Galaxies

12 pages, 866 KiB

Open AccessArticle

Production of Lithium and Heavy Elements in AGB Stars Experiencing PIEs

by Arthur Choplin, Lionel Siess, Stephane Goriely and Sebastien Martinet

Galaxies 2024, 12(5), 66; https://doi.org/10.3390/galaxies12050066 - 18 Oct 2024

Viewed by 507

Asymptotic giant branch (AGB) stars can experience proton ingestion events (PIEs), leading to a rich nucleosynthesis. During a PIE, the intermediate neutron capture process (i-process) develops, leading to the production of trans-iron elements. It is also suggested that lithium is produced during these [...] Read more.

Asymptotic giant branch (AGB) stars can experience proton ingestion events (PIEs), leading to a rich nucleosynthesis. During a PIE, the intermediate neutron capture process (i-process) develops, leading to the production of trans-iron elements. It is also suggested that lithium is produced during these events. We investigate the production of lithium and trans-iron elements in AGB stars experiencing a PIE with

1 < M_{ini} / M_{⊙} < 3

and

- 3 < [Fe / H] < 0

. We find that lithium is produced in all PIE models with surface abundances

3 <

A(Li)

< 5

. The surface enrichment and overall AGB lithium yield increases with decreasing stellar mass. The lithium enrichment is accompanied by a production of ¹³C with

3 <^{12}

C/¹³C

< 9

at the surface just after the PIE. AGB stars experiencing PIE may be related to J-type carbon stars whose main features are excesses of lithium and¹³C. In addition to Li and ¹³C, heavy elements (e.g., Sr, Ba, Eu, Pb) are significantly produced in low-metallicity stars up to [Fe/H]

≃ - 1

. The yields of our models are publicly available. Additionally, of interest to the Li nucleosynthesis, we provide an updated fitting formula for the ⁷Be(

e^{-}, ν_{e}

)⁷Li electron capture rate. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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11 pages, 498 KiB

Open AccessArticle

LAMOST Spectroscopy and Gaia Photo-Astrometry for an Interstellar Extinction Study

by Oleg Malkov, Aleksandra Avdeeva and Dana Kovaleva

Galaxies 2024, 12(5), 65; https://doi.org/10.3390/galaxies12050065 - 17 Oct 2024

Viewed by 455

Abstract

The aim of this work is to establish the present accuracy and convergence of available estimates of galactic extinction. We determine the galactic interstellar extinction in selected high-latitude areas of the sky based on Gaia DR3 astrometry and photometry and spectroscopic data from [...] Read more.

The aim of this work is to establish the present accuracy and convergence of available estimates of galactic extinction. We determine the galactic interstellar extinction in selected high-latitude areas of the sky based on Gaia DR3 astrometry and photometry and spectroscopic data from the LAMOST survey. For this purpose, we choose 42 northern high-latitude sky areas surrounding supernovae that allowed establishing the accelerated expansion of the universe. We compare our results with the estimates accepted in that paper and find that they agree well, within observational errors. Simultaneously, the estimates for galactic extinction by other authors along the same sightlines show systematic differences, which can cause the distance to the extragalactic object to change by ±3–5%. Full article

(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)

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8 pages, 851 KiB

Open AccessReview

Synthesis of Organic and Inorganic Compounds in Asymptotic Giant Branch Stars

by Sun Kwok

Galaxies 2024, 12(5), 64; https://doi.org/10.3390/galaxies12050064 - 11 Oct 2024

Viewed by 493

Abstract

After the synthesis of carbon in the core of asymptotic giant branch (AGB) stars, carbon is dredged up to the surface by convection. Many carbon-based molecules are formed in the subsequently developed stellar wind. These include acetylene, which can link together to form [...] Read more.

After the synthesis of carbon in the core of asymptotic giant branch (AGB) stars, carbon is dredged up to the surface by convection. Many carbon-based molecules are formed in the subsequently developed stellar wind. These include acetylene, which can link together to form benzene in post-AGB evolution. The emergence of the spectral signatures of aromatic and aliphatic compounds in the transition phase between AGB stars and planetary nebulae suggests that complex organic compounds can be formed in the circumstellar environment over very short (10³ yr) timescales. We suggest that the carrier of the family of unidentified infrared emission bands is an amorphous carbonaceous compound—mixed aromatic/aliphatic nanoparticles (MAONs). The implications of the synthesis of complex organics in evolved stars are discussed. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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10 pages, 2012 KiB

Open AccessArticle

M 1-92: The Death of an AGB Star Told by Its Isotopic Ratios

by Elisa Masa, Javier Alcolea, Miguel Santander-García, Valentín Bujarrabal, Carmen Sánchez Contreras and Arancha Castro-Carrizo

Galaxies 2024, 12(5), 63; https://doi.org/10.3390/galaxies12050063 - 10 Oct 2024

Cited by 1 | Viewed by 447

Abstract

Ongoing improvements in the sensitivity of sub-mm- and mm-range interferometers and single-dish radio telescopes allow for the increasingly detailed study of AGB and post-AGB objects in molecular species other than

{}^{12}{CO}

and

{}^{13}{CO}

. With a new update introduced in the [...] Read more.

Ongoing improvements in the sensitivity of sub-mm- and mm-range interferometers and single-dish radio telescopes allow for the increasingly detailed study of AGB and post-AGB objects in molecular species other than

{}^{12}{CO}

and

{}^{13}{CO}

. With a new update introduced in the modelling tool SHAPE + shapemol, we can now create morpho-kinematical models to reproduce observations of these AGB and post-AGB circumstellar shells in different molecular species, allowing for an accurate description of their physical features as well as their molecular abundances and isotopic ratios. The pre-planetary nebula M1-92 (Minkowski’s Footprint) is one of the most complex objects of this kind, with a wide range of physical conditions and more than 20 molecular species detected. We model this nebula, reproducing the observational data from IRAM-30m and HSO/HiFi spectra and NOEMA interferometric maps, trying to understand the unusual evolution of its central star in the last phases of its life. The results show interesting features that tell us the story of its death. According to standard evolution models, a

{}^{17}O

/

{}^{18}O

isotopic ratio of 1.6 implies a stellar initial mass of ∼1.7

M_{⊙}

. Such a star should have turned C-rich by the end of the AGB phase, in striking contrast to the O-rich nature of the nebula. The most plausible way of reconciling this discrepancy is that M1-92 resulted from a sudden massive ejection event, interrupting the AGB evolution of the central source and preventing its transformation into a C-rich star. We also detect a changing

{}^{12}C

/

{}^{13}C

ratio across the nebula, which is particularly relevant in the inner equatorial region traced by

{HCO}^{+}

and

H^{13}

{CO}^{+}

, indicating an isotopic ratio variation taking place at some point during the last 1200 yr. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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11 pages, 3346 KiB

Open AccessArticle

Exploring the Ionized Core of the Proto-Planetary Nebula CRL 618 and Its Vicinity with ALMA

by José Pablo Fonfría, Carmen Sánchez Contreras, Daniel Tafoya, Patricia Fernández-Ruiz, Arancha Castro-Carrizo, Javier Alcolea and Valentín Bujarrabal

Galaxies 2024, 12(5), 62; https://doi.org/10.3390/galaxies12050062 - 10 Oct 2024

Viewed by 471

Abstract

Proto- and young planetary nebulae comprise dense circumstellar envelopes made of molecular gas and dust, some of which hide compact ionized cores that host stellar systems with hot objects, and show high-velocity bipolar outflows launched from inside their cores by means of still [...] Read more.

Proto- and young planetary nebulae comprise dense circumstellar envelopes made of molecular gas and dust, some of which hide compact ionized cores that host stellar systems with hot objects, and show high-velocity bipolar outflows launched from inside their cores by means of still unknown mechanisms. We present high-angular-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations (HPBW ≃ 30–50 mas) of CRL 618 at 1.35 mm covering the H30

α

recombination line as well as ≃150 molecular lines. The ionized core is resolved, showing a size of ≃0

.^{″} 8 \times 0 .^{″} 5

and is elongated along the east–west direction. This region exhibits a remarkable incomplete ring-like structure with two bright spots to the north and south that are separated by ≃

0_{.}^{″} 2

and shows deprojected velocity gradients ranging from 0.2 to 0.6 km

s^{- 1}

{au}^{- 1}

. The 1 mm wavelength continuum emission is mostly produced by free–free emission with a small contribution from dust with an average spectral index of 0.28 (

S_{ν} \propto ν^{α}

). The ionized core can roughly be modeled as a tilted hollow cylinder with a denser, incomplete equatorial band lacking its back side. Molecular emission traces the neutral component of the same structures enclosing the ionized matter. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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8 pages, 829 KiB

Open AccessArticle

Temporal Changes in the Infrared Spectra of Magellanic Carbon Stars

by G. C. Sloan, K. E. Kraemer, B. Aringer, J. Cami, K. Eriksson, S. Höfner, E. Lagadec, M. Matsuura, I. McDonald, E. Montiel, R. Sahai and A. A. Zijlstra

Galaxies 2024, 12(5), 61; https://doi.org/10.3390/galaxies12050061 - 9 Oct 2024

Viewed by 480

Abstract

The Medium-Resolution Spectrometer on the Mid-Infrared Instrument on the JWST obtained spectra of three carbon stars in the Large Magellanic Cloud. Two of the spectra differ significantly from spectra obtained ∼16–19 years earlier with the Infrared Spectrograph on the Spitzer Space Telescope. The [...] Read more.

The Medium-Resolution Spectrometer on the Mid-Infrared Instrument on the JWST obtained spectra of three carbon stars in the Large Magellanic Cloud. Two of the spectra differ significantly from spectra obtained ∼16–19 years earlier with the Infrared Spectrograph on the Spitzer Space Telescope. The one semi-regular variable among the three has changed little. The long-period Mira variable in the sample shows changes consistent with its pulsation cycle. The short-period Mira shows dramatic changes in the strength of its molecular absorption bands, with some bands growing weaker and some stronger. Whether these variations result from its pulsation cycle or its evolution is not clear. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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17 pages, 506 KiB

Open AccessArticle

Polarization from a Radially Stratified GRB Outflow

by Augusto César Caligula do Espírito Santo Pedreira, Nissim Fraija, Antonio Galván-Gámez, Boris Betancourt Kamenetskaia, Simone Dichiara, Maria G. Dainotti, Rosa L. Becerra and Peter Veres

Galaxies 2024, 12(5), 60; https://doi.org/10.3390/galaxies12050060 - 4 Oct 2024

Viewed by 512

Abstract

While the dominant radiation mechanism of gamma-ray bursts (GRBs) remains a question of debate, synchrotron emission is one of the foremost candidates to describe the multi-wavelength afterglow observations. As such, it is expected that GRBs should present some degree of polarization across their [...] Read more.

While the dominant radiation mechanism of gamma-ray bursts (GRBs) remains a question of debate, synchrotron emission is one of the foremost candidates to describe the multi-wavelength afterglow observations. As such, it is expected that GRBs should present some degree of polarization across their evolution—presenting a feasible means of probing these bursts’ energetic and angular properties. Although obtaining polarization data is difficult due to the inherent complexities regarding GRB observations, advances are being made, and theoretical modeling of synchrotron polarization is now more relevant than ever. In this manuscript, we present the polarization for a fiduciary model, where the synchrotron FS emission evolving in the radiative–adiabatic regime is described by a radially stratified off-axis outflow. This is parameterized with a power-law velocity distribution and decelerated in a constant-density and wind-like external environment. We apply this theoretical polarization model for two select GRBs, presenting upper limits in their polarization—GRB 170817A, a known off-axis GRB with radio polarization upper limits, and GRB 190014C, an on-axis case, where the burst was seen from within the half-opening angle of the jet, with observed optical polarization—in an attempt to constrain their magnetic field geometry in the emitting region. Full article

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10 pages, 4820 KiB

Open AccessReview

Probing Magnetic Fields in Young Supernova Remnants with IXPE

by Patrick Slane, Riccardo Ferrazzoli, Ping Zhou and Jacco Vink

Galaxies 2024, 12(5), 59; https://doi.org/10.3390/galaxies12050059 - 2 Oct 2024

Viewed by 714

Abstract

Synchrotron emission from the shocked regions in supernova remnants provides, through its polarization, crucial details about the magnetic field strength and orientation in these regions. This, in turn, provides information on particle acceleration in these shocks. Due to the rapid losses of the [...] Read more.

Synchrotron emission from the shocked regions in supernova remnants provides, through its polarization, crucial details about the magnetic field strength and orientation in these regions. This, in turn, provides information on particle acceleration in these shocks. Due to the rapid losses of the highest-energy relativistic electrons, X-ray polarization measurements allow for investigations of the magnetic field to be carried outvery close to the sites of particle acceleration. Measurements of both the geometry of the field and the levels of turbulence implied by the observed polarization degree thus provide unique insights into the conditions leading to efficient particle acceleration in fast shocks. The Imaging X-ray Polarimetry Explorer (IXPE) has carried out observations of multiple young SNRs, including Cas A, Tycho, SN 1006, and RX J1713.7−3946. In each, significant X-ray polarization detections provide measurements of magnetic field properties that show some common behavior but also considerable differences between these SNRs. Here, we provide a summary of results from IXPE studies of young SNRs, providing comparisons between the observed polarization and the physical properties of the remnants and their environments. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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16 pages, 1466 KiB

Open AccessArticle

Orbital Precession in Janis–Newman–Winicour Spacetime

by Bobur Turimov, Khurshid Karshiboev, Ahmadjon Abdujabbarov, Samik Mitra and Shavkat Karshiboev

Galaxies 2024, 12(5), 58; https://doi.org/10.3390/galaxies12050058 - 28 Sep 2024

Cited by 1 | Viewed by 619

Abstract

We have investigated the Janis–Newman–Winicour spacetime through three fundamental tests of theories of gravity, namely, gravitational lensing, perihelion shift, and redshift due to gravitational force. Focusing initially on the circular motion of a massive particle within the equatorial plane, the analysis disregards external [...] Read more.

We have investigated the Janis–Newman–Winicour spacetime through three fundamental tests of theories of gravity, namely, gravitational lensing, perihelion shift, and redshift due to gravitational force. Focusing initially on the circular motion of a massive particle within the equatorial plane, the analysis disregards external scalar field interactions. The Janis–Newman–Winicour (JNW) spacetime’s unique parameters, mass (M) and the scalar parameter (n), are examined, revealing an intriguing relationship between the innermost stable circular orbit position of the test particle and the scalar field parameter. The study also explores photon motion around a gravitational object in JNW spacetime, revealing the expansion of the photon sphere alongside a diminishing shadow, influenced by the external scalar field. Despite these complexities, gravitational bending of light remains consistent with general relativity predictions. The investigation extends to perihelion precession, where the trajectory of a massive particle in JNW spacetime exhibits eccentricity-dependent shifts, distinguishing it from Schwarzschild spacetime. Finally, oscillatory motion of massive particles in JNW spacetime is explored, providing analytical expressions for epicyclic frequencies using perturbation methods. The study concludes with the application of MCMC analyses to constrain the JNW spacetime parameters based on observational data. Full article

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18 pages, 3470 KiB

Open AccessArticle

The Radioactive Elements in the Atmosphere of HD25354—Are They the Result of the Symmetric Decay of the Chemical Elements of the Island of Stability?

by Volodymyr Yushchenko, Vira Gopka, Alexander Yushchenko, Aizat Demessinova, Yeuncheol Jeong, Yakiv Pavlenko, Angelina Shavrina, Faig Musaev and Nazgul Alimgazinova

Galaxies 2024, 12(5), 57; https://doi.org/10.3390/galaxies12050057 - 27 Sep 2024

Viewed by 637

Abstract

In this research, we investigated the observed spectra of the hot peculiar star HD25354 with an effective temperature

T_{eff}

= 12,800 K, identified the lines of radioactive chemical elements, including the elements with short decay time, and estimated the abundances of these [...] Read more.

In this research, we investigated the observed spectra of the hot peculiar star HD25354 with an effective temperature

T_{eff}

= 12,800 K, identified the lines of radioactive chemical elements, including the elements with short decay time, and estimated the abundances of these elements. We tried to confirm or reject the existence of promethium lines and lines of other radioactive elements which were detected in previous investigations of this star and explain the physical mechanisms which are responsible for the synthesis of these elements in the stellar atmosphere. We used two high-dispersion spectra of HD25354 observed with the 2 m telescope of Terskol observatory with resolving power near R = 60,000, and a signal to noise ratio near 200. The spectrum of the star from the archive of the 1.93 m telescope of Haute-Provence observatory was also used. The observations were compared with synthetic spectra and the abundance of promethium was found using the best four lines of this element in the observed spectra:

log N

(Pm) = 5.84 ± 0.16 in the scale logN(H) = 12. It is comparable to the abundances of stable lanthanides in the atmosphere of this star. The abundance of thorium derived from two lines of double-ionized thorium is

log N

(Th) = 3.59 ± 0.15. The upper limits for technetium, radium, actinium, uranium, and americium abundances are found to be equal to 4.0, 3.0, 1.25, 3.5, and 4.0, respectively. Maybe the existence of promethium lines and lines of other unstable chemical elements in the spectra of HD25354, as well as the other stars of our Galaxy, Magellanic Clouds, and Fornax dwarf galaxy, can be explained by contamination of its atmosphere by the products of kilonova outburst and by symmetric decay of chemical elements with long decay times located at the island of stability (atomic numbers Z = 110–128) of transfermium elements. Maybe the decay of superheavy elements of the island of stability can be one of the reasons for the enhanced abundances of rare earth lanthanides in different types of stars. Full article

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11 pages, 278 KiB

Open AccessArticle

Clock Fields and Logarithmic Decay of Dark Energy

by Robert Brandenberger, Vincent Comeau, Leonardo Fossati and Lavinia Heisenberg

Galaxies 2024, 12(5), 56; https://doi.org/10.3390/galaxies12050056 - 26 Sep 2024

Cited by 4 | Viewed by 474

Abstract

We investigate the physical measurability of the infrared instability of a de Sitter phase in the formalism recently proposed. We find that the logarithmic decay of the effective cosmological constant is only measurable if an additional clock field is introduced. Full article

13 pages, 3623 KiB

Open AccessArticle

Slowly Rotating Peculiar Star BD00°1659 as a Benchmark for Stratification Studies in Ap/Bp Stars

by Anna Romanovskaya, Tatiana Ryabchikova, Yury Pakhomov, Ilya Potravnov and Tatyana Sitnova

Galaxies 2024, 12(5), 55; https://doi.org/10.3390/galaxies12050055 - 25 Sep 2024

Viewed by 432

Abstract

We present the results of a self-consistent analysis of the magnetic silicon star BD+00°1659, based on its high-resolution spectra taken from the ESPaDOnS archive (R = 68,000). This narrow-lined star shows the typical high Si abundance and Si ii– iii anomaly, [...] Read more.

We present the results of a self-consistent analysis of the magnetic silicon star BD+00°1659, based on its high-resolution spectra taken from the ESPaDOnS archive (R = 68,000). This narrow-lined star shows the typical high Si abundance and Si ii– iii anomaly, making it an ideal prototype for investigating the vertical distribution of Si and Fe in the stellar atmosphere. The derived abundances, ranging from helium to lanthanides, confirm the star’s classification as a silicon Bp spectral type. Silicon and iron are represented by lines of different ionisation stages (Fe i– iii, Si i– iii), indicating an ionisation imbalance interpreted as evidence of atmospheric stratification. Our stratification analysis reveals that there is a jump in iron and silicon abundances of 1.5 dex at atmospheric layers with an optical depth of

log τ_{5000}

= −0.85–−1.00. Non-LTE calculations for iron in this stratified atmosphere show minor non-LTE effects. Our results can be applied to studying the impact of stratification on the emergent flux in rapidly rotating Si stars with similar atmospheric parameters and abundance anomalies (for example, MX TrA), where direct stratification analysis is challenging due to line blending. Full article

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23 pages, 2586 KiB

Open AccessReview

IXPE View of BH XRBs during the First 2.5 Years of the Mission

by Michal Dovčiak, Jakub Podgorný, Jiří Svoboda, James F. Steiner, Philip Kaaret, Henric Krawczynski, Adam Ingram, Vadim Kravtsov, Lorenzo Marra, Fabio Muleri, Javier A. García, Guglielmo Mastroserio, Romana Mikušincová, Ajay Ratheesh and Nicole Rodriguez Cavero

Galaxies 2024, 12(5), 54; https://doi.org/10.3390/galaxies12050054 - 25 Sep 2024

Viewed by 764

Abstract

Accreting stellar-mass black holes represent unique laboratories for studying matter and radiation under the influence of extreme gravity. They are highly variable sources going through different accretion states, showing various components in their X-ray spectra from the thermal emission of the accretion disc [...] Read more.

Accreting stellar-mass black holes represent unique laboratories for studying matter and radiation under the influence of extreme gravity. They are highly variable sources going through different accretion states, showing various components in their X-ray spectra from the thermal emission of the accretion disc dominating in the soft state to the up-scattered Comptonisation component from an X-ray corona in the hard state. X-ray polarisation measurements are particularly sensitive to the geometry of the X-ray scatterings and can thus constrain the orientation and relative positions of the innermost components of these systems. The IXPE mission has observed about a dozen stellar-mass black holes with masses up to 20 solar masses in X-ray binaries with different orientations and in various accretion states. The low-inclination sources in soft states have shown a low fraction of polarisation. On the other hand, several sources in soft and hard states have revealed X-ray polarisation higher than expected, which poses significant challenges for theoretical interpretation, with 4U 1630–47 being one of the most puzzling sources. IXPE has measured the spin of three black holes via the measurement of their polarisation properties in the soft emission state. In each of the three cases, the new results agree with the constraints from the spectral observations. The polarisation observations of the black hole X-ray transient Swift J1727.8–1613 across its entire outburst has revealed that the soft-state polarisation is much weaker than the hard-state polarisation. Remarkably, the observations furthermore show that the polarisation of the bright hard state and that of the 100 times less luminous dim hard state are identical within the accuracy of the measurement. For sources with a radio jet, the electric field polarisation tends to align with the radio jet, indicating the equatorial geometry of the X-ray corona, e.g., in the case of Cyg X–1. In the unique case of Cyg X–3, where the polarisation is perpendicular to the radio jet, the IXPE observations reveal the presence and geometry of obscuring material hiding this object from our direct view. The polarisation measurements acquired by the IXPE mission during its first 2.5 years have provided unprecedented insights into the geometry and physical processes of accreting stellar-mass black holes, challenging existing theoretical models and offering new avenues for understanding these extreme systems. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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8 pages, 534 KiB

Open AccessReview

IXPE Observations of Magnetar Sources

by Roberto Turolla, Roberto Taverna, Silvia Zane and Jeremy Heyl

Galaxies 2024, 12(5), 53; https://doi.org/10.3390/galaxies12050053 - 18 Sep 2024

Cited by 1 | Viewed by 539

Abstract

Among the more than 60 sources observed in the first two years of operations, IXPE addressed four magnetars, neutron stars believed to host ultra-strong magnetic fields. We report here the main implication coming from IXPE measurements for the physics of magnetars. Polarimetric observations [...] Read more.

Among the more than 60 sources observed in the first two years of operations, IXPE addressed four magnetars, neutron stars believed to host ultra-strong magnetic fields. We report here the main implication coming from IXPE measurements for the physics of magnetars. Polarimetric observations confirmed the expectations of high polarization degrees, up to ≈

80 %

, values which have not been detected in any other source so far, providing further proof (independent from the P-

\dot{P}

estimate) that magnetars host indeed ultra-magnetized neutron stars. Polarization measurements also indicate that softer X-rays likely come from surface regions where the overlying atmosphere underwent magnetic condensation. The agreement of the phase-dependent polarization angle with a simple rotating vector model strongly supports the presence of vacuum birefringence around the star. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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19 pages, 1250 KiB

Open AccessArticle

Testing a Nonlinear Solution of the Israel–Stewart Theory

by Miguel Cruz, Norman Cruz, Esteban González and Samuel Lepe

Galaxies 2024, 12(5), 52; https://doi.org/10.3390/galaxies12050052 - 12 Sep 2024

Cited by 1 | Viewed by 570

Abstract

In this work, we test the ability of an exact solution, found in the framework of a nonlinear extension of the Israel–Stewart theory, to fit the supernovae Ia, gravitational lensing, and black hole shadow data. This exact solution is a generalization of one [...] Read more.

In this work, we test the ability of an exact solution, found in the framework of a nonlinear extension of the Israel–Stewart theory, to fit the supernovae Ia, gravitational lensing, and black hole shadow data. This exact solution is a generalization of one previously found for a dissipative unified dark matter model in the context of the near-equilibrium description of dissipative processes, where we do not have the full regime of the nonlinear picture. This generalized solution is restricted to the case where a positive entropy production is guaranteed and is tested under the condition that ensures its causality, local existence, and uniqueness. From the observational constraints, we found that this generalized solution is a good candidate in the description of the observational late-time data used in this work, with best-fit values of

H_{0} = {73.2}_{- 0.9}^{+ 0.8} \frac{k m / s}{M p c}

,

q_{0} = - {0.41}_{- 0.03}^{+ 0.03}

,

{\hat{ξ}}_{0} = {0.88}_{- 0.17}^{+ 0.09}

,

ϵ = {0.34}_{- 0.04}^{+ 0.03}

, and

k = {0.27}_{- 0.20}^{+ 0.37}

, at a

1 σ (68.3 %)

of confidence level. We show that the nonlinear regime of the Israel–Stewart theory consistently describes the recent accelerated expansion of the universe without the inclusion of some kind of dark energy component and also provides a more realistic description of the fluids that make up the late universe. Full article

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17 pages, 712 KiB

Open AccessArticle

The Density and Ionization Profiles of Optically Dark and High-Redshift GRBs Probed by X-ray Absorption

by Eka Puspita Arumaningtyas, Hasan Al Rasyid, Maria Giovanna Dainotti and Daisuke Yonetoku

Galaxies 2024, 12(5), 51; https://doi.org/10.3390/galaxies12050051 - 28 Aug 2024

Viewed by 485

Abstract

The X-ray column density (

N_{HX}

) of gamma-ray bursts (GRBs) can probe the local environment of their progenitors over a wide redshift range. Previous work has suggested an increasing trend as a function of redshift. The relevance of this current analysis [...] Read more.

The X-ray column density (

N_{HX}

) of gamma-ray bursts (GRBs) can probe the local environment of their progenitors over a wide redshift range. Previous work has suggested an increasing trend as a function of redshift. The relevance of this current analysis relies on investigating the selection bias method, such as the effect of the X-ray spectrum in high-redshift GRBs, which complicates the measurement of small

N_{HX}

; this has yet to be fully evaluated or discussed elsewhere. In this work, we evaluated these effects through simulations to define appropriate observational limits in the

N_{HX}

versus redshift plane. We then applied a one-sided nonparametric method developed by Efron and Petrosian. Within the framework of this method, we investigated the redshift dependence of

N_{HX}

and the local distribution function. Our results show that the evolution of

N_{HX}

with redshift firmly exists with a significance of more than four sigma and follows a power law of

{(1 + z)}^{1.39 (+ 0.22, - 0.27)}

. Based on these analyses and previous studies, the GRB progenitor mass varies but is more massive in the high-redshift environment and has a higher gas column density. This suggests that part of the luminosity evolution of GRBs, which has been widely reported, may be due to the evolution of the progenitor’s mass. Using the same method, we demonstrate that optically dark GRBs show a consistent evolution:

{(1 + z)}^{1.15 (+ 0.67, - 0.83)}

. By applying the Kolmogorov–Smirnov (KS) test, it is shown that optically dark GRBs have statistically identical flux and photon index distributions compared to normal GRBs, but the

N_{HX}

is systematically larger. This result suggests that the darkness of some GRB populations is not due to an intrinsic mechanism, but rather because a higher density surrounds them. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts in Multiwavelength: Theory, Observational Correlations and GRB Cosmology)

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