are discussed. Full article
(This article belongs to the Special Issue What’s New under the Binary Suns)
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8 pages, 559 KiB  
Article
On Variation Mechanisms in Recurrent Nova IM Normae
by R. E. Wilson
Galaxies 2022, 10(5), 96; https://doi.org/10.3390/galaxies10050096 - 14 Sep 2022
Viewed by 1080
Abstract
Light curves by Woudt and Warner (WW) of recurrent nova IM Nor show eclipse-like dips that they saw as too wide for eclipses alone, and interpreted as mainly a reflection effect due to irradiation of the companion (mass donor) star with some amplitude [...] Read more.
Light curves by Woudt and Warner (WW) of recurrent nova IM Nor show eclipse-like dips that they saw as too wide for eclipses alone, and interpreted as mainly a reflection effect due to irradiation of the companion (mass donor) star with some amplitude increase due to eclipse of IM Nor’s disk. A mainly reflection interpretation cannot be made to work because reflection does not produce dips over a restricted phase range but a somewhat distorted sinusoid that extends over the entire orbital cycle. Here, the dip features are interpreted in two ways, with testing via quantitative light curve modeling that includes an equipotential disk. One way is as alternating eclipses of and by the disk that surrounds this cataclysmic variable’s accreting white dwarf, rather than purely a succession of disk-by-star eclipses. WW’s estimated period of 0.d1026 was accordingly doubled to 0.d2052, with the observed dips now half of their previous width in phase, and with the modeled eclipses matching the observed dips in width and shape. In the 2nd interpretation, a toroidal disk’s capability to produce very wide eclipses is demonstrated computationally. Furthermore, much of the perceived eclipse width can be recognized as an apparent effect due to tidal stretching of the companion star and the disk. In overview, disk eclipses and tidal variation combine with reflection to produce a light curve waveform of approximately the observed shape and duration. Eclipses, tides, and reflection all have essential roles in the 2nd interpretation and no change from WW’s period is needed. Radial velocity observations will be crucial for identification of the correct resolution of the ”excessively wide eclipse” problem. Full article
(This article belongs to the Special Issue What’s New under the Binary Suns)
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21 pages, 1761 KiB  
Article
Properties of MAXI J1348-630 during Its Second Outburst in 2019
by Riya Bhowmick, Dipak Debnath, Kaushik Chatterjee, Arghajit Jana and Sujoy Kumar Nath
Galaxies 2022, 10(5), 95; https://doi.org/10.3390/galaxies10050095 - 07 Sep 2022
Cited by 2 | Viewed by 1336
Abstract
The newly discovered galactic black hole candidate (BHC) MAXI J1348-630 showed two major outbursts in 2019, just after its discovery. Here, we provide a detailed spectral and temporal analysis of the less-studied second outburst using archive data from multiple satellites, namely Swift, MAXI, [...] Read more.
The newly discovered galactic black hole candidate (BHC) MAXI J1348-630 showed two major outbursts in 2019, just after its discovery. Here, we provide a detailed spectral and temporal analysis of the less-studied second outburst using archive data from multiple satellites, namely Swift, MAXI, NICER, NuSTAR and AstroSat. The outburst continued for around two and a half months. Unlike the first outburst from this source, this second outburst was a ‘failed’ one. The source did not transition to soft or intermediate spectral states. During the entire outburst, the source was in the hard state with high dominance of non-thermal photons. The presence of strong shocks are inferred from spectral fitting using a TCAF model. In NuSTAR spectra, weak reflection is observed from spectral fitting. Low-frequency quasi-periodic oscillations are also detected in AstroSat data. Full article
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23 pages, 631 KiB  
Article
Gravitational-Wave Instabilities in Rotating Compact Stars
by Eric L. Bratton II, Zikun Lin, Fridolin Weber, Milva G. Orsaria, Ignacio F. Ranea-Sandoval and Nathaniel Saavedra
Galaxies 2022, 10(5), 94; https://doi.org/10.3390/galaxies10050094 - 30 Aug 2022
Cited by 3 | Viewed by 1872
Abstract
It is generally accepted that the limit on the stable rotation of neutron stars is set by gravitational-radiation reaction (GRR) driven instabilities, which cause the stars to emit gravitational waves that carry angular momentum away from them. The instability modes are moderated by [...] Read more.
It is generally accepted that the limit on the stable rotation of neutron stars is set by gravitational-radiation reaction (GRR) driven instabilities, which cause the stars to emit gravitational waves that carry angular momentum away from them. The instability modes are moderated by the shear viscosity and the bulk viscosity of neutron star matter. Among the GRR instabilities, the f-mode instability plays a historically predominant role. In this work, we determine the instability periods of this mode for three different relativistic models for the nuclear equation of state (EoS) named DD2, ACB4, and GM1L. The ACB4 model for the EoS accounts for a strong first-order phase transition that predicts a new branch of compact objects known as mass-twin stars. DD2 and GM1L are relativistic mean field (RMF) models that describe the meson-baryon coupling constants to be dependent on the local baryon number density. Our results show that the f-mode instability associated with m=2 sets the limit of stable rotation for cold neutron stars (T1010 K) with masses between 1M and 2M. This mode is excited at rotation periods between 1 and 1.4 ms (∼20% to ∼40% higher than the Kepler periods of these stars). For cold hypothetical mass-twin compact stars with masses between 1.96M and 2.10M, the m=2 instability sets in at rotational stellar periods between 0.8 and 1 millisecond (i.e., ∼25% to ∼30% above the Kepler period). Full article
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38 pages, 5725 KiB  
Review
The Structure of Gamma Ray Burst Jets
by Om Sharan Salafia and Giancarlo Ghirlanda
Galaxies 2022, 10(5), 93; https://doi.org/10.3390/galaxies10050093 - 30 Aug 2022
Cited by 17 | Viewed by 2679
Abstract
Due to relativistic bulk motion, the structure and orientation of gamma-ray burst (GRB) jets have a fundamental role in determining how they appear. The recent discovery of the GW170817 binary neutron star merger and the associated GRB boosted the interest in the modeling [...] Read more.
Due to relativistic bulk motion, the structure and orientation of gamma-ray burst (GRB) jets have a fundamental role in determining how they appear. The recent discovery of the GW170817 binary neutron star merger and the associated GRB boosted the interest in the modeling and search for signatures of the presence of a (possibly quasi-universal) jet structure in long and short GRBs. In this review, following a pedagogical approach, we summarize the history of GRB jet structure research over the last two decades, from the inception of the idea of a universal jet structure to the current understanding of the complex processes that shape the structure, which involves the central engine that powers the jet and the interaction of the latter with the progenitor vestige. We put some emphasis on the observable imprints of jet structure on prompt and afterglow emission and on the luminosity function, favoring intuitive reasoning over technical explanations. Full article
(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)
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29 pages, 3386 KiB  
Review
TeV Dark Matter Searches in the Extragalactic Gamma-ray Sky
by Moritz Hütten and Daniel Kerszberg
Galaxies 2022, 10(5), 92; https://doi.org/10.3390/galaxies10050092 - 29 Aug 2022
Cited by 4 | Viewed by 2221
Abstract
High-energetic gamma rays from astrophysical targets constitute a unique probe for annihilation or decay of heavy particle dark matter (DM). After several decades, diverse null detections have resulted in strong constraints for DM particle masses up to the TeV scale. While the gamma-ray [...] Read more.
High-energetic gamma rays from astrophysical targets constitute a unique probe for annihilation or decay of heavy particle dark matter (DM). After several decades, diverse null detections have resulted in strong constraints for DM particle masses up to the TeV scale. While the gamma-ray signature is expected to be universal from various targets, uncertainties of astrophysical origin strongly affect and weaken the limits. At the same time, spurious signals may originate from non-DM related processes. The many gamma-ray targets in the extragalactic sky being searched for DM play a crucial role to keep these uncertainties under control and to ultimately achieve an unambiguous DM detection. Lately, a large progress has been made in combined analyses of TeV DM candidates towards different targets by using data from various instruments and over a wide range of gamma-ray energies. These approaches not only resulted in an optimal exploitation of existing data and an improved sensitivity, but also helped to level out target- and instrument-related uncertainties. This review gathers all searches in the extragalactic sky performed so far with the space-borne Fermi-Large Area Telescope, the ground-based imaging atmospheric Cherenkov telescopes, and the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC). We discuss the different target classes and provide a complete list of all analyses so far. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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