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#### On the Cooling of Compact Stars in Light of the HESS J1731-347 Remnant
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-**Authors:** D. Nanopoulos, P. Laskos-Patkos, Ch. C. Moustakidis
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-**Subjects:** Subjects:
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High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc); Nuclear Theory (nucl-th)
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-**Arxiv link:**https://arxiv.org/abs/2512.21406
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-**Pdf link:**https://arxiv.org/pdf/2512.21406
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-**Abstract**
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Recent analyses on the central compact object in the HESS J1731-347 supernova remnant reported not only surprising structural properties (mass $M$ and radius $R$), but also an interesting thermal evolution. More precisely, it has been estimated that $M=0.77^{+0.20}_{-0.17}M_\odot$ and $R=10.4^{+0.86}_{-0.78}$ km (at the $1\sigma$ level), while a redshited surface temperature of $153^{+4}_ {-2}$ keV at an age of 2-6 kyrs has been reported. In the present work, we conduct an in-depth investigation on the possible nature (hadronic, hybrid, quark) of this compact object by attempting to not only explain its mass and radius but also the corresponding estimations for its temperature and age. In the case of hybrid stars we also examine possible effects of the symmetry energy on the activation of different neutrino emitting process, and hence on the resulting cooling curves. We found that the reported temperature and age may be compatible to hadronic stellar configurations regardless of whether pairing effects are included. In the scenario of hybrid stars, we found that the strange quark matter core has to be in a superconducting state in order to reach an agreement with the observational constraints. In addition, the hadronic phase must be soft enough so that the direct Urca process is not activated. Furthermore, we have shown that the considered cooling constraints can be reconciled within the framework of strange stars. However, quark matter has to be in a superconducting state and the quark direct Urca process needs to be blocked.
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#### Superradiant and dynamical spin-down of neutron stars with gravitational wave implications
High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR); General Relativity and Quantum Cosmology (gr-qc)
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-**Arxiv link:**https://arxiv.org/abs/2512.21955
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-**Pdf link:**https://arxiv.org/pdf/2512.21955
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-**Abstract**
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Neutron stars such as pulsars and magnetars lose angular momentum primarily through electromagnetic dipole radiation, gravitational waves, $r$-mode oscillation, and also affected by fallback accretion processes. However, anomalous spin variations, particularly sudden enhanced spin-down rates, indicate additional spin-down mechanisms. We propose superradiant spin-down as a potential explanation for these events. By modelling the interplay between conventional and superradiant spin-down channels, we evaluate their impact on neutron star rotational evolution. We also discuss gravitational-wave emission produced by quadrupole deformation, $r$-mode oscillations, and axion-induced bosonic clouds around an isolated neutron star, highlighting their potential as distinct multimessenger probes in upcoming detectors.
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#### Radio Supernovae
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-**Authors:** Esha Kundu
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-**Subjects:** Subjects:
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High Energy Astrophysical Phenomena (astro-ph.HE)
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-**Arxiv link:**https://arxiv.org/abs/2512.22117
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-**Pdf link:**https://arxiv.org/pdf/2512.22117
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-**Abstract**
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Supernovae (SNe), the catastrophic end of stars' lives, are among the most energetic phenomena in the universe. Mapping the aftermath of the explosions to the properties of pre-SN stars is challenging due to the lack of knowledge about the evolution of different types of stars. The immediate surroundings of pre-SN stars carry the signature of the progenitors, and radio observations are the best way to examine the ambient media. Since radio emission originates from the interaction of supersonic SN ejecta with the relatively stationary circumstellar medium, with a few years of radio study, the mass-loss history of progenitor stars can be probed from just before the explosion of the star to thousands of years before the onset of the SN. Moreover, this can provide crucial details about the explosions, which are poorly understood to date. In this paper, we review the radio properties of different types of core-collapse explosions and thermonuclear runaways to understand their mass-loss evolution--which allows us to unravel the imprints of the progenitors on the surrounding media and thus the nature of the exploded stars. Additionally, we discuss the current state of the art in this field, including existing and the next-generation radio facilities with enhanced capabilities that provide further details about these explosions.
#### On the Cooling of Compact Stars in Light of the HESS J1731-347 Remnant
18
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-**Authors:**D. Nanopoulos, P. Laskos-Patkos, Ch. C. Moustakidis
19
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-**Subjects:** Subjects:
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High Energy Astrophysical Phenomena (astro-ph.HE); Astrophysics of Galaxies (astro-ph.GA)
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-**Arxiv link:**https://arxiv.org/abs/2512.20993
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High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc); Nuclear Theory (nucl-th)
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-**Arxiv link:**https://arxiv.org/abs/2512.21406
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-**Pdf link:**https://arxiv.org/pdf/2512.20993
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-**Pdf link:**https://arxiv.org/pdf/2512.21406
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-**Abstract**
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We construct a generic X-ray spectral model for the reflection component from the clumpy torus and dusty gas in the polar region (polar dusty gas) in an active galactic nucleus (AGN), designated as Inclusive spectral energy distribution Model of Polar dust And Clumpy Torus for X-ray (IMPACTX). To calculate the spectra, we utilize the Monte-Carlo based, 3-dimensional radiative transfer code SKIRT. The adopted geometry is the same as that of the IMPACT model (Ogawa et al. in prep.), consisting of a clumpy torus defined by Nenkova et al. (2008) and paraboloid-shell polar dusty gas launched at the inner radius of the torus. We discuss the dependence of the X-ray spectrum on geometrical parameters in comparison with the conventional torus-only model (XCLUMPY). As an example, we have simultaneously applied the IMPACTX and IMPACT models to the X-ray and infrared spectra of the nearby Seyfert 2 galaxy NGC 4388, and have found a solution that can well reproduce both spectra. This demonstrates the importance of using both X-ray and infrared data to constrain the nuclear structure of an AGN.
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#### DRAGNs in the Forest: Identifying Artifacts with Random Forest Models in the VLASS DRAGNs Catalog
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-**Authors:**Verene Einwalter, Eric J. Hooper, Melissa E. Morris, Sarah Bach, Yjan A. Gordon
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Recent analyses on the central compact object in the HESS J1731-347 supernova remnant reported not only surprising structural properties (mass $M$ and radius $R$), but also an interesting thermal evolution. More precisely, it has been estimated that $M=0.77^{+0.20}_{-0.17}M_\odot$ and $R=10.4^{+0.86}_{-0.78}$ km (at the $1\sigma$ level), while a redshited surface temperature of $153^{+4}_ {-2}$ keV at an age of 2-6 kyrs has been reported. In the present work, we conduct an in-depth investigation on the possible nature (hadronic, hybrid, quark) of this compact object by attempting to not only explain its mass and radius but also the corresponding estimations for its temperature and age. In the case of hybrid stars we also examine possible effects of the symmetry energy on the activation of different neutrino emitting process, and hence on the resulting cooling curves. We found that the reported temperature and age may be compatible to hadronic stellar configurations regardless of whether pairing effects are included. In the scenario of hybrid stars, we found that the strange quark matter core has to be in a superconducting state in order to reach an agreement with the observational constraints. In addition, the hadronic phase must be soft enough so that the direct Urca process is not activated. Furthermore, we have shown that the considered cooling constraints can be reconciled within the framework of strange stars. However, quark matter has to be in a superconducting state and the quark direct Urca process needs to be blocked.
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#### Superradiant and dynamical spin-down of neutron stars with gravitational wave implications
Instrumentation and Methods for Astrophysics (astro-ph.IM); Astrophysics of Galaxies (astro-ph.GA)
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-**Arxiv link:**https://arxiv.org/abs/2512.20999
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High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR); General Relativity and Quantum Cosmology (gr-qc)
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-**Arxiv link:**https://arxiv.org/abs/2512.21955
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-**Pdf link:**https://arxiv.org/pdf/2512.20999
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-**Pdf link:**https://arxiv.org/pdf/2512.21955
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-**Abstract**
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The Quick Look data products from the Very Large Array Sky Survey (VLASS) contain widespread imaging artifacts arising from the simplified imaging algorithm used in their production. The catalog of double radio sources associated with active galactic nuclei (DRAGNs) found in the VLASS first epoch Quick Look release using the DRAGNhunter algorithm suffers from contamination from these artifacts. These sources contain two or three individual components, each of which can be an artifact. We train random forest models to classify these DRAGNs based on the number of artifacts they contain, ranging from zero to three artifacts. We optimize our models and mitigate the class imbalance of our dataset with judicious training set selection, and the best of our models achieves a weighted F1 score of $97.01\%^{+1.12\%}_{-1.32\%}$. Using our classifications, we produce a catalog of VLASS DRAGNs from which an estimated 99.3% complete catalog of 97.7% artifact-free sources can be extracted.
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#### The transport of angular momentum for massive stars I. Formation of slowly rotating WNE stars
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-**Authors:**Jijuan Si, Yan Li, Xue-Feng Li, Zhi Li
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Neutron stars such as pulsars and magnetars lose angular momentum primarily through electromagnetic dipole radiation, gravitational waves, $r$-mode oscillation, and also affected by fallback accretion processes. However, anomalous spin variations, particularly sudden enhanced spin-down rates, indicate additional spin-down mechanisms. We propose superradiant spin-down as a potential explanation for these events. By modelling the interplay between conventional and superradiant spin-down channels, we evaluate their impact on neutron star rotational evolution. We also discuss gravitational-wave emission produced by quadrupole deformation, $r$-mode oscillations, and axion-induced bosonic clouds around an isolated neutron star, highlighting their potential as distinct multimessenger probes in upcoming detectors.
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#### Radio Supernovae
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-**Authors:**Esha Kundu
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-**Subjects:** Subjects:
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Solar and Stellar Astrophysics (astro-ph.SR)
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-**Arxiv link:**https://arxiv.org/abs/2512.21074
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High Energy Astrophysical Phenomena (astro-ph.HE)
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-**Arxiv link:**https://arxiv.org/abs/2512.22117
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-**Pdf link:**https://arxiv.org/pdf/2512.21074
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-**Pdf link:**https://arxiv.org/pdf/2512.22117
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-**Abstract**
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The evolutionary scenario of early-type nitrogen-sequence Wolf-Rayet (WNE) stars predicts a slowly rotating subclass that typically forms after the red supergiant (RSG) phase. Their slow rotation rates are attributed to stellar winds that remove angular momentum transferred outward during core contraction. We incorporate improved prescriptions for internal gravity waves and the magnetic Tayler instability into single massive star evolution models. Our simulations successfully produce slowly rotating WNE stars and determine optimal parameters for both mechanisms ($A \ge 10$ for internal gravity waves (IGWs), $\alpha = 0.01$ for revised Tayler instability (TSF)). The results demonstrate that the efficiency of angular momentum transfer in massive stars is significantly enhanced compared to low-mass stars, both processes can self-consistently explain the slow rotation of WNE stars, confirming their efficiency in angular momentum redistribution and providing crucial theoretical support for the existence of this predicted stellar population.
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#### Asteroseismology and Dynamics Reveal Interior Structure and Coeval Evolution in the Triply Post-Main-Sequence system DG Leo
$\delta$ Scuti stars in binary or multiple systems serve as crucial probes for studying stellar pulsation and evolution. However, many such systems are not ideal for asteroseismology due to uncertainties in mass transfer with close companions and the challenges of dynamically measuring all components' physical properties. The triple system DG~Leo, comprising an inner binary and a distant $\delta$ Scuti star, is an ideal target due to its well-separated pulsator. By combining new \textit{TESS} photometry with archival spectroscopy, our dynamical analysis shows that the system's three components share similar masses, radii, and luminosities within errors, occupying coincident Hertzsprung--Russell diagram positions, indicative of coeval evolution. By fitting seven observed $\delta$ Scuti frequencies through asteroseismic modeling with dynamically constrained theoretical grids, we simultaneously trace the pulsating star's evolution and constrain the triple system's evolutionary stage, with the derived fundamental parameters showing consistency with the dynamical solutions. Our analysis reveals that all three components of DG~Leo are in the post-main-sequence phase, with a system age of $0.7664^{+0.1402}_{-0.1258}$~Gyr. Additionally, the $\delta$ Scuti component shows multiple non-radial modes with significant mixed-character frequencies, providing precise constraints on its convective core extent ($R_{\mathrm{cz}}/R = 0.0562^{+0.0137}_{-0.0021}$).
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#### A Near-Infrared and Optical Study of NGC 5822: An Open Cluster Hosting Barium-stars and Lithium-Enriched Giant Stars
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-**Authors:** N. Holanda, V. Loaiza-Tacuri, A. Sonally, S. Bijavara Seshashayana, M. P. Roriz, C. F. Martinez, M. Borges Fernandes, C. B. Pereira, O. J. Katime Santrich, S. Daflon
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-**Subjects:** Subjects:
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Solar and Stellar Astrophysics (astro-ph.SR)
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-**Arxiv link:**https://arxiv.org/abs/2512.21289
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-**Pdf link:**https://arxiv.org/pdf/2512.21289
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-**Abstract**
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We present a chemical abundance study of giant stars in the Galactic open cluster NGC 5822, which hosts two barium stars (#002 and #201) and three lithium-enriched giants (#006, #102, and #240). Using high-resolution optical and near-infrared ($H$ and $K$ band) spectra from FEROS and IGRINS, we determine atmospheric parameters and abundances for 23 elements (Li, C, N, O, F, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, Cr, Fe, Ni, Y, Ce, Nd, Yb, and Pb). This includes species not yet studied in this cluster, such as F, P, K, Yb, and Pb, as well as oxygen isotopic ratios $^{16}$O/$^{17}$O and $^{16}$O/$^{18}$O. Membership was assessed using astrometry and chemical abundances, providing insight into the evolutionary stages of Li-enriched giants and cluster parameters (age, distance, extinction). However, the identification of Ba-stars remains challenging due to their binary nature and less reliable astrometric solutions. The cluster's abundances are broadly consistent with expectations for the Galactic thin disk. The mean fluorine abundance agrees with chemical evolution models predicting that young clusters (<2 Gyr) exhibit elevated [F/Fe], with production from SN II, SN Ia, AGB, and Wolf-Rayet stars. No distinct chemical or rotational features were found to explain the lithium enrichment, likely occurring either during the red clump phase or near the RGB tip. For the Ba-stars, nucleosynthesis models combined with the cluster's turn-off mass suggest polluting companion masses of 3.00 and 3.75 $M_{\odot}$ for stars #002 and #201. These results highlight the importance of open clusters as laboratories for chemically peculiar stars.
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Supernovae (SNe), the catastrophic end of stars' lives, are among the most energetic phenomena in the universe. Mapping the aftermath of the explosions to the properties of pre-SN stars is challenging due to the lack of knowledge about the evolution of different types of stars. The immediate surroundings of pre-SN stars carry the signature of the progenitors, and radio observations are the best way to examine the ambient media. Since radio emission originates from the interaction of supersonic SN ejecta with the relatively stationary circumstellar medium, with a few years of radio study, the mass-loss history of progenitor stars can be probed from just before the explosion of the star to thousands of years before the onset of the SN. Moreover, this can provide crucial details about the explosions, which are poorly understood to date. In this paper, we review the radio properties of different types of core-collapse explosions and thermonuclear runaways to understand their mass-loss evolution--which allows us to unravel the imprints of the progenitors on the surrounding media and thus the nature of the exploded stars. Additionally, we discuss the current state of the art in this field, including existing and the next-generation radio facilities with enhanced capabilities that provide further details about these explosions.
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