Galaxies

18 pages, 5609 KiB

Open AccessReview

Gamma Ray Pulsars and Opportunities for the MACE Telescope

by Atul Pathania, Krishna Kumar Singh and Kuldeep Kumar Yadav

Galaxies 2023, 11(4), 91; https://doi.org/10.3390/galaxies11040091 - 17 Aug 2023

Viewed by 1034

Rapidly rotating neutron stars with very strong surface magnetic fields are observed to emit pulsed emission in the whole range of electromagnetic spectrum from radio to high-energy gamma rays. These so-called pulsars are known for their exceptional rotational stability. The radio emission from [...] Read more.

Rapidly rotating neutron stars with very strong surface magnetic fields are observed to emit pulsed emission in the whole range of electromagnetic spectrum from radio to high-energy gamma rays. These so-called pulsars are known for their exceptional rotational stability. The radio emission from pulsars is generally believed to be powered by the rotational energy of neutron stars. More than 3000 pulsars have been currently known from radio observations; however, only about 10% are observed in the high-energy gamma ray band. The Fermi-LAT observations in the energy range above 100 MeV have discovered more than 300 pulsars. However, the origin of high-energy non-thermal radiation from pulsars is not completely understood and remains an active area of research. In this contribution, we report a summary of observational features of the gamma ray pulsars and briefly discuss observability for the MACE gamma ray telescope, which has just started its regular science operation at Hanle in India. Six gamma ray pulsars, other than the well-known Crab and Geminga, are identified as probable candidates for MACE observations. Full article

(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)

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17 pages, 2043 KiB

Open AccessArticle

Infrared Spectroscopy of Be Stars: Influence of the Envelope Parameters on Brackett-Series Behaviour

by Yanina Roxana Cochetti, Anahi Granada, María Laura Arias, Andrea Fabiana Torres and Catalina Arcos

Galaxies 2023, 11(4), 90; https://doi.org/10.3390/galaxies11040090 - 17 Aug 2023

Cited by 1 | Viewed by 989

Abstract

The IR spectra of Be stars display numerous hydrogen recombination lines, constituting a great resource for obtaining information on the physical and dynamic structures of different regions within the circumstellar envelope. Nevertheless, this spectral region has not been analysed in depth, and there [...] Read more.

The IR spectra of Be stars display numerous hydrogen recombination lines, constituting a great resource for obtaining information on the physical and dynamic structures of different regions within the circumstellar envelope. Nevertheless, this spectral region has not been analysed in depth, and there is a lack of synthetic spectra with which to compare observations. Therefore, we computed synthetic spectra with the HDUST code for different disc parameters. Here, we present our results on the spectral region that includes lines of the Brackett series. We discuss the dependence of the line series strengths on several parameters that describe the structure of the disc. We also compared model line profiles, fluxes, and EWs with observational data for two Be stars (MX Pup and

π

Aqr). Even though the synthetic spectra adequately fit our observations of both stars and allow us to constrain the parameters of the disc, there is a discrepancy with the observed data in the EW and flux measurements, especially in the case of MX Pup. It is possible that by including Brackett lines of higher terms or adding the analysis of other series, we may be able to better constrain the parameters of the observed disc. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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31 pages, 44660 KiB

Open AccessReview

Multidimensional Simulations of Core Convection

by Daniel Lecoanet and Philipp V. F. Edelmann

Galaxies 2023, 11(4), 89; https://doi.org/10.3390/galaxies11040089 - 31 Jul 2023

Cited by 4 | Viewed by 1175

Abstract

The cores of main sequence intermediate- and high-mass stars are convective. Mixing at the radiative–convective boundary, waves excited by the convection, and magnetic fields generated by convective dynamos all influence the main sequence and post-main sequence evolution of these stars. These effects must [...] Read more.

The cores of main sequence intermediate- and high-mass stars are convective. Mixing at the radiative–convective boundary, waves excited by the convection, and magnetic fields generated by convective dynamos all influence the main sequence and post-main sequence evolution of these stars. These effects must be understood to accurately model the structure and evolution of intermediate- and high-mass stars. Unfortunately, there are many challenges in simulating core convection due to the wide range of temporal and spatial scales, as well as many important physics effects. In this review, we describe the latest numerical strategies to address these challenges. We then describe the latest state-of-the-art simulations of core convection, summarizing their main findings. These simulations have led to important insights into many of the processes associated with core convection. Two outstanding problems with multidimensional simulations are, 1. it is not always straightforward to extrapolate from simulation parameters to the parameters of real stars; and 2. simulations using different methods sometimes appear to arrive at contradictory results. To address these issues, next generation simulations of core convection must address how their results depend on stellar luminosity, dimensionality, and turbulence intensity. Furthermore, code comparison projects will be essential to establish robust parameterizations that will become the new standard in stellar modeling. Full article

(This article belongs to the Special Issue The Structure and Evolution of Stars)

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10 pages, 7483 KiB

Open AccessArticle

Searching for Short-Timescale Transients in Gamma-ray Telescope Data

by Annanay Jaitly, Dmitriy Kostunin and Karin Cescon

Galaxies 2023, 11(4), 88; https://doi.org/10.3390/galaxies11040088 - 24 Jul 2023

Viewed by 847

Abstract

Astrophysical sources show variability in their emissions over a range of timescales, with transients such as fast radio bursts (FRBs) and magnetar giant flares (MGFs) showing variability on timescales as short as a few milliseconds. Recent advances in gamma-ray astronomy such as telescopes’ [...] Read more.

Astrophysical sources show variability in their emissions over a range of timescales, with transients such as fast radio bursts (FRBs) and magnetar giant flares (MGFs) showing variability on timescales as short as a few milliseconds. Recent advances in gamma-ray astronomy such as telescopes’ high temporal resolution and relatively high uptime, combined with follow-up programs between different facilities, should allow serendipitous observations of burst-like phenomena. Even so, no very-high-energy gamma-ray counterparts for FRBs have been detected so far, and there is a general lack of software tools suited to search for such phenomena. We present a tool capable of searching gamma-ray telescope data for transient phenomena over arbitrary timescales—it is based on the Gammapy package and recursively scans the given field of view for clusters of events within user-defined time and angular-separation intervals. The generalized implementation allows for its application in many other cases and multiple gamma-ray telescopes. The main features and methodology of the developed tool are presented here, along with an analysis of the open gamma ray telescope data performed using it. Full article

(This article belongs to the Special Issue The New Era of Real-Time Multi-Messenger Astronomy)

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25 pages, 894 KiB

Open AccessReview

Dynamics of Powerful Radio Galaxies

by Ross J. Turner and Stanislav S. Shabala

Galaxies 2023, 11(4), 87; https://doi.org/10.3390/galaxies11040087 - 19 Jul 2023

Cited by 4 | Viewed by 1034

Abstract

Analytical models describing the dynamics of lobed radio sources are essential for interpretation of the tens of millions of radio sources that will be observed by the Square Kilometre Array and pathfinder instruments. We propose that historical models can be grouped into two [...] Read more.

Analytical models describing the dynamics of lobed radio sources are essential for interpretation of the tens of millions of radio sources that will be observed by the Square Kilometre Array and pathfinder instruments. We propose that historical models can be grouped into two classes in which the forward expansion of the radio source is driven by either the jet momentum flux or lobe internal pressure. The most recent generation of analytical models combines these limiting cases for a more comprehensive description. We extend the mathematical formalism of historical models to describe source expansion in non-uniform environments, and directly compare different model classes with each other and with hydrodynamic numerical simulations. We quantify differences in predicted observable characteristics for lobed radio sources due to the different model assumptions for their dynamics. We have made our code for the historical models analysed in this review openly available to the community. Full article

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70 pages, 2653 KiB

Open AccessReview

Cosmic Ray Processes in Galactic Ecosystems

by Ellis R. Owen, Kinwah Wu, Yoshiyuki Inoue, H.-Y. Karen Yang and Alison M. W. Mitchell

Galaxies 2023, 11(4), 86; https://doi.org/10.3390/galaxies11040086 - 16 Jul 2023

Cited by 3 | Viewed by 2648

Abstract

Galaxy evolution is an important topic, and our physical understanding must be complete to establish a correct picture. This includes a thorough treatment of feedback. The effects of thermal–mechanical and radiative feedback have been widely considered; however, cosmic rays (CRs) are also powerful [...] Read more.

Galaxy evolution is an important topic, and our physical understanding must be complete to establish a correct picture. This includes a thorough treatment of feedback. The effects of thermal–mechanical and radiative feedback have been widely considered; however, cosmic rays (CRs) are also powerful energy carriers in galactic ecosystems. Resolving the capability of CRs to operate as a feedback agent is therefore essential to advance our understanding of the processes regulating galaxies. The effects of CRs are yet to be fully understood, and their complex multi-channel feedback mechanisms operating across the hierarchy of galaxy structures pose a significant technical challenge. This review examines the role of CRs in galaxies, from the scale of molecular clouds to the circumgalactic medium. An overview of their interaction processes, their implications for galaxy evolution, and their observable signatures is provided and their capability to modify the thermal and hydrodynamic configuration of galactic ecosystems is discussed. We present recent advancements in our understanding of CR processes and interpretation of their signatures, and highlight where technical challenges and unresolved questions persist. We discuss how these may be addressed with upcoming opportunities. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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22 pages, 3833 KiB

Open AccessReview

What Drives the Ionized Gas Outflows in Radio-Quiet AGN?

by Mainak Singha, Christopher P. O’Dea and Stefi A. Baum

Galaxies 2023, 11(4), 85; https://doi.org/10.3390/galaxies11040085 - 12 Jul 2023

Cited by 4 | Viewed by 1483

Abstract

We review the mechanisms driving the ionized gas outflows in radio-quiet (RQ) AGN. Although it constitutes ∼90% of the AGN population, what drives these outflows in these AGNs remains an open question. High-resolution imaging and integral field unit (IFU) observation is key to [...] Read more.

We review the mechanisms driving the ionized gas outflows in radio-quiet (RQ) AGN. Although it constitutes ∼90% of the AGN population, what drives these outflows in these AGNs remains an open question. High-resolution imaging and integral field unit (IFU) observation is key to spatially resolving these outflows, whereas radio observations are important to comprehend the underlying radiative processes. Radio interferometric observations have detected linear, collimated structures on the hundreds of pc scale in RQ AGN, which may be very similar to the extended radio jets in powerful galaxies. Proper motions measured in some objects are sub-relativistic. Other processes, such as synchrotron radiation from shock-accelerated gas around the outflows could give rise to radio emissions as well. Near the launching region, these outflows may be driven by the thermal energy of the accretion disk and exhibit free–free emission. IFU observations on the other hand have detected evidence of both winds and jets and the outflows driven by them in radio-quiet AGN. Some examples include nearby AGN such as Mrk 1044 and HE 1353-1917. An IFU study of nearby (z

< 0.06

) RQ AGN has found that these outflows may be related to their radio properties on <100 pc scale, rather than their accretion properties. Recent JWST observations of RQ AGN XID 2028 have revealed that radio jets and wind could inflate bubbles, create cavities, and trigger star formation. Future high-resolution multi-wavelength observations and numerical simulations taking account of both jets and winds are hence essential to understand the complex interaction between radio-quiet AGN and the host from sub-pc to kpc scales. Full article

(This article belongs to the Special Issue The Symbiosis between Radio Source and Galaxy Evolution)

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8 pages, 363 KiB

Open AccessCommunication

Probing Neutrino Production in Blazars by Millimeter VLBI

by Yuri Y. Kovalev, Alexander V. Plavin, Alexander B. Pushkarev and Sergey V. Troitsky

Galaxies 2023, 11(4), 84; https://doi.org/10.3390/galaxies11040084 - 10 Jul 2023

Cited by 4 | Viewed by 1145

Abstract

The advancement of neutrino observatories has sparked a surge in multi-messenger astronomy. Multiple neutrino associations among blazars are reported while neutrino production sites are located within their central (sub)parsecs. Yet, many questions remain on the nature of those processes. The next generation Event [...] Read more.

The advancement of neutrino observatories has sparked a surge in multi-messenger astronomy. Multiple neutrino associations among blazars are reported while neutrino production sites are located within their central (sub)parsecs. Yet, many questions remain on the nature of those processes. The next generation Event Horizon Telescope (ngEHT) is uniquely positioned for these studies, as its high frequency and resolution can probe both the accretion disk region and the parsec-scale jet. This opens up new opportunities for connecting the two regions and unraveling the proton acceleration and neutrino production in blazars. We outline observational strategies for ngEHT and highlight what it can contribute to the multi-messenger study of blazars. Full article

(This article belongs to the Special Issue From Vision to Instrument: Creating a Next-Generation Event Horizon Telescope for a New Era of Black Hole Science)

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27 pages, 1325 KiB

Open AccessArticle

Searching for Phase-Locked Variations of the Emission-Line Profiles in Binary Be Stars

by Anatoly S. Miroshnichenko, Raghav Chari, Stephen Danford, Peter Prendergast, Alicia N. Aarnio, Ivan L. Andronov, Lidiia L. Chinarova, Aidan Lytle, Ainash Amantayeva, Ilfa A. Gabitova, Nadezhda L. Vaidman, Sayat S. Baktybayev and Serik A. Khokhlov

Galaxies 2023, 11(4), 83; https://doi.org/10.3390/galaxies11040083 - 04 Jul 2023

Cited by 3 | Viewed by 1395

Abstract

There is growing evidence that many Be stars are parts of binary systems. As the B-type primaries are very fast rotators and their spectral lines may be distorted by the circumstellar material, it is not easy to measure their radial velocity directly from [...] Read more.

There is growing evidence that many Be stars are parts of binary systems. As the B-type primaries are very fast rotators and their spectral lines may be distorted by the circumstellar material, it is not easy to measure their radial velocity directly from the spectral lines. It has been shown that some Be binaries exhibit peak intensity variations consisting of double-peaked H

α

lines that are phase-locked with orbital periods. We searched for such variations in the spectra of 12 Be stars, including several known and suspected binaries. Our results include confirmation of the orbital periods in

ν

Geminorum,

ϵ

Capricorni,

κ

Draconis, 60 Cygni, and V2119 Cygni, its refinement in o Puppis, as well as suggesting hints for binarity in o Aquarii, BK Camelopardalis, and 10 Cassiopeae. Monitoring of the H

α

line profile variations in

β

Canis Minoris for over the last 10 years gives further support to the existence of a 182.5-day period found earlier in a smaller set of data. A similar but still preliminary period (179.6 days) was found in the H

α

line profile variations in

ψ

Persei. It is shown for the first time that

ν

Geminorum exhibits phase-locked variations in the H

α

emission peak intensity ratio and, therefore, is a part of the inner binary in this triple system. Our results show that the mentioned phase-locked peak intensity variations are observed in more Be binary systems than previously known and can be used to search for binarity of Be stars when application of other methods is inconclusive. Full article

(This article belongs to the Special Issue New Insights on Binary Stars)

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22 pages, 1371 KiB

Open AccessArticle

Observational Implications of OJ 287’s Predicted 2022 Disk Impact in the Black Hole Binary Model

by Mauri J. Valtonen, Lankeswar Dey, Achamveedu Gopakumar, Staszek Zola, Anne Lähteenmäki, Merja Tornikoski, Alok C. Gupta, Tapio Pursimo, Emil Knudstrup, Jose L. Gomez, Rene Hudec, Martin Jelínek, Jan Štrobl, Andrei V. Berdyugin, Stefano Ciprini, Daniel E. Reichart, Vladimir V. Kouprianov, Katsura Matsumoto, Marek Drozdz, Markus Mugrauer, Alberto Sadun, Michal Zejmo, Aimo Sillanpää, Harry J. Lehto, Kari Nilsson, Ryo Imazawa and Makoto Uemura Show full author list Hide full author list

Galaxies 2023, 11(4), 82; https://doi.org/10.3390/galaxies11040082 - 03 Jul 2023

Cited by 3 | Viewed by 981

Abstract

We present a summary of the results of the OJ 287 observational campaign, which was carried out during the 2021/2022 observational season. This season is special in the binary model because the major axis of the precessing binary happens to lie almost exactly [...] Read more.

We present a summary of the results of the OJ 287 observational campaign, which was carried out during the 2021/2022 observational season. This season is special in the binary model because the major axis of the precessing binary happens to lie almost exactly in the plane of the accretion disc of the primary. This leads to pairs of almost identical impacts between the secondary black hole and the accretion disk in 2005 and 2022. In 2005, a special flare called “blue flash” was observed 35 days after the disk impact, which should have also been verifiable in 2022. We did observe a similar flash and were able to obtain more details of its properties. We describe this in the framework of expanding cloud models. In addition, we were able to identify the flare arising exactly at the time of the disc crossing from its photo-polarimetric and gamma-ray properties. This is an important identification, as it directly confirms the orbit model. Moreover, we saw a huge flare that lasted only one day. We may understand this as the lighting up of the jet of the secondary black hole when its Roche lobe is suddenly flooded by the gas from the primary disk. Therefore, this may be the first time we directly observed the secondary black hole in the OJ 287 binary system. Full article

(This article belongs to the Special Issue Distant Glowing Objects: Quest for Quasars)

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11 pages, 3172 KiB

Open AccessArticle

Investigating Possible Correlations between Gamma-Ray and Optical Lightcurves for TeV-Detected Northern Blazars over 8 Years of Observations

by Atreya Acharyya and Alberto C. Sadun

Galaxies 2023, 11(4), 81; https://doi.org/10.3390/galaxies11040081 - 01 Jul 2023

Cited by 1 | Viewed by 1109

Abstract

Blazars are a subclass of active galactic nuclei (AGN) having relativistic jets aligned within a few degrees of our line-of-sight and form the majority of the AGN detected in the TeV regime. The Fermi-Large Area Telescope (LAT) is a pair-conversion telescope, sensitive to [...] Read more.

Blazars are a subclass of active galactic nuclei (AGN) having relativistic jets aligned within a few degrees of our line-of-sight and form the majority of the AGN detected in the TeV regime. The Fermi-Large Area Telescope (LAT) is a pair-conversion telescope, sensitive to photons having energies between 20 MeV and 2 TeV, and is capable of scanning the entire gamma-ray sky every three hours. Despite the remarkable success of the Fermi mission, many questions still remain unanswered, such as the site of gamma-ray production and the emission mechanisms involved. The Asteroid Terrestrial-impact Last Alert System (ATLAS) is a high cadence all sky survey system optimized to be efficient for finding potentially dangerous asteroids, as well as in tracking and searching for highly variable and transient sources, such as AGN. In this study, we investigate possible correlations between the Fermi-LAT observations in the 100 MeV–300 GeV energy band and the ATLAS optical data in the R-band, centered at 679 nm, for a sample of 18 TeV-detected northern blazars over 8 years of observations between 2015 and 2022. Under the assumption that the optical and gamma-ray flares are produced by the same outburst propagating down the jet, the strong correlations found for some sources suggest a single-zone leptonic model of emission. Full article

(This article belongs to the Special Issue Distant Glowing Objects: Quest for Quasars)

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12 pages, 756 KiB

Open AccessArticle

A Mini Atlas of H-Band Spectra of Southern Symbiotic Stars

by Paula Esther Marchiano, María Laura Arias, Michaela Kraus, Michalis Kourniotis, Andrea Fabiana Torres, Lydia Sonia Cidale and Marcelo Borges Fernandes

Galaxies 2023, 11(4), 80; https://doi.org/10.3390/galaxies11040080 - 22 Jun 2023

Viewed by 853

Abstract

Symbiotic stars are interacting binary systems composed of an evolved star (generally a late-type red giant) and a degenerate or dwarf companion in orbit close enough for mass transfer to occur. Understanding the status of the late-type star is important for developing binary [...] Read more.

Symbiotic stars are interacting binary systems composed of an evolved star (generally a late-type red giant) and a degenerate or dwarf companion in orbit close enough for mass transfer to occur. Understanding the status of the late-type star is important for developing binary models for the symbiotic systems as it affects the transfer of matter needed to activate the hot component. Infrared observations have been very useful in probing the nature of late-type stars in symbiotic systems. This work presents a set of symbiotic stars observed with SOAR/OSIRIS (R∼3000) in the H-band. We aimed to search for possible molecular circumstellar emission, to characterize the cool companion in these systems, and to confront the new findings with those obtained from the previous K-band classifications. We detected molecular emission from just one object, BI Cru, which displays the second-overtone CO-bands. To fit the observed photospheric CO absorption bands, we used the MARCS atmosphere models. We present our results as a mini atlas of symbiotic stars in the near-infrared region to facilitate the comparison among different observed symbiotic systems. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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Galaxies, Volume 11, Issue 4 (August 2023) – 12 articles

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