Solar and Stellar Astrophysics
New Trends in Astrophysical Self-Organized Criticality
Markus J. Aschwanden
This review is focused on recent Self-Organized Criticality (SOC) literature of astrophysical phenomena, covering the last decade of (2015-2025), while previous SOC literature (1987-2014) is reviewed elsewhere. The selection of literature is mostly based on searches with the NASA-supported Astrophysics Data System (ADS). The discussed astrophysical SOC phenomena are subdivided into
solar flares,
solar atmosphere (photosphere, chromosphere, corona), heliospheric systems (coronal mass ejections,
solar wind,
solar energetic particles), planetary systems (asteroids and small bodies, lunar cratering, Saturnian ring systems, magnetospheric systems),
stellar flares, and
galactic systems (
pulsar glitches, gamma ray bursts, soft gamma-ray repeaters, supergiant fast X-ray transients, fast transient
radio bursts, magnetars, blazars, black holes).
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Cosmology and Nongalactic Astrophysics
The Local Distance Network: a community consensus report on the measurement of the Hubble constant at 1% precision
H0DN Collaboration: Stefano Casertano, Gagandeep Anand, Richard I. Anderson et al. (+34 more)
The direct, empirical determination of the local value of the Hubble constant (H0) has markedly advanced thanks to improved instrumentation, measurement techniques, and distance
estimators. However, combining determinations from different
estimators is non-trivial, due to correlated calibrations and different analysis methodologies. Using covariance weighting and leveraging the broad and comprehensive community of experts, we constructed a rigorous and transparent Distance Network (DN) to find a consensus value and uncertainty for the local H0. All critically reviewed the available data sets, spanning parallaxes, detached eclipsing binaries, masers, Cepheids, the TRGB, Miras, JAGB
stars, SN Ia, Surface Brightness Fluctuations, SN II, the Fundamental Plane, and Tully-Fisher relations and voted for indicators to define a `baseline' DN and others to assess robustness and sensitivity of the results. We provide open-source software and data products to support full transparency and future extensions of this effort. Our conclusions: 1) Local H0 is robustly determined, with first-rank indicators internally consistent within their uncertainties; 2) A covariance-weighted combination yields an uncertainty of 1.1% (baseline) or 0.9% (all
estimators); 3) The contribution from SNe Ia is consistent across four current compilations of optical magnitudes or using NIR-only magnitudes; 4) Removing either Cepheids or TRGB has minimal effect; 5) Replacing SNe Ia with
galaxy-based indicators changes H0 by less than 0.1 km/s/Mpc, while doubling its uncertainty; 6) The baseline result is H0=73.50+/-0.81 km/s/Mpc. Compared to early Universe results, our result differs by 7.1sigma from flat {\Lambda}CDM with Planck+SPT+ACT and 5.0 sigma with BBN+BAO (DESI2). A networked approach is invaluable for enabling further progress in accuracy and precision without overreliance on any single method, sample or group.
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All authors H0DN Collaboration: Stefano Casertano, Gagandeep Anand, Richard I. Anderson, Rachael Beaton, Anupam Bhardwaj, John P. Blakeslee, Paula Boubel, Louise Breuval, Dillon Brout, Michele Cantiello, Mauricio Cruz Reyes, Geza Csörnyei, Thomas de Jaeger, Suhail Dhawan, Eleonora Di Valentino, Lluís Galbany, Héctor Gil-Marín, Dariusz Graczyk, Caroline Huang, Joseph B. Jensen, Pierre Kervella, Bruno Leibundgut, Bastian Lengen, Siyang Li, Lucas Macri, Emre Özülker, Dominic W. Pesce, Adam Riess, Martino Romaniello, Khaled Said, Nils Schöneberg, Dan Scolnic, Teresa Sicignano, Dorota M. Skowron, Syed A. Uddin, Licia Verde, Antonella Nota
High Energy Astrophysical Phenomena
Unveiling the soft X-ray source population towards the inner Galactic disk with XMM-Newton
Tong Bao, Gabriele Ponti, Frank Haberl et al. (+5 more)
Across the
Galactic disk lies a diverse population of X-ray sources, with the fainter end remaining poorly understood due to past survey sensitivity limits. We aim to classify and characterize faint X-ray sources detected in the eROSITA All-Sky Survey (eRASS1) towards the inner
Galactic disk ($350^\circ < l < 360^\circ$, $-1^\circ < b < 1^\circ$) using deeper XMM-Newton observations (typical exposure of $\sim 20\,\text{ks}$). We analyzed 189 eRASS1 sources, combining X-ray spectral fitting ($0.2$–$10\,\text{keV}$) with
Gaia astrometric and photometric data for robust classification. Our results show that the eRASS1 catalog towards the
Galactic disk is overwhelmingly dominated by coronal sources ($\sim 74\%$), primarily active
stars and binaries, with $\sim 8\%$ being wind-powered massive
stars and $\sim 18\%$ being accreting compact objects. We propose an empirical hardness-ratio cut ($\text{HR} > -0.2$) to efficiently isolate these non-coronal sources. By stacking the classified population and comparing with the
Galactic Ridge X-ray Emission (GRXE), we estimate that $\sim 6\%$ of the GRXE flux in the $0.5$–$2.0\,\text{keV}$ band is resolved into point sources above the eRASS1 flux limit ($\sim 5\times 10^{-14}\,\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$). This resolved soft-band emission is dominated by active
stars, while hard-band flux originates primarily from X-ray binaries. We conclude that the eRASS1 catalog retains a non-negligible population of compact objects that can be effectively distinguished using X-ray color selection.
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All authors Tong Bao, Gabriele Ponti, Frank Haberl, Samaresh Mondal, Mark R. Morris, Kaya Mori, Shifra Mandel, Xiao-jie Xu
Astrophysics of Galaxies
Spatial-Kinematic Absorption Models of the Circumgalactic Medium. II. Ionized Gas Phases and Absorption Lines
Christopher W. Churchill
In this two-paper series, we present a straightforward mathematical model for synthesizing
quasar absorption line profiles from sight lines through idealized, spatial-kinematic models of the circumgalactic medium (CGM) and their host
galaxies. In Paper I, we developed the spatial components of the
galaxy/CGM structures (disk,
halo, wind,
accretion) and their 3D velocity fields. We derived the formalism for arbitrary observed orientation of the
galaxy/CGM model and
quasar line of sight positioning. In this paper, following a brief review of Paper I, we present the formalism for populating the
galaxy/CGM structures with multiphase photoionized and collisionally ionized gas and for generating HI and metal-line absorption profiles. Example absorption line systems through a fiducial
galaxy/CGM model are presented. These flexible spatial-kinematic absorption models (SKAMs) can be directly applied to and/or easily modified/expanded for studying individual or ensembles of observed absorption line systems, for exploring various competing theoretical scenarios of the
baryon cycle as studied through
quasar absorption line systems, and/or serving as pedagogical tools for developing physical intuition. We briefly describe a SKAM GUI that is in early stages of development.
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Earth and Planetary Astrophysics
The Dynamical Environment within the Habitable Zone of the Gaia-4 and Gaia-5 Planetary Systems
Stephen R. Kane
Exoplanetary systems exhibit a broad range of architectures which, in turn, enable a variety of dynamical environments. Many of the known planetary systems do not transit the host
star, and so we measure the minimum masses of their
planets, making it difficult to fully assess the dynamical environment within the system. Astrometry can resolve the mass ambiguity and thus allow a more complete dynamical analysis of systems to be conducted.
Gaia-4 and
Gaia-5 are two such systems, whose study with radial velocities and data from the
Gaia mission revealed that each
star harbors a massive
planet on a highly eccentric orbit. In this work, we provide the results of a dynamical analysis of each system, including calculations of the Habitable Zone (HZ), from which we show that the presence of the known companions largely exclude the presence of
planets within the HZ. We discuss the diagnostics of potential past
planet-
planet scattering events, and the occurrence of similar systems whereby a
giant planet on an eccentric orbit can substantially disrupt orbital integrity of terrestrial
planets. These "wrecking ball" systems have an impact on the target selection for planned direct imaging missions that seek to identify potentially habitable environments.
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Astrophysics of Galaxies
Does Machine Learning Work? A Comparative Analysis of Strong Gravitational Lens Searches in the Dark Energy Survey
J. Gonzalez, T. Collett, K. Rojas et al. (+9 more)
We present a systematic comparison of three independent machine learning (ML)-based searches for strong gravitational lenses applied to the Dark Energy Survey (Jacobs et al. 2019a,b; Rojas et al. 2022; Gonzalez et al. 2025). Each search employs a distinct ML architecture and training strategy, allowing us to evaluate their relative performance, completeness, and complementarity. Using a visually inspected sample of 1651 systems previously reported as lens candidates, we assess how each model scores these systems and quantify their agreement with expert classifications. The three models show progressive improvement in performance, with F1-scores of 0.31, 0.35, and 0.54 for Jacobs, Rojas, and Gonzalez, respectively. Their completeness for moderate- to high-confidence lens candidates follows a similar trend (31%, 52%, and 70%). When combined, the models recover 82% of all such systems, highlighting their strong complementarity. Additionally, we explore ensemble strategies: average, median, linear regression, decision trees, random forests, and an Independent
Bayesian method. We find that all but averaging achieve higher maximum F1 scores than the best individual model, with some ensemble methods improving precision by up to a factor of six. These results demonstrate that combining multiple, diverse ML classifiers can substantially improve the completeness of lens samples while drastically reducing false positives, offering practical guidance for optimizing future ML-based strong lens searches in wide-field surveys.
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All authors J. Gonzalez, T. Collett, K. Rojas, K. Bechtol, J. A. Acevedo Barroso, A. Melo, A. More, D. Sluse, C. Tortora, P. Holloway, N. E. P. Lines, A. Verma
Solar and Stellar Astrophysics
Unsupervised learning for variability detection with Gaia DR3 photometry. The main sequence-white dwarf valley
P. Ranaivomanana, C. Johnston, G. Iorio et al. (+4 more)
The unprecedented volume and quality of data from space- and ground-based
telescopes present an opportunity for machine learning to identify new classes of variable
stars and peculiar systems that may have been overlooked by traditional methods. Extending prior methodological work, this study investigates the potential of an unsupervised learning approach to scale effectively to larger
stellar populations, including objects in crowded fields, and without the need for pre-selected catalogues, specifically focusing on 13 405 sources selected from
Gaia DR3 and lying in the selected region of the CMD. Our methodology incorporates unsupervised clustering techniques based primarily on statistical features extracted from
Gaia DR3 epoch photometry. We used the t-distributed
stochastic neighbour
embedding (t-SNE)
algorithm to identify variability classes, their subtypes, and spurious variability induced by instrumental effects. The clustering results revealed distinct groups, including hot subdwarfs, cataclysmic variables (CVs), eclipsing binaries, and objects in crowded fields, such as those in the Andromeda (M31) field. Several potential
stellar subtypes also emerged within these clusters. Notably, objects previously labelled as RR Lyrae were found in an unexpected region of the CMD, potentially due to either unreliable astrometric measurements (e.g., due to binarity) or alternative evolutionary pathways. This study emphasises the robustness of the proposed method in finding variable objects in a large region of the
Gaia CMD, including variable hot subdwarfs and CVs, while demonstrating its efficiency in detecting variability in extended
stellar populations. The proposed unsupervised learning framework demonstrates scalability to large
datasets and yields promising results in identifying
stellar subclasses.
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All authors P. Ranaivomanana, C. Johnston, G. Iorio, P.J. Groot, M. Uzundag, T. Kupfer, C. Aerts
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Theory-agnostic searches of non-gravitational modes in the ringdown
Francesco Crescimbeni
We present a theory-agnostic framework to search for extra-fundamental modes in the ringdown phase of black hole mergers. These modes, distinct from standard General Relativity (GR) deviations, originate from modifications of the theory of
gravity. Applying our method to the events GW150914, GW190521, and GW200129, we find no significant evidence for extra modes beyond those expected in GR.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph
WIMP Shadows: Phenomenology of Secluded Dark Matter in Three Minimal BSM Scenarios
Mattia Di Mauro and Yanhan Wang
We present a comprehensive study of secluded dark matter (DM) $\chi$, where the relic abundance is set by annihilations into lighter dark mediators $\phi$ that couple only feebly to the Standard Model (SM). In contrast to canonical WIMPs, which are now strongly constrained by direct and indirect searches, secluded models still achieve the observed relic abundance via thermal freeze-out into hidden-sector mediators, while predicting highly suppressed present-day signals. We analyze three minimal models: (i) a $U(1)_X$ gauge
boson ($A'$) with kinetic mixing; (ii) a scalar DM candidate $S$ with a scalar mediator $K$ that has a trilinear vertex; and (iii) a Dirac
fermion $\chi$ whose mass arises from a
Higgs-mixed singlet $H_p$. For each model we derive annihilation and scattering rates in both WIMP-like and secluded regimes, and solve the Boltzmann equations: a single-species equation for the WIMP case and a coupled $\chi$-$\phi$ system for the secluded case to account for possible early departure of the mediator from thermal
equilibrium with the SM bath. In this regard, we provide explicit lower limits on the portal coupling $\epsilon$ required to keep the mediator in thermal
equilibrium with the SM bath and to ensure mediator decay before BBN. We show that for portal couplings $\epsilon \ll 10^{-3}$ the relic density is dominantly controlled by DM annihilation into mediator pairs, while
spin-independent scattering lies well below current limits and remains viable even for future experiments approaching the irreducible
neutrino background floor. Indirect constraints are typically weak due to $p$-wave suppression, off-resonance $s$-channels, and cascade spectra controlled by $\epsilon^2$. Finally, we highlight the most promising
collider tests, which remain sensitive despite tiny portal couplings.
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Cosmology and Nongalactic Astrophysics
Clustering properties of the CatWISE2020 quasar catalogue and their impact on the cosmic dipole anomaly
Sebastian von Hausegger, Nathan Secrest, Harry Desmond et al. (+3 more)
The
cosmic dipole anomaly – the observation of a significant mismatch between the dipole observed in the matter distribution and that expected given the kinematic interpretation of the
cosmic microwave background dipole – poses a serious challenge to the Cosmological Principle upon which the standard model of cosmology rests. Measurements of the dipole ($\ell=1$) in a given sample crucially depend on having control over other large-scale power ($\ell > 1$) so as to avoid biases, in particular those potentially caused by correlations among multipoles during fitting, and those by local source clustering. Currently, the most powerful catalogue that exhibits the
cosmic dipole anomaly is the sample of 1.6~million mid-infrared
quasars derived from CatWISE2020. We therefore analyse clustering properties of this catalogue by performing an inference analysis of large-scale multipoles in real space, and by computing its angular power spectrum on small scales to test for convergence with $\Lambda$CDM. After accounting for the known trend of the
quasar number counts with ecliptic latitude, we find that any other large-scale power is consistent with noise, find no evidence for the presence of an octupole ($\ell=3$) in the data, and quantify the clustering dipole's proportion to be marginal. Our results therefore reaffirm the anomalously high dipole in the distribution of
quasars.
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All authors Sebastian von Hausegger, Nathan Secrest, Harry Desmond, Mohamed Rameez, Roya Mohayaee, Subir Sarkar
Astrophysics of Galaxies
Galactic Alchemy: Deep Learning Map-to-Map Translation in Hydrodynamical Simulations
Philipp Denzel, Yann Billeter, Frank-Peter Schilling et al. (+1 more)
We present the first systematic study of multi-domain map-to-map translation in
galaxy formation simulations, leveraging deep generative models to predict diverse
galactic properties. Using high-resolution magneto-hydrodynamical simulation data, we compare conditional generative
adversarial networks and
diffusion models under unified preprocessing and evaluation, optimizing architectures and
attention mechanisms for physical fidelity on
galactic scales. Our approach jointly addresses seven astrophysical domains - including dark matter, gas, neutral hydrogen,
stellar mass, temperature, and
magnetic field strength - while introducing physics-aware evaluation metrics that quantify structural realism beyond standard computer
vision measures. We demonstrate that translation difficulty correlates with physical coupling, achieving near-perfect fidelity for mappings from gas to dark matter and mappings involving astro-chemical components such as total gas to HI content, while identifying fundamental challenges in weakly constrained tasks such as gas to
stellar mass mappings. Our results establish GAN-based models as competitive counterparts to state-of-the-art
diffusion approaches at a fraction of the computational cost (in training and inference), paving the way for scalable, physics-aware generative frameworks for forward modelling and observational reconstruction in the SKA era.
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All authors Philipp Denzel, Yann Billeter, Frank-Peter Schilling, Elena Gavagnin
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
The symplectic geometry of the black hole photon shell
Judy Shir and Shahar Hadar
The unstably bound, critical null geodesics of the Kerr spacetime form a distinguished class of orbits whose properties govern observables such as the
photon ring and the high-frequency component of black-hole ringdown. This set of orbits defines a codimension-two submanifold of the null-geodesic phase space known as the
photon shell. In this work we investigate the
photon shell's intrinsic
symplectic geometry. Using the induced
symplectic form, we construct the canonical volume form on the shell and compute the differential phase-space volume it encloses as a function of radius – equivalently, the radial density of states. In the near-extremal limit the
photon shell bifurcates into near-horizon and far-region components; we find that approximately $3\%$ of the shell's phase-space volume resides in the near-horizon component. We also analyze a thickening of the
photon shell that includes near-critical orbits, and compute its differential phase-space volume. Beyond their intrinsic theoretical interest, these results may inform the interpretation of high-resolution observations of spinning black holes.
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High Energy Astrophysical Phenomena
Study of Neutron Star Properties under the Two-Flavor Quark NJL Model
Chunran Zhu and Bolin Li
The Equation of State (EOS) of matter within
neutron stars is a central topic in
nuclear physics and astrophysics.This study investigates
hadron-
quark hybrid
stars by integrating the density-dependent DDME2 relativistic mean-field model for hadronic matter with a two-
flavor Nambu-Jona-Lasinio (NJL) model for
quark matter.A quintic
polynomial interpolation is employed to construct a smooth ($C^2$ continuity) and thermodynamically consistent crossover between the phases.We systematically explore the parameter space to reconcile the tension between the high stiffness required by massive
pulsars and the softness demanded by
tidal deformability and radius constraints.Our analysis demonstrates that to simultaneously satisfy the mass measurement of PSR J0740+6620 and the compact radius constraints from NICER (e.g., PSR J0437-4715), the
hadron-
quark crossover must initiate in the vicinity of
nuclear saturation density.This result suggests that the early percolation of
quark degrees of freedom is a necessary feature to accommodate current multi-messenger observations.
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Instrumentation and Methods for Astrophysics
Re-envisioning Euclid Galaxy Morphology: Identifying and Interpreting Features with Sparse Autoencoders
John F. Wu and Michael Walmsley
Sparse Autoencoders (SAEs) can efficiently identify candidate monosemantic features from pretrained neural networks for
galaxy morphology. We demonstrate this on Euclid Q1 images using both supervised (Zoobot) and new self-supervised (MAE) models. Our publicly released MAE achieves superhuman image reconstruction performance. While a Principal Component Analysis (PCA) on the supervised model primarily identifies features already aligned with the
Galaxy Zoo decision tree, SAEs can identify interpretable features outside of this framework. SAE features also show stronger
alignment than PCA with
Galaxy Zoo labels. Although challenges in interpretability remain, SAEs provide a powerful engine for discovering astrophysical phenomena beyond the confines of human-defined classification.
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High Energy Astrophysical Phenomena
Unlocking the radio-gamma spectrum of the pulsar wind nebula around PSR J1124-5916 in SNR G292.0+1.8
A. Lemière, G. Castelletti and N. L. Maza
We present the first detection of GeV $\gamma$-ray emission potentially associated with the
pulsar wind nebula (PWN) hosted by the young core-collapse
supernova remnant G292.0+1.8, based on a detailed time-resolved analysis of
Fermi-LAT data. By isolating the unpulsed component from the dominant magnetospheric radiation of PSR~J1124$-$5916, we successfully disentangle a candidate nebular emission in the GeV range, characterise its morphology and extract its spectrum. This identification places G292.0+1.8 among the few systems in which the
pulsar and PWN contributions have been spectrally resolved at high energies, offering new insight into their respective emission mechanisms. We characterise the $\gamma$-ray spectrum of the
pulsar and model the broadband spectral energy distribution (SED) of the PWN using
radio, X-ray, and GeV data. The emission is well described by a single electron population with two spectral breaks: one intrinsic to the injection spectrum and another produced by synchrotron cooling in a
magnetic field of $\sim$15~$\mu$G. Notably, the inferred
magnetic field and the low TeV flux of the nebula resemble those of 3C~58, suggesting that similar low-field environments can arise in young PWNe. The high-energy portion of the SED is now tightly constrained by our GeV detection and existing TeV upper limits. Compared to our model, earlier predictions tend to underpredict the $\gamma$-ray flux, while others that succeed in reproducing the GeV component often overpredict the TeV emission. This mismatch underscores the challenges in modelling particle acceleration and radiation processes in young PWNe and establishes G292.0+1.8 as a valuable
benchmark for testing and refining such models.
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Astrophysics of Galaxies
Molecular Gas in Major Mergers Hosting Dual and Single AGN at <10 kpc Nuclear Separations
Makoto A. Johnstone, Ezequiel Treister, Franz E. Bauer et al. (+24 more)
We present high-resolution ($\sim$50$-$100 pc) Atacama Large Millimeter Array (ALMA) observations of $^{12}$CO(2-1) or $^{12}$CO(1-0) emission in seven local ($z$ $\lesssim$ 0.05) major mergers – five of which are dual active
galactic nuclei (AGN) systems, and two of which are single AGN systems. We model the molecular gas kinematics through rotating disk profiles using a
Bayesian Markov chain Monte Carlo approach. The residuals were then used to isolate non-rotating components of the molecular gas – the most likely contributor to future SMBH growth. We find that more massive SMBHs have higher surface densities of non-rotating molecular gas within their sphere of influence. This potential molecular gas supply, however, does not correlate with the current
accretion efficiency of the SMBHs, suggesting that only a fraction of the observed non-rotating gas is currently reaching the SMBH. Finally, we tentatively find no significant differences in the
nuclear molecular gas masses of single AGN and dual AGN hosts, both within the SMBH sphere of influence and within the central kiloparsec. Our results indicate that the probability of occurrence of the dual AGN phenomenon is likely dependent on AGN variability and/or obscuration rather than the availability of molecular gas in the
nuclear regions.
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All authors Makoto A. Johnstone, Ezequiel Treister, Franz E. Bauer, Chin-Shin Chang, Claudia Cicone, Michael J. Koss, Ignacio del Moral-Castro, Francisco Muller-Sanchez, George C. Privon, Claudio Ricci, Nick Scoville, Giacomo Venturi, Loreto Barcos-Muñoz, Lee Armus, Laura Blecha, Caitlin Casey, Julia Comerford, Aaron Evans, Taiki Kawamuro, Anne M. Medling, Hugo Messias, Neil Nagar, Alejandra Rojas, David Sanders, Benny Trakhtenbrot, Vivian U, Meg Urry
Astrophysics of Galaxies
Jun-Sung Moon and Teppei Okumura
The
tidal torque theory (TTT) predicts that
galaxy spins are correlated with the surrounding
tidal field, reflecting how angular momentum is acquired during structure formation. We present a new observational test of this prediction using the final data release of the Sloan Digital Sky Survey IV Mapping Nearby
Galaxies at Apache Point Observatory integral field spectroscopy survey, which enables direct
spin measurements from
stellar and ionized gas kinematics for a sample of 6325 disk
galaxies. We utilize the three-dimensional
tidal field reconstructed from the
galaxy distribution, providing a physically defined reference frame for the analysis. We find that massive
galaxies tend to align their
spins parallel to the intermediate axis of the
tidal field, consistent with the prediction of the TTT, while also showing a tendency to align perpendicular to the major axis. In contrast, low-mass
galaxies exhibit the opposite trend, with a transition mass of $M_* \sim 10^{10}-10^{10.5}M_\odot$. No significant
alignment is detected with respect to the minor axis across all
stellar masses. We further examine the dependence on morphology and environment, finding that S0 and early-type spiral
galaxies exhibit stronger
alignment signals than late-type spirals. The
alignment trend becomes particularly pronounced in regions of high
tidal anisotropy and high overdensity. A mutual information analysis identifies these environmental factors as the dominant drivers of the observed trends. Our results provide new empirical evidence for the connection between
galaxy spins and the
cosmic tidal field.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
String Decomposition and Gravitational Waves in High-quality Axion Gauge Theories
Camilla Mupo and Yue Zhang
The QCD
axion can successfully solve the strong CP problem under the condition that the Peccei-Quinn
symmetry is respected to extremely high standard. We explore a class of gauge theories that accommodate a high-quality
axion, known as the Barr-Seckel models, paying special
attention to the cosmology of topological defects. In models with domain wall number equal to unity, we show that
axion strings with winding number larger than one can always be decomposed into a number of unit-winding
axion strings and pure gauge strings. This mechanism enables the
axion string-wall network to be destroyed timely to render a viable cosmology. The subsequent decay of gauge strings into gravitational waves generically produces a double-plateau in frequency space allowing the mechanism to be tested by upcoming experiments.
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Instrumentation and Methods for Astrophysics
Magrathea v2: A planetary interior modeling platform in C++
David R. Rice, Chenliang Huang, Robert Royer et al. (+5 more)
Magrathea is an open-source C++ code for modeling the internal structure of differentiated
planets. The initial release, Huang et al. (2022), introduced the base solver, a modular framework for defining equations of state (EOS) within phase diagrams for each layer, and a plan for expanding the code. Many of those updates are now implemented. Magrathea v2 is a more versatile platform that supports a wider range of compositions, adds new tools for composition
retrieval, and makes it easier for users to adapt the code to their own models.
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All authors David R. Rice, Chenliang Huang, Robert Royer, Mangesh Daspute, Krishang Mittal, Baptiste Journaux, Jason H. Steffen, Allona Vazan
High Energy Astrophysical Phenomena
ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger
Mansi M. Kasliwal, Tomas Ahumada, Robert Stein et al. (+61 more)
On August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported gravitational waves from a sub-threshold binary
neutron star merger. If astrophysical, this event would have a surprisingly low chirp mass, suggesting that at least one
neutron star was below a
solar mass. The Zwicky Transient Facility mapped the coarse localization and discovered a transient, ZTF25abjmnps (AT2025ulz), that was spatially and temporally coincident with the gravitational wave trigger. The first week of follow-up suggested properties reminiscent of a GW170817-like kilonova. Subsequent follow-up suggests properties most similar to a young, stripped-envelope, Type IIb
supernova. Although we cannot statistically rule out chance coincidence, we undertake due diligence analysis to explore the possible association between ZTF25abjmnps and S250818k. Theoretical models have been proposed wherein sub-
solar neutron star(s) may form (and subsequently merge) via
accretion disk fragmentation or core fission inside a core-collapse
supernova i.e. a “superkilonova". Here, we qualitatively discuss our multi-wavelength
dataset in the context of the superkilonova picture. Future higher significance gravitational wave detections of sub-
solar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multi-messenger association.
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All authors Mansi M. Kasliwal, Tomas Ahumada, Robert Stein, Viraj Karambelkar, Xander J. Hall, Avinash Singh, Christoffer Fremling, Brian D. Metzger, Mattia Bulla, Vishwajeet Swain, Sarah Antier, Marion Pillas, Malte Busmann, James Freeburn, Sergey Karpov, Aleksandra Bochenek, Brendan O'Connor, Daniel A. Perley, Dalya Akl, Shreya Anand, Andrew Toivonen, Sam Rose, Theophile Jegou du Laz, Chang Liu, Kaustav Das, Sushant Sharma Chaudhary, Tyler Barna, Aditya Pawan Saikia, Igor Andreoni, Eric C. Bellm, Varun Bhalerao, S. Bradley Cenko, Michael W. Coughlin, Daniel Gruen, Daniel Kasen, Adam A. Miller, Samaya Nissanke, Antonella Palmese, Jesper Sollerman, Niharika Sravan, G.C. Anupama, Smaranika Banerjee, Sudhanshu Barway, Joshua S. Bloom, Tomas Cabrera, Tracy Chen, Chris Copperwheat, Alessandra Corsi, Richard Dekany, Nicholas Earley, Matthew Graham, Patrice Hello, George Helou, Lei Hu, Yves Kini, Ashish Mahabal, Frank Masci, Tanishk Mohan, Natalya Pletskova, Josiah Purdum, Yu-Jing Qin, Nabeel Rehemtulla, Anirudh Salgundi, Yuankun Wang
High Energy Astrophysical Phenomena
Hydrodynamic Simulations of Tidal Disruption Encores
Ian P.A. Johnson, Taeho Ryu and Rosalba Perna
We present hydrodynamic simulations with the moving-mesh code AREPO of
Tidal Disruption Encores (TDEEs) in
nuclear star clusters (NSCs). TDEEs arise when a
stellar-mass black hole (sBH) disrupts a
star within the NSC, producing debris that is unbound from the sBH but remains gravitationally bound to the central massive black hole (MBH), leading to a delayed secondary
flare. We find that the morphology and thermodynamics of the fallback material depend sensitively on the disruption geometry, MBH mass, and sBH-MBH separation. We identify two distinct morphological outcomes: ring encores, where debris circularize into a
torus, and direct encores, where streams plunge toward the MBH, with encore luminosities peaking at times corresponding to the freefall timescale and one orbital period, respectively. Across all simulated cases, we find these events exhibit luminosities of $10^{40}-10^{42}$ erg/s with lightcurves characteristic of their morphology. Our work greatly improves the predictions of TDEE lightcurves and empowers observations to probe into NSC dynamics and sBH population while providing possible explanations for anomalous TDE-like
flares.
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High Energy Astrophysical Phenomena
AT2025ulz and S250818k: Deep X-ray and radio limits on off-axis afterglow emission and prospects for future discovery
Brendan O'Connor, Roberto Ricci, Eleonora Troja et al. (+15 more)
The first joint electromagentic (EM) and gravitational wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 years since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported a low-significance (high false alarm rate) binary
neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz ($z=0.08484$) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with Swift, XMM-Newton, Chandra, and the Very Large Array to search for non-thermal afterglow emission. Our deep X-ray and
radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope
supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary
neutron star merger at the distance of AT2025ulz ($\approx 400$ Mpc) based on our X-ray and
radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles of $\theta_\textrm{v}\approx 12.5$ degrees. We briefly discuss the prospects for the future discovery of off-axis afterglows.
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All authors Brendan O'Connor, Roberto Ricci, Eleonora Troja, Antonella Palmese, Yu-Han Yang, Geoffrey Ryan, Hendrik van Eerten, Muskan Yadav, Xander J. Hall, Ariel Amsellem, Rosa L. Becerra, Malte Busmann, Tomas Cabrera, Simone Dichiara, Lei Hu, Ravjit Kaur, Keerthi Kunnumkai, Ignacio Magana Hernandez
High Energy Astrophysical Phenomena
Continuous gravitational waves from magnetized white dwarfs: Quantifying the detection plausibility by LISA
Mayusree Das, Banibrata Mukhopadhyay and Tomasz Bulik
White
dwarfs (WDs) are frequently observed to have strong
magnetic fields up to $10^9$ G and expected to have a possible internal field as high as $\sim 10^{14}$ G. High internal fields can significantly deform a WD's
equilibrium structure, generating a quadrupole moment. If the rotation axis is misaligned with the
magnetic axis, the deformation can lead to the emission of continuous gravitational waves (CGWs). We examine the potential for detecting CGWs from magnetized WDs with future space-based detectors such as LISA, ALIA, DECIGO, Deci-Hz, BBO and TianQin. We model the field-induced deformation and compute the resulting GW strain, incorporating amplitude decay due to angular momentum loss from electromagnetic and gravitational radiation. This sets a timescale for detection -`active timescale' of $10^{5-6}$ yr, requiring observation while the object remains sufficiently young. Our results suggest that LISA could detect a few dozens of highly magnetized WDs across the
Galaxy during its mission. As a specific case, we investigate ZTF J1901+1458- a compact, massive, fast-rotating, and strongly magnetized WD with
spin period $\sim416$ s and inferred surface field $\sim10^{9}$ G. We find that this object would be detectable by LISA with four years of continuous data. This highlights the potential of CGW observations to probe
magnetic field structure in WDs and their role in type Ia
supernova progenitors.
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High Energy Astrophysical Phenomena
AT2025ulz and S250818k: Leveraging DESI spectroscopy in the hunt for a kilonova associated with a sub-solar mass gravitational wave candidate
Xander J. Hall, Antonella Palmese, Brendan O'Connor et al. (+47 more)
On August 18th, 2025, the LIGO–Virgo–KAGRA collaboration reported a sub-threshold gravitational wave candidate detection consistent with a sub-
solar-mass
neutron star merger, denoted S250818k. An optical transient, AT2025ulz, was discovered within the localization region. AT2025ulz initially appeared to meet the expected behavior of kilonova (KN) emission, the telltale signature of a binary
neutron star merger. The transient subsequently rebrightened after $\sim$\,$5$ days and developed spectral features characteristic of a Type IIb
supernova. In this work, we analyze the observations of the host
galaxy of AT2025ulz obtained by the Dark Energy Spectroscopic Instrument (DESI). From the DESI spectrum, we obtain a secure
redshift of $z = 0.084840 \pm 0.000006$, which places the transient within $2\sigma$ of the gravitational wave distance and results in an integral overlap between the gravitational wave alert and the transient location of $\log_{10}\mathcal{I} \approx 3.9-4.2$. Our analysis of the host
galaxy's spectral energy distribution reveals a
star-forming, dusty
galaxy with
stellar mass ${\sim} 10^{10}~M_\odot$, broadly consistent with the population of both short gamma-ray bursts and core-collapse
supernova host
galaxies. We also present our follow-up of DESI-selected candidate host
galaxies using the Fraunhofer
Telescope at the Wendelstein Observatory, and show the promise of DESI for associating or rejecting candidate electromagnetic counterparts to gravitational wave alerts. These results emphasize the value of DESI's extensive spectroscopic
dataset in rapidly characterizing host
galaxies, enabling spectroscopic host subtraction, and guiding targeted follow-up.
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All authors Xander J. Hall, Antonella Palmese, Brendan O'Connor, Daniel Gruen, Malte Busmann, Julius Gassert, Lei Hu, Ignacio Magana Hernandez, Jessica Nicole Aguilar, Ariel Amsellem, Steven Ahlen, John Banovetz, Segev BenZvi, Davide Bianchi, David Brooks, Francisco Javier Castander, Todd Claybaugh, Andrei Cuceu, Arjun Dey, Peter Doel, Jennifer Faba-Moreno, Simone Ferraro, Andreu Font-Ribera, Jaime E. Forero-Romero, Gaston Gutierrez, Laurent Le Guillou, Dick Joyce, Theodore Kisner, Anthony Kremin, Ofer Lahav, Claire Lamman, Martin Landriau, Michael Levi, Axel de la Macorra, Marc Manera, Aaron Meisner, Ramon Miquel, John Moustakas, Seshadri Nadathur, Francisco Prada, Ignasi Perez-Rafols, Graziano Rossi, Eusebio Sanchez, David Schlegel, Michael Schubnell, David Sprayberry, Gregory Tarle, Benjamin Alan Weaver, Rongpu Zhou, Hu Zou
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-th
Higher Curvature Inflation and the Species Scale
Joaquin Masias
We study the scalar potentials that arise from higher
curvature corrections in general $f(R)$ theories of
gravity and their connection to a dynamical species scale. Starting from general considerations in arbitrary dimensions, we show that at large field values, the scalar potential generated by an infinite series of
curvature terms and the field dependent species scale arising from circle compactification both decay exponentially, in complementary ways. We identify conditions under which these two effects precisely balance out, giving rise to exponentially flat, plateau-like potentials. We additionally find a precise
embedding of Starobinsky inflation consistent with the Swampland program, and we discuss possible implications the mechanism proposed could have for M and string theory.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Cosmic Overture Echoed in a Stellar Final Act: Implications of Nanohertz Gravitational Waves for Core Collapse Supernova Neutrinos
Hooman Davoudiasl, Peter B. Denton and Anna M. Suliga
The growing evidence for nanohertz gravitational waves, from NANOGrav and other observations, may be pointing to a cosmological first-order phase transition at temperatures of $\mathcal{O}(10-100)\;\mathrm{MeV}$. Such an interpretation requires beyond the Standard Model dynamics in this energy range. If so, it may well be the case that
galactic core-collapse
supernova explosions would produce the key components related to the first-order phase transition, leaving detectable imprints on the spectrum of
neutrinos emitted in the initial few seconds of the collapse. This might provide further evidence in support of the early universe interpretation of the nanohertz gravitational wave signal. The scenario proposed here is also suggestive of a low-mass seesaw mechanism to explain
neutrino masses. We outline the prospects for future observations of
Galactic supernovae to uncover the signals of this scenario, with further confirmation from future
pulsar timing array measurements of
primordial nanohertz gravitational waves.
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Earth and Planetary Astrophysics
Sensitivity to Sub-Io-sized Exosatellite Transits in the MIRI LRS Lightcurve of the Nearest Substellar Worlds
Andrew Householder, Mary Anne Limbach, Beth Biller et al. (+20 more)
JWST's unprecedented sensitivity enables precise spectrophotometric monitoring of substellar worlds, revealing atmospheric variability driven by mechanisms operating across different pressure levels. This same precision now permits exceptionally sensitive searches for transiting exosatellites, small terrestrial companions to these worlds. Using a novel simultaneous dual-band search method to address host variability, we present a search for transiting exosatellites in an 8-hour JWST/MIRI LRS lightcurve of the nearby ($2.0\,pc$) substellar binary WISE J1049-5319AB, composed of two $\sim30 M_{\rm Jup}$ brown
dwarfs separated by $3.5\,au$ and viewed near edge-on. Although we detect no statistically significant transits, our injection-recovery tests demonstrate sensitivity to satellites as small as $0.275\,R_{\oplus}$ ($0.96\,R_{\rm Io}$ or $\sim$1 lunar radius), corresponding to 300ppm transit depths, and satellite-to-host mass ratios $>$$10^{-6}$. This approach paves the way for detecting Galilean-moon analogs around directly imaged brown
dwarfs, free-floating
planets, and wide-orbit
exoplanets, dozens of which are already scheduled for JWST lightcurve monitoring. In our
Solar System, each
giant planet hosts on average 3.5 moons above this threshold, suggesting that JWST now probes a regime where such companions are expected to be abundant. The technique and sensitivities demonstrated here mark a critical step toward detecting exosatellites and ultimately enabling constraints on the occurrence rates of small terrestrial worlds orbiting $1\text{-}70$$M_{\rm Jup}$ hosts.
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All authors Andrew Householder, Mary Anne Limbach, Beth Biller, Brooke Kotten, Mikayla J. Wilson, Johanna M. Vos, Andrew Skemer, Andrew Vanderburg, Ben J. Sutlieff, Xueqing Chen, Ian J. M. Crossfield, Nicolas Crouzet, Trent Dupuy, Jacqueline Faherty, Pengyu Liu, Elena Manjavacas, Allison McCarthy, Caroline V. Morley, Philip S. Muirhead, Natalia Oliveros-Gomez, Genaro Suárez, Xianyu Tan, Yifan Zhou
Earth and Planetary Astrophysics
Twinkle Twinkle Little Star, Roman Sees Where You Are: Predicting Exoplanet Transit Yields in the Rosette Nebula with the Nancy Grace Roman Space Telescope
Ritvik Sai Narayan, Melinda Soares-Furtado, Mary Anne Limbach et al. (+3 more)
Young
stars host only a small fraction of the known
exoplanet population because their photometric variability,
magnetic activity, and frequent placement in dense, poorly-resolved regions hamper
exoplanet detections. Yet, measuring
planets at these ages is crucial since these phases are when dynamical processes that drive planetary migration are most active. We assess the expected yield of a hypothetical Nancy Grace Roman Space
Telescope transit survey of the Rosette Nebula, a ${\sim}10$ Myr
star-forming region with a dense and diverse
stellar population. Using the Roman Exposure Time Calculator to quantify sensitivity to Rosette members, we establish detection thresholds for companions and evaluate yields via Monte Carlo injection-recovery simulations, accounting for nebular extinction and youth-driven
stellar variability. We predict the detection of $33 \pm 9$ young transiting
exoplanets orbiting
stellar hosts in a month-long survey, and $29 \pm 8$ in a two-week survey. The extended baseline primarily improves sensitivity to longer-period
planets orbiting FGK
stars, while most M
dwarf detections are well-sampled within two weeks. Irrespective of the temporal baseline, transit detections are dominated by of 1-2 $R_\oplus$ super-Earths and sub-Neptunes with $P\lesssim8$ days. Such a sample would substantially expand the census of only three detected transiting
planets younger than 20 Myr around
stars less massive than the Sun, probing an age regime in which planetary radii remain inflated, the stability of close-in orbits is uncertain, and planetary migration may still be ongoing. This survey offers a path to constrain early planetary
evolution and establish
prime follow-up targets for JWST, Rubin, and the Habitable Worlds Observatory.
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All authors Ritvik Sai Narayan, Melinda Soares-Furtado, Mary Anne Limbach, Nishanth Ramanujam, Andrew Vanderburg, Johanna M. Vos
Astrophysics of Galaxies
The SARAO MeerKAT Galactic Plane Survey compact source catalogue
Mubela Mutale, Mark A. Thompson, Gwenllian M. Williams et al. (+11 more)
We present a catalogue of compact sources detected in the SARAO MeerKAT 1.3 GHz
Galactic Plane Survey (SMGPS). We extract 510599 compact sources, with areas less than five 8" beams, from the survey maps covering the regions $252\deg< l < 358^\deg$ and $2\deg < l < 61\deg$ at $|b| \leq 1.5^\deg$, which have an angular resolution of 8" and a sensitivity of $\sim$ 10-30 $\mu$Jy beam$^{-1}$. In this paper, we describe the source identification and characterisation methods, present the quality assurance of the catalogue, explore the nature of the catalogue sources, and present initial science highlights. We limit our catalogue to sources with a signal-to-noise ratio $\geq 5$, as the catalogue is $\sim$90 per cent complete, and has a false positive rate of less than 1 per cent at this threshold. The bulk of the catalogue sources are previously unknown to the literature, with the majority of unknown sources at sub-mJy levels. Initial science highlights from the catalogue include the detection of 213
radio quiet WISE HII region candidates, previously undetected in
radio continuum studies. We show images that compare the SMGPS compact sources to CORNISH ultracompact HII regions, thus highlighting the sensitivity and unprecedented uv-coverage of the SMGPS, and the potential synergy of the SMGPS with other surveys.
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All authors Mubela Mutale, Mark A. Thompson, Gwenllian M. Williams, Andrew J. Rigby, Melvin G. Hoare, James S. Urquhart, Michael Bietenholz, Cristobal Bordiu, Fernando Camilo, William D. Cotton, Sharmila Goedhart, Willice O. Obonyo, Simone Riggi, Aiyuan Yang
Cosmology and Nongalactic Astrophysics
Gravothermal collapse of self-interacting dark-matter halos with anisotropic velocity distributions
Marc Kamionkowski and Kris Sigurdson
Self-gravitating
galactic halos composed of self-interacting dark matter exhibit the formation of a highly dense core at the
galactic center–a gravothermal collapse. Analytic models to describe this
evolution have been developed and calibrated to numerical simulations initialized with isotropic particle velocity distributions, an assumption not necessarily warranted by the theory of
halo formation. Here we study the dependence of the timescale for gravothermal collapse on the velocity distribution. To do so, we consider self-consistent initial conditions for
halos with the same density distribution but with different velocity distributions. We consider models with constant anisotropy and with an anisotropy that increases with radius. The velocity distributions that we explore have collapse times that differ from that assuming isotropic distributions by more than a factor of two. We argue that these variations may depend on the global changes in velocity-dispersion profiles in these anisotropic models and not just on the degree of anisotropy.
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