High Energy Astrophysical Phenomena · Cross-listed from nucl-th
Formation of bound composite vortices of a singly-quantized $^1$S$_0$ vortex and half-quantized $^3$P$_2$ vortices in the $^1$S$_0$-$^3$P$_2$ coexisting phase in neutron stars
Tatsuhiro Hattori, Muneto Nitta and Kazuyuki Sekizawa
Pulsar glitches are believed to originate from the dynamics of quantized
vortices in the
neutron superfluid interior. The outer core of a
neutron star hosts a $^3\text{P}_2$
spin-triplet superfluid, whose half-integer
quantum vortices (HQVs) are qualitatively different from the $^1\text{S}_0$ singly quantized
vortices (SQVs) in the inner crust. It has recently been proposed that the coupling between these two
vortex species gives rise to a large-scale
vortex network, providing a candidate mechanism for the diversity of observed
pulsar glitch phenomena. Using the Gross–Pitaevskii equations for the $^1\text{S}_0$ and $^3\text{P}_2$ condensates, we perform two-dimensional simulations of one SQV and two HQVs in a coexistence phase near the crust-core boundary, varying the density–density and Josephson coupling constants. We find that the Josephson term, arising from the relative phase between the two condensates, induces a strong attractive interaction between the two HQVs and the SQV, which dominates over the density–density coupling. When pinning potentials are applied to the HQVs and the SQV at spatially separated locations, this attraction is found to be sufficiently strong to drive
vortex depinning. These results suggest that two HQVs and one SQV can form a tightly bound composite
vortex at the crust-core boundary, with implications for the glitch mechanism in
neutron stars.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Accurate waveforms for generic planar-orbit binary black holes: The multipolar effective-one-body model SEOBNRv6EHM
Aldo Gamboa, Alessandra Buonanno, Lorenzo Pompili et al. (+8 more)
Accurate and computationally efficient waveform models are required to infer the parameters of compact binaries from their gravitational wave (GW) emission. Among these parameters, orbital eccentricity serves as a smoking gun for dynamical formation channels and must be accounted for to avoid systematic errors in GW analyses. Here, we present SEOBNRv6EHM, a time-domain, multipolar waveform model for binaries on generic planar orbits, calibrated to quasi-circular (QC) numerical-relativity (NR) simulations from the SXS collaboration. In addition to the dominant $(2,2)$ mode, the model provides the $(2,1)$, $(3,3)$, $(3,2)$, $(4,4)$, and $(4,3)$ multipoles for the full inspiral-merger-ringdown process of coalescing binaries, as well as for dynamical captures and scattering encounters. The model is built within the effective-one-body (EOB) framework, and it employs novel resummations of the radiation-reaction force and waveform modes. We validate its accuracy through comparisons against 592 QC, 319 eccentric, one dynamical-capture, and two scattering SXS NR waveforms, and through scattering-angle comparisons against 61 SXS NR simulations. For QC and small-eccentricity binaries, its accuracy is comparable to previous-generation SEOBNRv5 models. For highly eccentric systems, however, SEOBNRv6EHM attains unprecedented accuracy, with waveform mismatches remaining below or close to $ 2\% $ across the total mass range $ 20-200\, \mathrm{M}_\odot $ for eccentricities up to $\sim 0.9$ at 14 periastron passages before merger. Additionally, SEOBNRv6EHM achieves waveform-generation walltimes that are $ 2 - 6 $ times faster than other state-of-the-art EOB eccentric models, enabling efficient and accurate applications in GW astronomy.
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All authors Aldo Gamboa, Alessandra Buonanno, Lorenzo Pompili, Raffi Enficiaud, Michael Boyle, Lawrence E. Kidder, Oliver Long, Peter James Nee, Harald P. Pfeiffer, Antoni Ramos-Buades, Mark A. Scheel
High Energy Astrophysical Phenomena
Magnetic Configuration Imprints on Quasi-Periodic Variability in GRMHD Simulations of Thin Accretion Disks
Jing-Ze Xia, Hong-Xuan Jiang, Indu K. Dihingia et al. (+1 more)
The origin of quasi-periodic oscillations (QPOs) in black hole
accretion flow remains uncertain, particularly regarding the role of
magnetic field configurations in shaping disk structure and variability signatures. We investigate this using global two- and three-dimensional (2D and 3D) general relativistic magnetohydrodynamic (GRMHD) simulations of geometrically thin disks initialized with different multi-loop
magnetic field configurations. These configurations naturally produce a puffed-up inner region. We find that QPO-like variability arises in the effective viscosity and mass
accretion rate, with frequencies following the local radial epicyclic frequency and its harmonics. Time-series diagrams show coherent, inclined stripe-like patterns associated with inertial-acoustic perturbations, while power spectra exhibit narrow bands of enhanced variability linked to truncation radii associated with
magnetic fields. Cross-correlation analysis reveals a finite lag between pressure and Maxwell stress at these interfaces, consistent with viscous-epicyclic overstability. The
magnetic topology regulates both the truncation radius and the location of resonant cavities that sustain oscillations. As the disk becomes thicker, increased turbulent
diffusion suppresses the overstability and the associated QPO signals. We find that the QPO frequency ranges and their
evolution are consistent with observations of black hole X-ray binaries during outbursts. These results suggest that
magnetic field configurations play a pivotal role in shaping disk structure and variability in accreting black holes.
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All authors Jing-Ze Xia, Hong-Xuan Jiang, Indu K. Dihingia, Yosuke Mizuno
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
First-Order Perturbations of Covariant Maxwell Equations in Gravitational Waves
Lingyue Lou, Haorong Wu and Xi-Long Fan
We present a systematic theoretical framework for investigating first-order electromagnetic (EM) perturbations induced by gravitational waves (GWs). Beginning with the covariant Maxwell equations, we derive the complete first-order perturbation equations in terms of both the EM field tensor and the four-potential, demonstrating their equivalence alongside the residual gauge invariance under the Lorenz gauge condition. Furthermore, explicit first-order expressions for the induced electric and
magnetic fields, as well as the associated EM energy-momentum tensor, are obtained. As an explicit illustration, we analytically evaluate the interaction between a plane EM wave and a GW within the transverse-traceless gauge. By demonstrating that the maximum modulus of the coupling coefficient is on the order of $10^2$, we quantitatively establish that a typical astrophysical GW with a dimensionless strain of $h_0 \sim 10^{-21}$ generates a first-order EM response on the order of $10^{-19}$ relative to the incident field amplitude.
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Cosmology and Nongalactic Astrophysics
Testing cosmic anisotropy with the Combo correlation of gamma-ray bursts
Dong Zhao, Hao-Ran Duan and Jun-Qing Xia
We employ the sample of 244 gamma-ray bursts (GRBs; i.e., C244) with the Combo correlation to test
cosmic anisotropy. Meanwhile, the Pantheon sample is introduced to verify whether the GRB sample can suppress the fake anisotropic signals induced by inhomogeneous spatial distributions. In the dipole fitting (DF) method, under the dipole-modulated $\Lambda$CDM model, the C244 sample shifts the best-fitting longitude $l$ derived from the Pantheon sample by $54.09^\circ$ and reduces the uncertainty in $l$ by approximately $40\%$. Compared to the 118 GRBs (i.e., A118) with the $E_\mathrm{p}$-$E_\mathrm{iso}$ correlation, the shift in longitude $l$ increases by additional $21.35^\circ$. In the hemisphere comparison (HC) method, the preferred direction derived from the C244+Pantheon sample deviates from that of the Pantheon-only sample by more than $1\sigma$. In contrast, the preferred direction from the A118+Pantheon sample is consistent with the Pantheon-only result within the $1\sigma$ uncertainty. The preferred direction changes significantly as the number of GRBs increases from 118 to 244. Our results show that a larger GRB sample can reduce the fake anisotropic signals caused by inhomogeneous spatial distributions. Accordingly, we suggest that GRBs have the potential to provide a reliable probe of
cosmic anisotropy.
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Cosmology and Nongalactic Astrophysics
Measuring the Hubble constant with strongly lensed gravitational waves from space-based detector networks
Yong Yuan, Minghui Du, Wen-Fan Feng et al. (+4 more)
The measurement of the Hubble constant $H_0$ plays a central role in modern cosmology. In this work, we investigate the potential of strongly lensed gravitational-wave (SLGW) signals from massive binary black hole mergers to constrain $H_0$ using future space-based detector networks. We consider two observational scenarios: one in which the source
redshift is unknown, and another in which it is independently determined through electromagnetic observations. We show that meaningful constraints on $H_0$ can still be achieved without source-
redshift information, provided that the lens
redshift is known. For individual SLGW events, the joint Taiji+LISA analysis improves the measurement precision of $H_0$ by approximately a factor of two compared with the Taiji-only configuration. Extending the analysis to the population level, we combine five simulated SLGW events and find that the uncertainty in $H_0$, quantified by the 95% credible interval, reaches the $1.1\times10^{-1}$ level when the source
redshift is treated as unknown, and further improves to $4.2\times10^{-2}$ when the source
redshift is independently measured. Our results demonstrate that joint space-based gravitational-wave observations can substantially enhance the cosmological capability of SLGW events and provide a promising avenue for precision measurements of the Hubble constant.
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All authors Yong Yuan, Minghui Du, Wen-Fan Feng, Benyang Zhu, Qing Diao, Peng Xu, Xilong Fan
Cosmology and Nongalactic Astrophysics
Exploring non-Poisson satellite occupation in HOD models and its impact on 2- and 3-point galaxy clustering
Antoine Rocher
Understanding the connection between
galaxies and dark matter
halos is a central challenge in modern cosmology. The
Halo Occupation Distribution (HOD) framework provides a widely used statistical description of how
galaxies populate dark matter
halos, enabling precise modelling of
galaxy clustering. A common assumption in standard HOD models is that the number of satellite
galaxies follows a Poisson distribution at fixed
halo mass. In this work, we revisit this assumption and introduce the Conway-Maxwell-Poisson (CMP) distribution as a minimal extension of of the Poisson model, which add a single parameter, $\nu$, to explore sub- and super-Poisson behaviour. We derive analytical approximations for the CMP expectation parameter $\lambda$ and develop a numerical scheme that smoothly connects small- and large-$\lambda$ regimes, achieving $\sim5\%$ accuracy for $0.5 < \nu < 2$. Using the HODDIES package, we study the impact of non-Poisson satellite occupations on mock
galaxy catalogues and clustering statistics. Variations in the variance of the satellite occupation significantly affect small-scale clustering, producing deviations of up to $10\%$ in projected clustering and $5\%$ in the monopole and quadrupole. We further investigate higher-order statistics using counts-in-cylinders (CiC) and the tree-level
galaxy bispectrum. CiC statistics are highly sensitive to changes in the variance, with variations up to $\sim30\%$, while the tree-level
galaxy bispectrum (in the Sugiyama basis) is only weakly affected ($<2\%$ up to $k_\mathrm{max} = 0.3$). These results suggest that non-Poisson satellite statistics are important for small-scale analyses, but should have a limited impact on cosmological constraints from power spectrum and bispectrum measurements using large scales $k_\mathrm{max} < 0.3$.
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Earth and Planetary Astrophysics
Triaxial shapes and densities of G!kún||'hòmdímà, Haumea, and Varda from stellar occultations
Benjamin Proudfoot, Will Grundy, Flavia Luane Rommel et al. (+2 more)
The shapes and densities of mid-sized and large trans-Neptunian objects (TNOs) are pivotal for understanding a variety of important aspects of
planet formation. In this work, we present a
Bayesian shape modeling method which combines constraints from rotational light curves and satellite orbits to construct three-dimensional shape models of TNOs. We use it to reanalyze three
stellar occultations of the TNOs (229762) G!kún||'hòmdímà (2007 UK$_{126}$), (136108) Haumea, and (174567) Varda. By assuming that their satellites (or ring) orbit in their respective equatorial planes, we are able to derive unique shape models for both G!kún||'hòmdímà and Haumea. Our derived shape for G!kún||'hòmdímà is spheroidal with $a = b = 329^{+4}_{-3}$ km and $c = 294^{+11}_{-10}$ km, with a system density $\rho = 1007^{+50}_{-49}$ kg m$^{-3}$. For Haumea, we find $a = 1061^{+87}_{-71}$ km, $b = 844^{+5}_{-7}$ km, and $c = 514^{+18}_{-19}$ km, providing $\rho = 2050^{+157}_{-152}$ kg m$^{-3}$. For Varda, after updating its mutual orbit with its satellite Ilmarë, we find that currently published data are unable to fully constrain its three-dimensional shape. Intriguingly, Varda's elongated limb appears to point towards its satellite at the time of the occultation. With a $\sim$2% chance of such an
alignment happening randomly, this may be suggestive of a frozen-in
tidal and/or rotational bulge. Our work emphasizes the importance of how external constraints can improve occultation analyses. With continued observations of rotational light curves,
stellar occultations, and satellite orbits, these and other TNOs can have their shapes and densities further refined.
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All authors Benjamin Proudfoot, Will Grundy, Flavia Luane Rommel, Estela Fernández-Valenzuela, Darin Ragozzine
Earth and Planetary Astrophysics
Comparing telluric removal methods in their capability to recover injected exoplanet atmosphere signals with high resolution emission spectroscopy
Marcelo Aron Fetzner Keniger, Matteo Brogi, David Armstrong et al. (+1 more)
For ground-based high-resolution spectroscopic
exoplanet atmosphere studies, removing the contamination from telluric and
stellar lines is a crucial step in the analysis process. Despite that, there is no consensus in the literature on the most appropriate way to carry this out. Typically, tellurics are either directly modelled to a percent-level precision, or blindly detrended via Principal Component Analysis (PCA)
algorithms, the latter particularly common at infrared wavelengths. Here, we compare three different detrending methods, PCA, Molecfit and our own fitting
algorithm Astroclimes, measuring their performance in the context of removing telluric and
stellar lines to detect exoplanetary atmospheric signals. We specifically look for H2O detections, which are particularly affected by residual, time-correlated variability of the telluric spectrum. We use near-infrared CARMENES observations of the day-side of $\tau$ Bootis b to carry out injection and recovery tests. We find that while PCA can sometimes achieve higher SNR, it comes at the expense of stronger signal degradation. All methods struggle more for injected signals with lower orbital velocities, not just affecting the signal's magnitude but its location in velocity space as well. This behaviour is more prominent for PCA than for Astroclimes and Molecfit. These results highlight the importance of understanding the effects of different detrending methods on exoplanetary signals, which can lead to biases when characterising real detections. Finally, we report that our attempts to detect a previously claimed water signal from $\tau$ Bootis b all resulted in non-detections.
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All authors Marcelo Aron Fetzner Keniger, Matteo Brogi, David Armstrong, Siddharth Gandhi
Cosmology and Nongalactic Astrophysics
Dark Quest II: A Wide-Coverage Neural Network Emulator of the Nonlinear Matter Power Spectrum Across Extended Cosmologies
Satoshi Tanaka, Takahiro Nishimichi and Yosuke Kobayashi
DarkEmulator2 is a neural network emulator of the nonlinear matter power spectrum in a nine-dimensional $w_0 w_a \nu o \mathrm{CDM}$ parameter space, developed as the emulator component of the Dark Quest II (DQ2) program. It is trained on simulations generated with the Ginkaku code, whose numerical implementation, accuracy tests, and post-processing pipeline are described in the companion paper. The design follows a unified strategy: in addition to the cosmological parameter vector, we supplement the neural network's inputs with three families of physically motivated auxiliary quantities – the linear matter power spectrum, descriptors of the simulation resolution, and a low-dimensional summary of the initial Gaussian random field – that are expected to improve generalization across the parameter space. Training a single network jointly across three simulation resolution tiers allows the emulator to exploit a small number of high-resolution simulations while retaining broad coverage from lower-resolution simulations. For a $L_{\mathrm{box}}=1\,\hiGpc$ box with $N=3000^{3}$ particles, the emulator reproduces the simulated matter power spectrum to subpercent accuracy up to the particle Nyquist scale, $k_{\mathrm{Ny}}\simeq 10\,\hMpci$. The emulator remains accurate over the calibrated wavenumber range, while its highest-$k$ predictions depend on the simulation resolution and shot noise. We validate the emulator on independent test suites and, through a cross-comparison with several public emulators and widely used fitting formulas, characterize the inter-model consistency and the parameter-dependent trends in their residuals.
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Cosmology and Nongalactic Astrophysics
GINKAKU: Scalable Cosmological Structure Formation Simulation Code and Post-processing Pipeline
Takahiro Nishimichi, Satoshi Tanaka and Kohji Yoshikawa
We introduce GINKAKU, a new cosmological $N$-body code developed for the Dark Quest II (DQ2) simulation campaign and designed for controlled ensemble production across the cosmological model space required by next-generation
galaxy surveys, including massive
neutrinos and clustering dark energy. Built on the FDPS framework, GINKAKU couples a TreePM
gravity solver with a linear-response treatment of external source terms for components not evolved as $N$-body particles, formulated in the $N$-body gauge. This design incorporates massive-
neutrino perturbations, general-relativistic corrections, early-time radiation perturbations, and dark-energy clustering with non-unit effective sound speed at the linear level, while preserving Newtonian particle dynamics on subhorizon scales. The code is validated through internal convergence studies and cross-comparisons with GADGET, PKDGRAV3, and RAMSES on shared initial conditions: code-to-code differences in the nonlinear power spectrum can be reduced below $\sim1\%$ level by tuning internal accuracy parameters, and we identify a production-grade fiducial setting achieving this control at modest cost. We apply GINKAKU to an initial set of DQ2 production runs – eight cosmological models with $3,000^3$ particles in boxes up to $4\,h^{-1}\mathrm{Gpc}$ – processed by a renewed post-processing pipeline that reduces the inter-resolution spread of the
halo mass function to $\sim 1\%$ and includes
halo-shape measurements for intrinsic-
alignment statistics. The scale-dependent-growth cosmologies reproduce the expected nonlinear signatures of massive
neutrinos and clustering dark energy, demonstrating suitability for emulator-scale production. A total matter power spectrum emulator from these runs is presented in an accompanying paper. (abridged)
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Instrumentation and Methods for Astrophysics
Unsupervised Morphological Characterization of Gravitational-Wave Glitches in LIGO O4a Using Frozen DINOv2 Features
Luca Cirfeta
A central open question in gravitational-wave detector characterization is whether the O4a observing run has introduced glitch morphologies not present in earlier runs. We present gravi-signal-ml, an open-source pipeline for unsupervised morphological characterization of instrumental noise transients (glitches) in LIGO gravitational-wave data, applied to 1,277 hours of public O4a strain data from the Hanford and Livingston detectors. The pipeline extracts 384-dimensional visual
embeddings from Q-transform spectrograms using a frozen DINOv2
Vision Transformer with register
tokens (ViTS/14), requiring no labeled training data.
Embeddings are projected via PCA and UMAP with cosine metric, then clustered using a Dirichlet Process Mixture Model (DPMM). Cluster robustness is systematically assessed through ablation studies, stability analysis across hyperparameter perturbations, and morphological cross-check against an in-domain
Gravity Spy O3b reference index. A time-slide background test excludes statistically significant H1–L1 coincidences ($p \geq 0.1$) in all sessions. Across 188,000+ spectrograms, no morphologically novel glitch candidates were identified – all anomalous clusters map to known
Gravity Spy classes with cosine similarity $> 0.98$. L1
embeddings show consistently high robustness (ablation ARI $> 0.90$ in all four sessions), while H1 exhibits lower and more variable grayscale ablation ARI ($\sim 0.68$–$0.90$), suggesting a structural difference in the H1 noise
manifold under DINOv2 feature extraction. This null result, obtained with a fully validated pipeline, establishes a reproducible baseline for zero-shot glitch morphology characterization in O4a data. The pipeline and all results are publicly available at https://github.com/lucacirfeta/dante-gravi-signal-ml DOI: https://doi.org/10.5281/zenodo.20121860.
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Astrophysics of Galaxies
Isochrone Fitting of Galactic Globular Clusters – VIII. Homogeneous estimates of parameters for 27 clusters
G. A. Gontcharov, O. S. Ryutina, C. J. Bonatto et al. (+2 more)
We use the Stetson, HST, and
Gaia DR3 data sets for 22
Galactic globular clusters to select their members and fit their CMDs with isochrones from DSED and BaSTI for $[\alpha/$Fe$]=+0.4$ and an adopted helium mass fraction $Y$. As a result, we estimate the
metallicity [Fe/H], age, distance from the Sun, and reddening E(B-V) for these clusters. Special
attention is paid to identify variable
stars among the cluster members. We combine these results with our earlier estimates for five other clusters into a homogeneous set of parameters for 27 clusters and investigate relationships among these parameters and their statistical properties. In particular, we count the
giants on the red (RGB), horizontal (HB), and asymptotic branches using the clean data sets cross-identified to cover the entire cluster fields and count each
giant only ones. This allows us to calculate the $R$-parameter, the ratio between the numbers of the HB and RGB, with unprecedented accuracy and to examine its relations with other parameters. These relations are much stronger for accreted clusters rejecting a constant $R$ value for them, possibly because of their heterogeneous origins and environments. For all in-situ clusters, a constant value of $R=1.31^{+0.06}_{-0.05}$ is consistent with the data. According to previous calculations, this value may imply anomalous energy losses in HB
stars corresponding to an
axion–
photon coupling of $g_{a\gamma}=(0.66^{+0.11}_{-0.13})\times10^{-10}\,\mathrm{GeV}^{-1}$. However, the observed dependencies of $R$ on cluster parameters require improved statistics and a theoretical understanding of these dependencies. By comparing $R$-parameter estimates from HST and
Gaia in cluster centres and peripheries, respectively, we suggest that the peripheral HB population is depleted by about half when the cluster crosses the
Galactic disk, then the HB recovers over 60-80 Myr.
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All authors G. A. Gontcharov, O. S. Ryutina, C. J. Bonatto, S. S. Savchenko, S. V. Troitsky
Astrophysics of Galaxies
Spectroscopic metallicities and first {\alpha}-element abundances of RR Lyrae stars in Baade's Window
J. Olivares Carvajal, Á. Rojas-Arriagada, M. Zoccali et al. (+10 more)
RR Lyrae
stars in the bulge have been reported to be associated with the spheroidal, relatively metal-poor component. They offer a way to trace this component with precise distances. While a few studies of RR Lyrae spectra with medium/high resolution are now available, none of them target
stars in the
Galactic bulge. We present here a spectroscopic determination of Fe and {\alpha}-element abundances for RR Lyrae
stars in the
Galactic bulge, with the main goal of providing a
benchmark to calibrate other
metallicity indicators, appropriate for this specific
stellar population. We analyzed FLAMES/GIRAFEE spectra of 78 RR Lyrae
stars (60 ab-type and 18 c-type). We applied a full-spectrum fitting technique to obtain the spectroscopic
metallicity and overall {\alpha}-element abundance. Distances are derived by means of a period-luminosity-
metallicity relation, and orbits are computed by combining the radial velocities derived here with the proper motions from DR3.
Gaia The resulting
metallicities peak at [Fe/H] _median = -1.34 +- 0.04 and -1.44 +- 0.08 dex for ab and c-types respectively. The majority of the bulge RR Lyrae are metal-poor
stars with relatively high {\alpha}-element abundances around [{\alpha}/Fe] ~ 0.25 +- 0.03 dex. We used our spectroscopic measurements to test different methods for deriving
metallicities based on photometry, which utilize
Fourier parameters in the light curves of the RR Lyrae. The data suggest a possible correlation between the
metallicity difference and the [{\alpha}/Fe] ratio, which needs to be investigated further. There are some ab-type RR Lyrae that show
metallicities higher than -1 dex and low [{\alpha}/Fe] values. We studied these
stars kinematically and found a difference between three
stars with similar [{\alpha}/Fe] values and the main group, indicating that they may be slightly younger and correspond to the disk population.
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All authors J. Olivares Carvajal, Á. Rojas-Arriagada, M. Zoccali, B. Acosta-Tripailao, M. De Leo, R. Albarracín, M. Sanchez-Benavente, E. Valenti, G. Bono, S. Duffau, L. Sbordone, M. Fabrizio, V.F. Braga
Instrumentation and Methods for Astrophysics
Noise Suppression and Radio Frequency Interference Rejection for Self-Triggered Radio Detectors of Extensive Air Showers
Pengfei Zhang, Xin Xu, Hanrui Wang et al. (+8 more)
Self-triggered
radio detection of ultra-high-energy
cosmic rays and
neutrinos offers a scalable and cost-effective approach for next-generation astroparticle observatories, but remains challenging under realistic
radio-frequency interference (RFI) conditions. In the classical air-shower
radio band, the achievable sensitivity and trigger reliability are critically limited by the balance between external sky background noise and internal detector-unit noise, as well as by non-stationary anthropogenic interference. In this work, we present an end-to-end design and experimental characterization of a self-triggered
radio detector unit explicitly optimized to operate in a
galactic-noise-dominated regime. Rather than focusing on a single hardware component or trigger
algorithm, we adopt a system-level methodology that coherently integrates sky-noise modeling, RF-chain noise budgeting, electromagnetic compatibility (EMC) mitigation, and measurement-driven validation. By using the
galactic radio background as a quantitative reference, we assess the internal noise performance of the detector unit and demonstrate conditions under which extensive air shower (EAS)
radio signals can be distinguished from anthropogenic interference at the system-response level. We further introduce an indirect noise-quantification method to estimate the low-noise amplifier contribution within the complete RF chain based on differential internal-noise measurements evaluated at the ADC level. The proposed detector unit is validated through laboratory and on-site measurements, demonstrating operation close to the
galactic-noise limit in the core frequency band. These results provide a practical and transferable methodology for the design and deployment of large-scale self-triggered
radio arrays such as GRAND.
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All authors Pengfei Zhang, Xin Xu, Hanrui Wang, Xing Xu, Bohao Duan, Feng Wei, Hongwei Pan, Xishui Tian, Yi Zhang, Pengxiong Ma, Olivier Martineau-Huynh
High Energy Astrophysical Phenomena
Mass and radius measurements of the neutron star 47~Tuc X7 – A new bias-free method
C. Kazantsev, S. Guillot, L. Mauviard et al. (+2 more)
Neutron star (NS) radius measurements provide precious information to constrain the dense matter equation of state (EOS). Quiescent low-mass X-ray binaries (qLMXBs) have been used for this purpose, but a number of sources of systematic biases were uncovered, making other sources more favored for EOS studies. We aim to reintroduce qLMXBs as reliable sources of NS mass and radius measurements with a new method, free of systematic biases. We test our implementation on the qLMXB X7 in the globular cluster 47 Tucanae. We used X-PSI to perform the spectral analysis of the 47Tuc X7 observations. X-PSI accurately models the effects of the unknown NS rotation and possible surface anisotropies (two sources of biases in qLMXBs) on the NS spectra. The most significant source of bias on the radius is usually the chemical composition of the NS atmosphere, which, in the case of 47Tuc X7, is known to be hydrogen-rich. A broad range of masses and radii was explored. We obtain a NS radius at 1.4 $M_\odot$ of $R_{1.4} = 12.9\pm0.4$ km (68% credible interval). A shift of the radius by less than a % is measured compared to the model where these sources of systematic uncertainties are neglected. More importantly, including rotation and surface anisotropies in the modeling does not significantly broaden the radius posteriors. We also place strong constraints on the X-ray pulsed fraction (upper limit of 6.0% at a 99.97% credible level) caused by the possible presence of a hot spot. This suggests that, for 47Tuc X7, robust radius constraints can be obtained even without considering systematics, likely because of the deep exposures. We use the resulting M-R constraints from this NS to quantify the improvement on an EOS inference when combined with other measurements. We show that, using recently developed tools, qLMXBs can be exploited to infer reliable NS masses and radii, which can in turn constrain the EOS.
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All authors C. Kazantsev, S. Guillot, L. Mauviard, T. Salmi, N.A. Webb
Instrumentation and Methods for Astrophysics
FARSim: a fast RF-chain-aware trigger-screening surrogate for radio detection of ultra-high-energy cosmic rays
Xin Xu, Pengfei Zhang, Fufu Yang et al. (+3 more)
Radio arrays provide a scalable route to detecting extensive air showers from ultra-high-energy
cosmic rays, but trigger studies for candidate layouts are expensive when every energy, arrival direction, core position and trigger configuration is evaluated with full
radio simulations. We present FARSim, a fast surrogate framework that reuses a reduced library of ZHAireS reference footprints to reconstruct ground-plane
radio emission and to estimate trigger-relevant observables. The method combines vector geomagnetic and charge-excess field decomposition, geomagnetic-angle and energy scaling, geometrical projection, contour-based core sampling and event-rate integration. We validate the reconstructed field footprints and trigger regions against dedicated ZHAireS simulations, and quantify the computational gain obtained by replacing repeated full shower simulations with fast footprint queries. We further extend the peak-field surrogate to time-domain electric-field synthesis by combining the predicted three-component peak-field vector with geometry-dependent normalized pulse templates. Propagating these traces through an RF-chain response enables voltage-domain threshold and L1-trigger diagnostics. For the validation samples considered here, the time-domain extension reaches a median vector-waveform R^2 of 0.986 over 2112 held-out ZHAireS traces when tested at the true peak amplitude. FARSim is therefore intended as a rapid, physics-informed screening layer for array-layout and trigger studies; absolute exposure predictions and detector commissioning remain the role of full end-to-end simulations.
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All authors Xin Xu, Pengfei Zhang, Fufu Yang, Pengxiong Ma, Ramesh Koirala, Chao Zhang
Instrumentation and Methods for Astrophysics
Hybrid neural denoising for resource-efficient near- and sub-threshold radio triggering of extensive air showers
Qader Dorosti, Alperen Aksoy, Ilja Bekman et al. (+7 more)
Autonomous
radio self-triggering for extensive air showers requires strong rejection of
radio-frequency interference while preserving weak pulses within station-level hardware constraints. We present a hybrid neural trigger comprising a compact waveform denoiser followed by a classifier. The method is evaluated using experimentally measured high-interference background traces and detector-folded simulated air-shower pulses produced with the Pierre Auger Offline chain, with the
benchmark concentrated in the near-threshold regime. Hardware constraints are incorporated through hyperparameter optimisation, quantisation-aware training, and high-granularity fixed-point quantisation. Applying the conventional peak-envelope trigger after denoising increases its area under the receiver-operating-characteristic curve from 0.63 to 0.98. At a false-positive rate of 10^-4, the full denoiser-classifier chain retains about 41% of the held-out signal traces, compared with 27% for the classifier acting on the raw waveform and about 2% for the peak-envelope reference. The fixed-point firmware is synthesised, placed, and routed on representative field-programmable gate array targets, where it achieves timing closure with microsecond-scale latency and compact arithmetic-resource usage. These results establish hybrid neural denoising as a practical FPGA-compatible route toward
radio-only triggering of weak and inclined air-shower signals in noisy environments.
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All authors Qader Dorosti, Alperen Aksoy, Ilja Bekman, Markus Cristinziani, Eric-Teunis de Boone, Vesselin Dimitrov, Chimezie Eguzo, Stefan Heidbrink, Stefan van Waasen, Andre Zambanini
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Dark Matter Interpretation of the Super-Kamiokande Antineutrino Excess in $\mathrm{U}(1)_{L_\mu-L_\tau}$ model
Motoi Endo, Yushi Mura and Tenta Tsuji
Recent Super-Kamiokande analyses of the diffuse
supernova neutrino background, based on data across all SK phases, indicate a mild preference over the zero-DSNB hypothesis at the level of about $2.3\sigma$ with electron-like antineutrino events at $E_{\bar{\nu}_e} \simeq 20\,\mathrm{MeV}$. We investigate whether this excess can be explained by MeV-scale dark matter annihilation into
neutrinos in a $\mathrm{U}(1)_{L_\mu - L_\tau}$ model. The dark matter is a Dirac
fermion with $m_\chi \simeq 22\,\mathrm{MeV}$ that annihilates via a light $Z'$ mediator into $\nu_\mu \bar{\nu}_\mu$ and $\nu_\tau \bar{\nu}_\tau$, which are partly converted into $\bar{\nu}_e$ through
flavor oscillations. We find that this scenario simultaneously accounts for the excess and the observed relic abundance via thermal freeze-out. We further discuss the relevant laboratory and cosmological constraints, including
neutrino trident production, NA64-$\mu$, Borexino, and the contribution to $\Delta N_\mathrm{eff}$.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Using the Pericentre Precession of LAGEOS II to Constrain Quadratically Coupled Ultralight Dark Matter
Clare Burrage, Angus Macdonald and Elisa Todarello
It has been proposed that feebly interacting ultralight scalars may constitute the dark matter content of the universe. Models describing the interactions of a dark matter scalar with Standard Model fields may feature quadratic interactions at leading order, such that the scalar acquires an effective mass in the neighbourhood of a classical matter distribution. The effect of their introduction is to provide effective shifts in fundamental constants of physics, as well as to mediate scalar fifth forces between test bodies. We here demonstrate how these fifth forces can result in pericentre precession in the orbital motion of satellites around the Earth. We apply this to the measured pericentre precession of the LAGEOS II experiment, constraining the mass, and couplings to the light Standard Model fields, of a quadratically coupled ultra-light dark matter scalar. We observe such an experiment to be effective in constraining parameter space at strong couplings, where existing constraints from satellite and tabletop level experiments break down.
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Astrophysics of Galaxies
cBHBd: A fast code for the evolution of tidally limited star clusters and their binary black hole mergers
Fotios Fronimos Pouliasis, Nolan Dickson, Daniel Marín Pina et al. (+3 more)
The
evolution of
star clusters is driven by
stellar mass loss, two-body relaxation, and evaporation in the
Galactic tidal field. Fast modeling tools are crucial for exploring diverse initial conditions and predicting cluster populations and their contribution to gravitational wave (GW) sources over
cosmic timescales. We present an improved version of the clusterBHBdynamics (cBHBd) code, designed to evolve
star clusters containing
stars and
stellar-mass black holes (BHs). We improve the treatment of evaporation in the
Galactic tidal field and include the effects of
metallicity and
stellar mass functions. We also introduce new prescriptions for GW captures during BBH-BBH interactions and between resonant interactions due to distant encounters that increase BBH eccentricities. The updated cBHBd is validated against Cluster Monte Carlo (CMC) models and $N$-body simulations spanning a range of cluster properties. Seven model parameters are fitted to the CMC results with nested sampling. With the best-fit values, the
evolution of the cluster mass, half-mass radius, and BH population over 13 Gyr is reproduced to within $\sim10\%$. The new GW capture prescriptions allow cBHBd to reproduce BBH merger rates from CMC models of massive clusters ($\gtrsim10^5,M_\odot$) and direct $N$-body models of lower-mass clusters ($\lesssim10^5,M_\odot$) to within $\sim20\%$. The improved cBHBd provides a fast and flexible tool for large-scale
star cluster studies. With a runtime of about one second per cluster, it enables applications such as searches for globular cluster initial conditions,
stellar stream modeling, and GW population synthesis.
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All authors Fotios Fronimos Pouliasis, Nolan Dickson, Daniel Marín Pina, Mark Gieles, Vincent Hénault-Brunet, Fabio Antonini
High Energy Astrophysical Phenomena
Fermi-LAT View on Three Ultra-high-energy 1LHAASO Sources in the $52^{\circ}<l<55^{\circ}$ Region
Linjie Liu, Xian Hou, Pierrick Martin et al. (+1 more)
Using more than 17 yr of
Fermi-LAT data, we performed a detailed investigation of the complex $52^{\circ}<l<55^{\circ}$ region, which encompasses the three ultra-high-energy sources 1LHAASO J1928+1746u, 1LHAASO J1928+1813u, and 1LHAASO J1929+1846u. This region hosts multiple SNRs,
pulsars, GeV and TeV sources. Our analysis resolves the GeV emission into three pointlike sources (J1925+1729P, J1930+1851P, and J1932+1916P) and two extended sources (J1929+1732E and J1930+1826E), and improves significantly on the description based on the 4FGL-DR4 catalog. Source J1932+1916P is identified as the known gamma-ray
pulsar PSR J1932+1916, while J1925+1729P may be a new gamma-ray
pulsar candidate distinct from the known gamma-ray
pulsar PSR J1925+1720. This warrants future investigation and a search for pulsations. Source J1930+1851P coincides with the TeV source PWN/SNR G54.1+0.3 and its GeV-TeV spectrum is consistent with both leptonic and hadronic interpretations, although a leptonic origin in relation to the known PWN is more likely. The GeV-TeV spectrum of J1929+1732E is consistent with a hybrid lepto-hadronic scenario in which the TeV emission traces the PWN powered by the
pulsar PSR J1928+1746, while the GeV emission may result from interactions between particles escaped from the parent SNR and illuminating the gas environment. Similarly, J1930+1826E is likely connected to PWN/SNR G54.1+0.3 under a hadronic scenario involving escaped particles in their early propagation stage. Owing to spectral and/or morphological mismatches, the connection of these five GeV sources to the three LHAASO sources is not clear. This warrants deeper observations with HAWC and LHAASO, and a dedicated study of the modeling of the
Galactic diffuse emission. Future CTAO observations with higher angular resolution are expected to deliver crucial information for the study of this region.
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All authors Linjie Liu, Xian Hou, Pierrick Martin, Chuyuan Yang
Solar and Stellar Astrophysics
Detection of a large-scale organized 2 kG order magnetic field in the Herbig Ae star HD179218
S. P. Järvinen, S. Hubrig, M. Schöller et al. (+5 more)
While about two dozen Herbig Ae/Be
stars have been reported to be
magnetic, only two of them, HD101412 and HD190073, have had their
magnetic field geometries studied in the past. The knowledge of the
magnetic field structure is important to understanding how magnetospheric
accretion works in these
stars. We aim to study in detail the spectral and
magnetic variability of HD179218, which is necessary to put constraints on its
magnetic field geometry. We measured the mean longitudinal
magnetic field, $\langle B_{\rm z}\rangle$, from newly acquired and archival high-resolution spectropolarimetric observations of HD179218 using the least-squares deconvolution technique. Additionally, we studied the spectral variability of the hydrogen lines using dynamical spectra. Based on our analysis of the Stokes V spectra of HD179218, we report for the first time the definite detection of a
magnetic field. Using a slightly refined rotation period of P$_{rot}$ = 1.34102 d, we constrained its geometry as follows: an estimated
magnetic obliquity angle of $\beta$= 79.9 $\pm$ 0.7 deg and a dipole strength of B$_d$ = 2142 $\pm$ 52 G. The $\langle B_{\rm z}\rangle$ variation is best fitted by the superposition of a sine wave and of its first harmonic, but more spectropolarimetric observations are necessary to test the impact of the limited measurement precision and the uneven coverage of the rotation cycle. The strongest emission in the H$\alpha$ and H$\beta$ line profiles in the medium-resolution spectra acquired in 2025 was detected close to the phases of the best visibility of the
magnetic poles. HD179218 is the second Herbig Ae/Be
star after HD190073 for which a first snapshot of a magnetosphere is presented.
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All authors S. P. Järvinen, S. Hubrig, M. Schöller, I. Ilyin, H. N. Adigozalzade, N. Z. Ismailov, U. Z. Bashirova, S. A. Alishov
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-th
Fermionic Bubble Loop in Cosmological Collider Revisited: Exact signals from spectral and Mellin-Barnes methods
Shuntaro Aoki, Zhehan Qin, Masahide Yamaguchi et al. (+1 more)
Fermionic degrees of freedom are essential ingredients in cosmological
collider physics and are well motivated by many phenomenological models beyond the Standard Model, but their signals remain largely unexplored due to the difficulty of computing loop diagrams. In this work, we ask how fermionic bubble loops contribute to cosmological
collider signals and provide an exact answer for arbitrary couplings. We develop two parallel analytical methods whose agreement provides a non-trivial check of the result. The first method is similar in spirit to spectral decomposition and is built directly from an identity for the product of propagators, which turns the bubble signal into an infinite sum of tree-level exchange signals. The second method is based on the Mellin-Barnes representation, where the result is reconstructed from the residues of distinct families of poles. We also show that the fermionic bubble can be generated from the scalar bubble by the action of appropriate differential operators. As a phenomenologically important application, we consider Yukawa interactions between
fermions and the inflaton, finding that the resulting bispectrum signal vanishes identically. Through the spectral decomposition, this vanishing can be traced to a field redefinition of the associated tree-level counterparts.
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All authors Shuntaro Aoki, Zhehan Qin, Masahide Yamaguchi, Yuhang Zhu
Earth and Planetary Astrophysics
Molecular Similarity and Water Diversity in Coeval Binary Disks: JWST/MIRI Observations of Sz 65 and Sz 66
Jinghuai Yao, Ke Zhang, Andrea Banzatti et al. (+10 more)
We present JWST/MIRI Medium Resolution Spectrometer spectra of the wide-separation (projected separation $= 980$ au) binary protoplanetary disks Sz 65 (K7; $0.68~M_{\odot}$) and Sz 66 (M3; $0.30~M_{\odot}$), reduced using the uniform pipeline of the JWST Disk Infrared Spectral Chemistry Survey. Both disks show rich molecular emission, including H$_2$O, CO$_2$, HCN, C$_2$H$_2$, and OH. The scaled spectra of the two disks exhibit remarkably similar H$_2$O, CO$_2$, and HCN line emission in the 13–18 $\mu$m region, with the only notable difference being stronger C$_2$H$_2$ emission in the primary (Sz 65). Beyond 18 $\mu$m, the difference in H$_2$O line emission between the two disks increases. Both the flux ratios and the slab-model-derived mass ratios of cold to hot H$_2$O ($\sim$200 K to $\sim$750 K) and warm to hot H$_2$O ($\sim$450 K to $\sim$750 K) are significantly higher in the secondary (Sz 66). Because binary
stars share nearly the same age and
metallicity, and as both disks appear compact in millimeter emission ($<30$ au), we suggest that the excess cold H$_2$O in the secondary is best explained by its unstructured
dust disk, in contrast to the primary, which shows gaps at 6 and 20 au. The enhanced cold water in the secondary is consistent with efficient pebble drift across the water snow line and increased H$_2$O vapor from the sublimation of icy mantles. Our results demonstrate that wide-separation binaries can serve as powerful control samples for isolating the impact of individual disk properties on inner-disk chemistry and
evolution.
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All authors Jinghuai Yao, Ke Zhang, Andrea Banzatti, Naman S. Bajaj, Ilaria Pascucci, James Miley, Geoffrey A. Blake, Colette Salyk, John M. Carpenter, Paola Pinilla, Lucas A. Cieza, Miguel Vioque, Benoît Tabone
Astrophysics of Galaxies
Transient axion streams from disrupted miniclusters
Luca Visinelli and Momchil Naydenov
We investigate the formation and
evolution of
axion streams generated by the
tidal disruption of
axion miniclusters through
stellar encounters in the Milky Way
halo. Combining a large-scale Monte Carlo treatment of repeated
stellar flybys with a tracer reconstruction of the stripped debris, we follow the phase-space
evolution of the streams across a broad range of galactocentric radii and assess their contribution to the local dark matter distribution. We find that the kinetic energy of the stripped debris typically exceeds its residual self-gravitational binding energy at formation, so that the subsequent
evolution is dominated by anisotropic free expansion and orbital shear. As a result, stream densities can decrease by factors as large as $\sim10^{-9}$ over
Galactic timescales, strongly suppressing the steady-state abundance of dense streams near the
Solar circle. At the
Solar radius, only a small fraction of realizations yields a nonzero encounter probability over a 10-year exposure, implying that observable streams are dominated by rare recent and nearby disruption events rather than by a persistent population of long-lived overdense substructure. Despite this rapid dilution, the streams remain dynamically cold and produce detector-frame linewidths many orders of magnitude narrower than the cavity bandwidths of current haloscope experiments. For representative haloscope configurations, we find characteristic stream linewidths in the range $\Delta\nu_{\rm stream}\sim10^{-7}\mathrm{-}10^{1}\,{\rm Hz}$, while the corresponding Doppler drift remains well below the cavity response width.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Dynamical Resolution of the Cosmic Coincidence Problem in Non-Interacting Holographic Dark Energy via Einstein-Cartan Torsion
Yongjun Yun, Kyungduk Kim and Jungjai Lee
We investigate the
cosmic coincidence problem in non-interacting
holographic dark energy with the Hubble radius as the infrared cutoff in Einstein-Cartan
gravity. In general relativity, this cutoff gives a
dust-like equation of state in the non-interacting case, whereas interacting models require a phenomenological dark sector coupling and yield a constant density ratio. We show that the Einstein-Cartan torsion scalar $\Phi$, compatible with the cosmological principle, with the self-consistent scaling behavior $\Phi\sim a^{-3}$, makes the density ratio $r\equiv \rho_m/\rho_X$ dynamical even when the phenomenological interaction term is absent, $Q=0$. The same torsion contribution shifts the equation of state for
holographic dark energy toward negative values, allowing
cosmic acceleration, and realizes the observed order-unity density ratio within the weak torsion regime without tuning the
holographic free parameter. Thus, Einstein-Cartan torsion provides a
geometric mechanism that replaces the phenomenological dark sector interaction and offers a dynamical resolution of the
cosmic coincidence problem.
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Astrophysics of Galaxies
Double Dots: Compact Pairs Mark Little Red Dots and High-Redshift Broad-line AGNs
A.J. Barger and L.L. Cowie
We utilize JWST imaging of the massive
lensing cluster field A2744 to find close pairs of compact sources with separations less than 0.25". A large fraction of these "Double Dots" correspond to Little Red Dots (LRDs) or high-
redshift broad-line active
galactic nuclei (BLAGNs). Our analysis of 31 identified pairs reveals a median separation of 0.15". Statistical comparison against a uniform background shows that these are mostly physical pairs. We find that at least 16 of the 24 previously published LRDs in this field (~67%) are such pairs, as are both of the high-
redshift BLAGNs. We demonstrate that the presence of a companion can significantly contaminate the measured spectral energy distribution, potentially masking the characteristic “v-shape" used for LRD classification. Furthermore, our 2D spectroscopic analysis of several pairs reveals that BLAGN activity is not confined to the redder member of the pair but can originate in either one. Since most LRDs contain broad emission lines, our findings suggest that close pairs are extremely effective markers of
galaxies with broad lines at high
redshift. We speculate on possible mechanisms, concluding that we are likely seeing merger-driven
accretion.
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Solar and Stellar Astrophysics
Comparing explodability predictions from a parameter-optimized semi-analytic model with structure-based progenitor criteria
Yuxin U. Wu, Hideyuki Umeda and Koh Takahashi
Three-dimensional (3D) simulations of
neutrino-driven core-collapse
supernovae are among the most reliable tools for predicting explosion outcome. However, their high computational cost limits systematic surveys over large progenitor samples. We test how well a fast one-dimensional (1D) approach captures progenitor explodability. We use a parameter-optimized semi-analytic 1D explosion model based on Müller et al. (2016), calibrated to the 3D results of Burrows et al. (2024) as an adopted reference set. We compare the model's explodability predictions with commonly used structure-based criteria: compactness, the free-fall mass coordinate, and the two-parameter $\mu_4$-$M_4$ criterion. Our analysis shows that the semi-analytic model can reproduce the trends seen in this adopted 3D calibration set by adjusting physically meaningful parameters. This provides a more direct way to examine the physics that controls explodability than traditional structure-based criteria. We identify where the semi-analytic model agrees with these criteria, where it differs, and which physical trends explain the differences. This work clarifies the strengths and limitations of structure-based explodability criteria by evaluating them against a parameter-optimized,
neutrino-driven semi-analytic model.
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High Energy Astrophysical Phenomena
Can COSI detect $\gamma$-ray lines from rare isotopes produced in the astrophysical intermediate neutron-capture process?
Falk Herwig, Pavel Denissenkov and Eric Burns
We investigate the
nuclear $\gamma$-ray line emission from rare isotopes produced in the astrophysical intermediate
neutron-capture process ($i$ process) and assess the prospects of observing these emissions with $\gamma$-ray
telescopes. The astrophysical sites of the $i$ process remain uncertain, but two candidates with predicted rapid mass ejections at
metallicities of
stars in the
solar neighborhood are post-asymptotic
giant branch (post-AGB)
stars, such as Sakurai's object (V4334 Sagittarii), and rapidly-accreting white
dwarfs (RAWDs). Detailed 1D and 3D simulations indicate that the convective-reactive fluid dynamics responsible for $i$-process nucleosynthesis can lead to violent, non-radial outbursts resulting in mass ejections of $i$-process products. We calculate ejected yields of rare isotopes whose radioactive decays may produce detectable $\gamma$-ray lines, particularly in the 0.5-2 MeV range, focusing on $^{22}$Na, $^{89}$Sr, and $^{95}$Zr. We estimate the formation rates of these sources and the likelihood of detecting their $\gamma$-ray emissions within 1000 parsecs of the Sun. The probability of observing $i$-process emission lines during COSI's operational period is up to $\approx 1\%$, rising to $11\%$ for $^{89}$Sr if observed within a few days. Due to the long lifetime and large production of $^{22}$Na from
proton-capture reactions its detection is more likely, with a probability of $\approx 5\%$. Future space missions could increase the observation probability to several tens of percent. Detection of long-lived
neutron-rich isotopes such as $^{137}$Cs would provide the first direct $\gamma$-ray signature of intermediate
neutron-density nucleosynthesis, distinguishing the $i$ process from classical s- and r-process pathways. (abridged)
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